Feeding and Ranging Behaviour of a Mountain Gorilla Group (Gorilla gorilla beringeia) in the Tshibinda–Kahuzi Region (Zaïre)

Feeding and Ranging Behaviour of a Mountain Gorilla Group (Gorilla gorilla beringeia) in the Tshibinda–Kahuzi Region (Zaïre)

15 Feeding and Ranging Behaviour of a Mountain Gorilla Group (Gorilla gorilla beringet) in the Tshibinda-Kahuzi Region (Zaire) A L A N G. G O O D A L ...

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15 Feeding and Ranging Behaviour of a Mountain Gorilla Group (Gorilla gorilla beringet) in the Tshibinda-Kahuzi Region (Zaire) A L A N G. G O O D A L L Biology

Department,

Paisley

College

of Technology,

Strathclyde,

Scotland

1. I n t r o d u c t i o n a n d m e t h o d s . . . . . . . . . 1.1. I n t r o d u c t i o n 1.2. S t u d y a r e a 1.3. F i e l d m e t h o d s 2. F e e d i n g b e h a v i o u r . . . . . . . . . . 2.1. Dietetic diversity . . . . . . . . . 2.2. F o r a g i n g a n d feeding techniques . . . . . . . 2 . 3 . N u t r i t i v e values of v e g e t a t i o n s a m p l e s . . . . . . 2.4. T h e o r e t i c a l n u t r i t i o n a l r e q u i r e m e n t s a n d estimates of possible intakes of t h e s t u d y a n i m a l s . . . . . . . . . 3. Feeding a n d ranging behaviour . . . . . . . . 3 . 1 . H o m e r a n g e size a n d d a y j o u r n e y l e n g t h . . . . . 3.2. Differential utilization of h o m e r a n g e . . . . . . 3 . 3 . M o n t h l y v a r i a t i o n of q u a d r a t visits . . . . . . 4. Discussion . . . . . . . . . . . 4 . 1 . Dietetic diversity a n d t h e selection of food items . . . . 4.2. R e g i o n a l v a r i a t i o n s a n d t h e e v o l u t i o n of gorilla diets . . . 4.3. H o m e range utilization . . . . . . . . 4.4. Conservation . . . . . . . . . . 5. S u m m a r y . . . . . . . . . . . Acknowledgements . . . . . . . . . . a

450 450 451 454 456 456 4 5 7 459 462 464 464 466 469 4 7 1 4 7 1 474 475 4 7 7 478 479

T h e precise t a x o n o m i c position of this p o p u l a t i o n is still u n c l e a r a n d m a y n e e d further revision (Groves, pers. c o m m . ) . I n this p a p e r t h e most c o m m o n n o m e n c l a t u r e is used.

450

ALAN

G.

GOODALL

1. I N T R O D U C T I O N A N D M E T H O D S 1.1. Introduction T r u e " m o u n t a i n g o r i l l a s " {Gorilla gorilla beringei) a r e found only in t w o regions of c e n t r a l Africa—the V i r u n g a volcanoes a n d t h e K a h u z i r e g i o n (Groves, 1970). T h e r a n g e of t h e V i r u n g a gorillas is d o m i n a t e d b y l a r g e areas of Hagenia-Hypericum w o o d l a n d ( R o b y n s , 1947) ( S p i n a g e , 1972) w h i c h was m u c h m o r e extensive u n t i l recently. T h e r a n g e of h a b i t a t o c c u p i e d b y t h e K a h u z i gorillas, a n d t h a t of Gorilla gorilla grauen 28° 30'

29°30'

FIG. 1. G e o g r a p h i c regions a n d s t u d y a r e a s .

1 5 . GORILLA

GORILLA

BERINGE!

451

(Groves, 1970), is far m o r e v a r i a b l e a n d is p r o b a b l y m o r e similar to t h e h a b i t a t o c c u p i e d b y gorillas d u r i n g t h e Pleistocene w h e n m o n t a n e forest covered all areas over 500 m . T h i s p a p e r is c o n c e r n e d w i t h t h e feeding a n d r a n g i n g b e h a v i o u r of o n e g r o u p of Gorilla gorilla beringet w h i c h i n h a b i t s t h e T s h i b i n d a - K a h u z i r e g i o n of Z a i r e .

1.2. Study Area T h e m a i n s t u d y a r e a was a r o u n d T s h i b i n d a in t h e K a h u z i - B i e g a N a t i o n a l P a r k . T h i s lies a p p r o x i m a t e l y 100 k m south-west of t h e V i r u n g a volcanoes a n d is in t h e K i v u p r o v i n c e of Z a i r e (Fig. 1). T h e a l t i t u d e of t h e s t u d y a r e a was 2 0 0 0 - 2 5 0 0 m a n d t h e t e r r a i n consisted of m a n y hills covered in various s u b t y p e s of m o n t a n e forest. Several large, flat a r e a s w e r e covered b y Cyperus s w a m p s . T e n n a t u r a l v e g e t a t i o n types w e r e recognized ( L a m b i n o n , M a r i u s et al., pers. c o m m . ) . T h e s e a r e listed in T a b l e I t o g e t h e r w i t h d a t a o n a l t i t u d e a n d d o m i n a n t species. A n e l e v e n t h v e g e t a t i o n t y p e — c u l t i v a t i o n — w a s also recognized. T h i s b o r d e r e d t h e s t u d y a r e a a l o n g its eastern b o u n d a r y a n d consisted of t e a a n d coffee p l a n t a t i o n s t o g e t h e r w i t h crops of b e a n s , p o t a t o e s , b a n a n a s , manioc and maize. A d d i t i o n a l n o t a b l e n a t u r a l v e g e t a t i o n i n c l u d e d tree ferns (Cyathia manniana) o n w e t slopes a m o n g t h e p r i m a r y forest r e g i o n s ; Usnea lichen was c o m m o n o n m a n y trees in t h e p r i m a r y m o n t a n e forest—especially n e a r t h e s w a m p s ; Myrianthus holstii trees w e r e found in localized p a t c h e s a m o n g s t a m i x t u r e of old s e c o n d a r y / p r i m a r y forest w i t h i n t h e s t u d y a r e a s . L a r g e areas of ferns Pteridium africanum w e r e also p r e v a l e n t , especially in regions w h i c h h a d b e e n r e p e a t e d l y b u r n e d a n d cleared for c u l t i v a t i o n . S u c h areas w e r e most extensive o n t h e s o u t h - w e s t e r n b o r d e r s of t h e p a r k n e a r t h e village of N y a k a l o n g e . V e g e t a t i o n types 1-6 (inclusive) a r e primary forest types while v e g e t a t i o n types 7 - 1 0 (inclusive) a r e secondary v e g e t a t i o n types. T e m p e r a t u r e records a t L w i r o ( 1 6 8 0 m ) s h o w a m e a n m i n i m u m v a l u e d u r i n g t h e p e r i o d 1953-70 of 12-4° C, a n d a m e a n m a x i m u m v a l u e of 24-6° C. T e m p e r a t u r e records a t t h e highest station, M t B u k u l u m i s a (2100 m ) , d u r i n g 1971 s h o w a m i n i m u m v a l u e of 9-9° C a n d a m a x i m u m v a l u e of 19*2° C. D u r i n g t h e s a m e p e r i o d t h e relative h u m i d i t y v a r i e d b e t w e e n 4 6 % (at 15.00h in A u g u s t ) a n d 8 2 % (at 6.00h in M a y ) . T h e a m o u n t of p r e c i p i t a t i o n increases w i t h i n c r e a s e d a l t i t u d e , from a n a n n u a l m e a n from 16 years of 1590 m m a t L w i r o , to o n e of a p p r o x i m a t e l y 2300 m m p e r y e a r in t h e region of M t Bukulimisa. M o s t r a i n fell i n M a r c h / A p r i l a n d in S e p t e m b e r - D e c e m b e r ( M e t e o r o l o g i c a l records—IRSAC, Lwiro).

Polysciasfulva (Hiern) Conophyryngiajohnstonii (Stapf) Pannicum s p . (in d r y a r e a s ) Brillantaisia s p . ( i n w e t a r e a s ) Symphonia globulifera L . F . Syzygium guinense ( D . C . ) Coffea s p . Alchornea hirtella Toxocarpus racemosus Rapanea pulchra ( s a p l i n g ) Hypericum lanceolatum ( L a m ) ( s a p l i n g s ) Maytenus acuminatus ( L . F . )

Arundinaria

Xymalos s p . Draecona s p . Halophyllus s p . Rapania pulchra (Gilg a n d Schellenb/ Cyperus latifolius

2350 to 2600m

2250 to 2350m

2200 to 2250m

2 . B a m b o o forest

3 . S w a m p forest

4. Cyperus s w a m p

(Poir)

alpina ( K . S c h u m )

Albizzia gummifera ( G m e l ) Newtonia buchanani ( B a k ) Strombosia grandiflora ( H o o k ) Pteridium qfricanum ( A l s t o n ) is c o m m o n i n t h e u n d e r g r o w t h

O t h e r characteristic species

region

Podocarpus s p . Ficus s p . Chrysophyllum longipes ( E n g l . ) Parinari s p . Carappa grandiflora ( S p r a g u e ) Symphonia globulifera L . F .

D o m i n a n t species

Kahuzi

2000 to 2400m

Altitude

Vegetation types in the Mount

1. H i g h m o n t a n e forest (primary)

Vegetation type

Major

TABLE I

After M a r i u s , 1973.

10. H e r b a c e o u s (N.B. at roadsides a n d in recently cleared areas)

Meadows

Vernonia s p . Maesa lanceolatum Pennisetum purpureum Brillantaisia sp. Lobelia s p . (Κ. Schum)

Urera hypselodendron Basella alba ( L ) Helichrysum s p . Clematis s p . Cissus s p .

(Wedd)

Hyparrhenia s p . Setaria megaphylla

Imperata s p . Pennisetum purpureum

2300m

9. (Κ. Schum)

Pteridium s p . Graminae—many

Hagenia abyssinica ( G m e l ) ( N . B . t h i s f o r m a t i o n is m u c h smaller t h a n t h a t seen in the Virunga)

1800 to 2100m

8. O p e n v e g e t a t i o n dominated by Hagenia abyssinica sp.

Urera hypselodendron ( W e d d ) Basella alba ( L ) Serichostachys scandens Cissus s p . Lactuca s p .

Myrianthus holstii ( E n g l . ) jSleobotonia macrocalyx ( P a x ) Dombeya goetzenii ( K . S c h u m ) Vernonia s p . Hagenia abyssinica ( G m e l ) Alangium chinense ( L o u r )

22000000 ttoo 2400m

7. M i x e d s e c o n d a r y m o n t a n e forest

Alchemilla s p . Deschampsia s p .

Erica arborea ( L ) Senecio s p . Lobelia s p .

2600 to 3300m

5 . a n d 6. H e a t h t y p e s ( N . B . not p r e s e n t i n study area)

454

ALAN

G.

GOODALL

1.3. Field Methods D a t a w e r e collected from trail signs a n d direct o b s e r v a t i o n o n b o t h u n h a b i t u a t e d a n d h a b i t u a t e d gorillas (Goodall, 1974). P a r t i c u l a r family g r o u p s w e r e studied over periods r a n g i n g from o n e to seven m o n t h s — t h e longest period b e i n g spent w i t h o n e g r o u p in t h e T s h i b i n d a a r e a , a n d this b e c a m e t h e " m a i n s t u d y g r o u p " . I t was c o m p o s e d of o n e silverback m a l e , o n e blackback/silverback m a l e , t h r e e b l a c k b a c k e d males, four a d u l t females, t h r e e s u b a d u l t s ( u n k n o w n sexes), t w o juveniles, five infants, plus o n e s e m i p e r i p h e r a l silverback m a l e . A t t h e start of t h e s t u d y t h e g r o u p h a d b e e n p a r t i a l l y h a b i t u a t e d b y i n t e r m i t t e n t contacts m a d e b y M o n s i e u r A d r i e n D e s c h r y v e r ( C o n s e r v a t e u r — P a r e N a t i o n a l d u K a h u z i - B i e g a ) d u r i n g t h e previous t h r e e years. T h e g r o u p w a s followed b y t h e p r e s e n t a u t h o r , almost o n a d a y - t o - d a y basis, d u r i n g A p r i l - O c t o b e r (1972). E a c h d a y ' s trail m a d e b y t h e g r o u p d u r i n g t h e s t u d y p e r i o d w a s followed a n d p a c e d . If a d a y was missed from field w o r k t h e n t h e e n t i r e gorilla trail—from t h e p l a c e t h e y w e r e last seen to t h e i r p r e s e n t position—was followed a n d r e c o r d e d t h e n e x t d a y . I n this w a y 209 of t h e total 213 d a y j o u r n e y s w e r e r e c o r d e d a c c u r a t e l y . F o u r further trails in t h e b a m b o o forest w e r e so criss-crossed t h a t it was impossible t o r e c o r d their exact r o u t e . T h e location of all 213 n i g h t nest-sites was also r e c o r d e d . T h e gorillas' daily routes w e r e carefully m a p p e d o n t o a t r a n s p a r e n t overlay s u p e r i m p o s e d u p o n stereoscopic aerial p h o t o g r a p h s of t h e s t u d y a r e a (scale 1 : 28 000). T h i s m e t h o d e n a b l e d t h e locations to be p i n - p o i n t e d usually to w i t h i n 100 m o n t h e g r o u n d . T h e daily routes w e r e later t r a n s p o s e d o n t o detailed v e g e t a t i o n m a p s (scale 1 : 28 000). T h e s e w e r e divided i n t o o n e - c e n t i m e t r e s q u a r e q u a d r a t s (280 χ 280 m o n t h e g r o u n d ) for t h e purposes of c a l c u l a t i n g t h e h o m e r a n g e sizes of t h e m a i n study g r o u p a n d t h e relative cover of t h e various v e g e t a t i o n types w i t h i n t h e h o m e r a n g e , T a b l e H a . T h e n u m b e r of visits p a i d to each v e g e t a t i o n t y p e b y t h e g r o u p t h r o u g h o u t t h e study p e r i o d was r e c o r d e d daily, a n d m o n t h l y s u m m a r i e s w e r e p r e p a r e d , T a b l e l i b . P r o l o n g e d visual contacts w i t h t h e g r o u p (i.e. of m o r e t h a n 15 m i n u t e s d u r a t i o n ) w e r e n o t possible each d a y . T h i s was d u e m a i n l y t o t h e p r o b l e m s , a n d t h e real d a n g e r s , involved in m a k i n g e x t r e m e l y close c o n t a c t w i t h p a r t i a l l y h a b i t u a t e d gorillas in areas of dense v e g e t a t i o n . D u r i n g t h e s e v e n - m o n t h period, 103 p r o l o n g e d visual c o n t a c t s w e r e m a d e p r o v i d i n g 273-4 h o u r s of d e t a i l e d o b s e r v a t i o n t i m e as follows: A p r i l 41-24 h r ( 1 1 ) ; M a y 50-19 h r ( 1 6 ) ; J u n e 39-33 h r ( 1 7 ) ; J u l y 50-07 h r ( 1 3 ) ; A u g u s t 38-06 h r ( 1 7 ) ; S e p t e m b e r 30-33 h r ( 1 3 ) ;

15. GORILLA

GORILLA

455

BERING El

TABLE II (a)

The relative percentage cover of each vegetation type (1-11) in the home range of the main study group (Tshibinda-Kahuzi) Primary vegetation types 1

% cover in home range

(b)

11-8

2

3

5-8 2-1 (31-3)

Secondary vegetation types 4

7

8

9

10

11

11-6

62-6

0-2

3-1 (68-7)

2-8

0

The numbers and percentages of crossings of each vegetation type within the home range area of the main study group Nos. primary vegetation types

Month Apr.

/o no.

June

/o no. o/ /o no.

July

/o no.

Aug.

/o no.

Sept.

/o no.

Oct.

/o no.

May

/o Totals /o

1 8 14-5 3 5-2 24 24-0 18 20-5 24 22-0 26 23-0 24 22-4 20-2

2

3

1 2 1-8 3-6 2 2 3-4 3-4 5 9 5-0 9-0 1 2 1-1 2-3 8 9 7-3 8-3 21 12 10-6 18-6 31 1 29-0 0-9 11-1 5-7 (49-4)

Secondary vegetation types 4

7

8

3 5-5 2 3-4 13 13-0 6 6-8 14 12-8 21 18-6 19 17-8 12-4

29 52-7 31 53-4 26 26-0 31 35-2 29 26-6 25 22-1 27 25-2 31-4

0 0 3 5-2 2 2-0 4 4-5 4 3-7 0 0 0 0 2-1

9 0 0 0 0 1 1-0 3 3-4 2 1-8 0 0 0 0 1-4 (51-1)

10 12 21-8 15 25-9 20 20-0 22 25-0 19 17-4 8 7-1 5 4-7 16-0

11 0 0 0 0 0 0 1 0-2 0 0 0 0 0 0 0-2

Totals 55 58 100 88 109 113 107 (630)

O c t o b e r 23-35 h r (16) (N.B. t h e n u m b e r in b r a c k e t s refers to t h e n u m b e r of p r o l o n g e d visual c o n t a c t s , n u m e r o u s o t h e r brief e n c o u n t e r s are not included). P l a n t samples w e r e identified a t t h e h e r b a r i u m in I R S A C ( L w i r o ) . T h o s e i n t e n d e d for n u t r i t i o n a l analysis w e r e d r i e d for several h o u r s a t 96° C, cooled in a desiccator a n d t h e n sealed in s e p a r a t e p o l y t h e n e bags. T h e y w e r e l a t e r a n a l y s e d for c r u d e fibre c o n t e n t , t o t a l n i t r o g e n a n d

456

A L A N G. G O O D A L L

gross energy c o n t e n t . M o n t h l y s u m m a r i e s of all d a t a w e r e p r e p a r e d a n d t h e feeding a n d r a n g i n g b e h a v i o u r of t h e gorillas w e r e analysed in relation to these (Goodall, 1974).

2. FEEDING

BEHAVIOUR

2.1. Dietetic Diversity A t least 104 food p l a n t s , c o m p r i s i n g 42 families, w e r e e a t e n by gorillas of t h e two K a h u z i - B i e g a s t u d y areas. O n l y t w o of these lacked p e r s o n a l verification (they w e r e p o i n t e d o u t b y p y g m y t r a c k e r s ) . T h e r e m a i n d e r w e r e either observed to b e e a t e n or found as food r e m n a n t s o n t h e gorilla trail, or b o t h . T h e m a i n study g r o u p a t e over 160 p a r t s of at least 78 of t h e k n o w n food p l a n t s in t h e T s h i b i n d a a r e a . T h e c o m p l e t e list of food items comprises 33 vines, 25 h e r b s , 20 trees, 13 shrubs, six grasses, four ferns a n d t h r e e semiparasites. F u r t h e r details, i n c l u d i n g a c o m p l e t e species list, a r e given in G o o d a l l (1974). F r o m d a t a collected via trail signs a n d visual sightings t h e food p l a n t s w e r e given a r a t i n g for d e g r e e of relative i m p o r t a n c e in t h e diet a n d classified i n t o t h e following groups: 1. Main food items

2.

"Preferred"food items

3. Occasional food items (** a n d ***) 4. Rarely eaten food items (*) 5. Seasonal food items

P l a n t s w i t h a w i d e a n d plentiful distribution, e a t e n t h r o u g h o u t t h e y e a r b u t w i t h some seasonal v a r i a ­ tion in a m o u n t , e.g. Urera hypselodendron, Basella alba, Taccazafloribunda. P l a n t s w i t h a very p a t c h y distribution, often r a r e ; these a r e almost always e a t e n w h e n t h e y a r e e n c o u n t e r e d , usually in g r e a t q u a n t i t i e s , e.g. Achyrospermum omicranthum, Galineria coffeoides, Coffea sp., a n d species of semiparasitic Loranthus. P l a n t s w h i c h m a y h a v e either a w i d e or a p a t c h y distribution b u t m a y or m a y n o t b e e a t e n w h e n e n c o u n t e r e d , e.g. Hypoestes sp., Piper capense, Hagenia abyssinica. P l a n t s w i t h v a r i e d d i s t r i b u t i o n b u t only seen to b e e a t e n o n o n e or t w o occasions, e.g. Albizzia gummifera, Sericostachys scandens, Begonia sp., Dombeya goetzenii. P l a n t s w h i c h a r e e a t e n in large q u a n t i t i e s w h e n t h e y b e c o m e seasonally a b u n d a n t , e.g. fruits of Myrianthus holstii a n d Syzygium guinense, b a m b o o shoots (Arundinaria alpina).

15. GORILLA

GORILLA

BERING El

457

A l t h o u g h t h e m a i n s t u d y g r o u p h a d o p p o r t u n i t i e s to eat eggs or helpless chicks of several species of birds, I found n o i n d i c a t i o n t h a t these w e r e ever e a t e n . I n fact o n several occasions i n t a c t eggs a n d s q u a b s w e r e found o n or a t t h e side of t h e trail left b y feeding gorillas. W h i l e c h i m p a n z e e s in t h e s t u d y a r e a frequently d u g o p e n t h e u n d e r g r o u n d nests of Trigona bees a n d a t e h o n e y , g r u b s a n d a d u l t bees, t h e gorillas w e r e seen to pass n e a r to, a n d even over, nests of these stingless bees w i t h o u t a t t e m p t i n g to feed. N o traces of a n i m a l m a t t e r of a n y k i n d w e r e ever found in t h e m a n y s a m p l e s of d u n g w h i c h w e r e analysed.

2.2. Foraging and Feeding Techniques I n g e n e r a l t h e g r o u p w a s s p r e a d o u t while foraging, t h e m a x i m u m s p r e a d d e p e n d i n g u p o n t h e local v e g e t a t i o n types a n d t h e food items b e i n g e a t e n . W h e n feeding o n Ensete sp. (the wild b a n a n a ) most of t h e g r o u p c o n c e n t r a t e d a r o u n d o n e or t w o of these large trees for periods u p to o n e or t w o h o u r s u n t i l t h e p l a n t w a s almost c o m p l e t e l y d e m o l i s h e d . W h i l e foraging in either old s e c o n d a r y forest or b a m b o o forest, t h e g r o u p often s p r e a d o u t over distances of u p to 200 m , a n d sometimes split i n t o t w o s u b g r o u p s . A n i m a l s from t h e m a i n s t u d y g r o u p w e r e seen to feed in almost every conceivable position, from sitting o n t h e g r o u n d to h a n g i n g in t h e trees b y o n e foot a n d o n e h a n d , p a r t l y u p s i d e - d o w n a n d a n y t h i n g u p to 40 m a b o v e t h e g r o u n d . T h e feeding p o s t u r e a d o p t e d was g r e a t l y influenced b y t h e food i t e m b e i n g c o n s u m e d a n d t h e local v e g e t a t i o n t y p e . F o r e x a m p l e , t h e h e r b a c e o u s vine Basella alba, o n e of t h e m a i n food items in K a h u z i - B i e g a , usually o c c u r r e d in g r e a t q u a n t i t i e s in o p e n areas associated w i t h Pteridium ferns a n d o t h e r low h e r b a c e o u s g r o w t h . Gorillas w o u l d typically sit d o w n , r e a c h o u t w i t h o n e or b o t h h a n d s a n d pull in p a t c h e s of this vine w h i c h w e r e e n t a n g l e d in t h e n e a r b y v e g e t a t i o n . Leaves w e r e t h e n e i t h e r b i t t e n off directly or a n u m b e r w e r e collected m a n u a l l y a n d t h e n p l a c e d in t h e m o u t h . T h e leaves of m a n y vines, p a r t i c u l a r l y Urera hypselodendron, w e r e collected b y r a p i d l y p u l l i n g t h e stem b e t w e e n c l e n c h e d fingers a n d t h e resulting b u n c h was t h e n e a t e n (cf. C a s i m i r , 1975) a l t h o u g h s o m e t i m e s t h e y w e r e d i s c a r d e d a n d only t h e b a r k w a s e a t e n . T h i s b e h a v i o u r e m p h a s i z e s t h e i m p o r t a n c e of visual verification of all food items, for m a n y c a n easily be missed b y reliance o n trail signs a l o n e . F e e d i n g a n i m a l s sometimes stood b i p e d a l l y to r e a c h u p a n d pull d o w n low h a n g i n g vines from n e a r b y shrubs. W h e n feeding intensively t h e gorillas often p u s h e d leaves r a p i d l y i n t o t h e m o u t h w i t h o n e h a n d while collecting m o r e w i t h t h e o t h e r h a n d . T h e b a r k of w o o d y vines such as Urera hypselodendron w a s either b i t t e n off

458

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directly w i t h t h e incisor teeth or scraped off while t h e vine w a s pulled b e t w e e n clenched teeth. L a r g e q u a n t i t i e s w e r e e a t e n b y all gorillas except suckling infants. A m a r k e d vertical d i s t r i b u t i o n of feeding a n i m a l s was often observed, especially in s e c o n d a r y forest regions, w i t h t h e largest silverback usually feeding o n t h e g r o u n d a n d t h e rest of t h e g r o u p feeding a b o v e g r o u n d . S o m e i n d e p e n d e n t infants c l i m b e d highest ( u p to 40 m ) , while juveniles w e r e seen to c l i m b h i g h e r t h a n females, a n d t h e females h i g h e r t h a n m a l e s — t h o u g h o n e very large b l a c k b a c k e d (almost silverbacked) m a l e was frequently seen at heights of u p to 35 m . H o w e v e r , w h e n feeding o n t h e leaves of semiparasitic Loranthus sp. or t h e fruits of Myrianthus or Syzygium, all a n i m a l s w e r e seen to c l i m b a n d even t h e largest silverb a c k e d m a l e , w h o m u s t h a v e w e i g h e d at least 200 kg, w a s seen to c l i m b over 20 m h i g h — e v e n a m o n g h a n g i n g vines. O n l y a few h e r b a c e o u s p l a n t s such as Rubia cordifolia, Galium simense, t h r e e species of e p i p h y t i c ferns, a n d possibly a few h e r b a c e o u s vines o n occasions, w e r e seen to be e a t e n in their entirety. P a r t or p a r t s of o t h e r food p l a n t s , such as leaves—especially u p p e r leaves a n d g r o w i n g t i p s — b a r k , flowers, fruits, shoot, root, p i t h a n d even d r y w o o d , w e r e con­ s u m e d in v a r y i n g q u a n t i t i e s . W h i l e only o n e p a r t w a s e a t e n from t h e majority of food p l a n t s , t h r e e or four p a r t s w e r e e a t e n from some of t h e m a i n food items. T h e gorillas used their h a n d s , lips, teeth a n d j a w s in extremely well c o - o r d i n a t e d m o v e m e n t s in o r d e r to collect, p r e p a r e a n d ingest food items r a n g i n g from tiny succulent fruits to t o u g h fibrous b a r k or p i t h from m a t u r e trees. D e t a i l e d descriptions of t h e feeding tech­ n i q u e s used to d e a l w i t h this w i d e variety of food items a r e given b y Goodall(1974). Drinking. T h e gorillas of t h e m a i n s t u d y g r o u p a p p a r e n t l y o b t a i n e d most, if n o t all, of t h e i r w a t e r r e q u i r e m e n t s from t h e h e r b s a n d vines w h i c h w e r e e a t e n d u r i n g t h e r a i n y season; from t h e j u i c y Myrianthus holstii fruits a n d t h e s a t u r a t e d leaf bases of Cyperus latifolius d u r i n g t h e d r y season; from b a m b o o shoots d u r i n g t h e latter w e t p a r t of t h e y e a r a n d from t h e p i t h of wild b a n a n a s a t various times t h r o u g h o u t t h e y e a r . H o w e v e r , o n several occasions trail signs, especially in t h e s w a m p s d u r i n g t h e d r y season, strongly i n d i c a t e d t h a t gorillas h a d p u l l e d vegetation aside to expose a small s t r e a m or trickle of w a t e r . O n t w o s e p a r a t e occasions m e m b e r s of t h e m a i n s t u d y g r o u p w e r e seen to lick d e w from n e a r b y leaves while t h e y w e r e still in t h e nests early in t h e m o r n i n g . W h e n feeding o n t h e p i t h of t h e wild b a n a n a (Ensete sp.) t h e gorillas w e r e seen to ingest large a m o u n t s of t h e copious cell-sap present in this food item.

15. GORILLA

GORILLA

BERINGE/

459

2.3. Nutritive Values of Vegetation Samples A variety of vegetation collected from many parts of the study area was analysed for nutritive values in terms of gross available energy and protein. The results for 39 items known to be eaten by the study gorillas revealed gross energy values ranging from 1467-2472 kj/100 g (dry matter) and crude protein values from 2*18-47-68 g/100 g (dry matter). The percentage of crude fibre in 22 of these samples ranged from as low as 7-41% (in new bamboo shoots) to as high as 43-07% (see Table III). The mean nutritional values of the remaining 28 samples showed no significant differences, in terms of gross available energy and protein, to the known food plants (t-test). These plants appeared to be as readily available as known food items—in many cases growing with them in mixed stands. Some were eaten by Pan troglodytes and several species of Cercopithecus. Although no crude fibre analyses were carried out on these samples it seems unlikely that these were significantly different from known gorilla food items. It is obvious that whatever the gorillas actually gain from their food, in terms of both energy and protein, will depend not only on their intrinsic availability in various food items but also on the efficiency of the gorilla digestive system. Therefore, in order to arrive at what Mitchell (1964) calls "the truly metabolizable" energy of a diet, i.e. net available, various debits have to be made from the gross available values owing to losses via indigestibility, urine and the specific dynamic action (SDA) of a given ration. According to Dr T. Lawrence (pers. comm.) the losses due to these three factors are generally in the order of 70-80%, 5-10% and 15-20% respectively. Unfortunately it was impossible to collect accurate data on each of these losses for free-living gorillas, and no animals were available in captivity which were feeding on a "wild-type" diet. However, an attempt has been made (Goodall, 1974) to estimate values of the net available energy (and protein) in the plant samples from Kahuzi. There is a high negative correlation between the digestibility of vegetable diets and their crude fibre content. Regression curves of these relationships for various farm animals are given by Mitchell (1964). Since pigs are animals of comparable size to gorillas, and also are non-ruminants, it was decided to use the regression curve given for these animals in order to estimate the digestibility values of some of the Kahuzi plant samples. By this method it was found that they varied from as high as 84% digestibility in bamboo shoots and the leaves of the herbaceous vine Basella alba, to as low as 38% for the leaves of Smilax Q

Leaves

Leaves

G. braunii

All

woodfordioides

E.

Leaves

Leaves

cordifolia

R.

2191

1918

1601

1749

1595

Leaf base

Leaves

latifolius

C.

1897

1909

1780

2066

2162

2016

1802

2351

2056

Leaves

1716 1800

F r u i t (flesh)

Lactuca s p .

holstii

holstii

alpina

A.

M.

Shoots

Ensete s p .

M.

Leaves

Pith

Ensete s p .

racemosus

T.floribunda

T.

Fruit

Leaves

hypselodendron

U.

B. alba

hypselodendron

U.

Leaves

Bark

hypseledendron

U.

Name

Part eaten

2-46

2-79

2-26

3-28

1-47

1-76

3-15

7-63

0-59

3-46

2-03

2-98

4-82

4-77

1-87

2-85

Gross

15-37

17-43

14-12

20-50

9-18

11-00

19-68

47-68

3-68

21-62

12-68

18-62

39-06

29-81

11-68

17-81

protein ( N 2 x 6-25) (%)

11-41

29-83

16-58

24-72

7-41

23-17

27-26

16-46

7-29

25-83

37-93

14-99

Crude fibre (%)

80-0

55-0

72-0

68-0

84-0

64-0

58-0

72-0

84-0

60-0

43-0

75-0

Digestibility (%)

group

1370

569

12-5

3-3

6-6

12-0 994 1233

1-8 36-8

1474

8-6

11-2

30-0

12-8

6-0

12-0

Metabolizable protein/100 g

864

826

1209

1391

1008

500

1108

Metabolizable energy/100 g

to be eaten by the main study

TABLE III values of some plants found

Gross energy Gross N 2 kJ/lOOg (%)

Nutritive

***** ***** *** ***** **** *** ***** ***** ***** **** ***** ***** **** *** **** ****

Relative amount eaten

Fruit

Chrysophyllum

Leaves

kraussiana

omicrantham

S.

A.

Leaves

Leaves

R.

4-99

1746

Bark

4-33

2014

Leaves

D.

goetzenii

6-57

7-30

2005 1870

Leaves

Leaves

gummifera

A.

S. scandens

niamiamnensis

/.

Leaves

3-21 0-35

1537 2031

Leaves

Helichrysum

sp.

3-09

1892

Leaves

cordata

M.

sp.

6-77

1793

Leaves

punctata

M.

Eucalyptus

2-29

1698

All

G. simense

2-26

2040

Leaves

2-43

4-62

Embelia s p .

1753

2099

0-99

2-09

5-24

3-82

4-73

2-43

1-22

2-45

Leaves

Leaves

1-91 2-44 5-91

cordifolia

T.

Brillantaisia

sp.

1547

Stem

P.

purpureum

2141

F r u i t (flesh)

1875

1776

1878

2092

1880

S. guinense

abyssinicus

Hypoestes s p .

Leaves

Bark

Shefflera s p .

2472

1911 2062

sp.

1820

All Leaves

(1)

All

Polypodiumsp.

Poly podium s p . (2) P. capense

19-09 27-11

12-47

41-10

21-23

21-17

12-17

34-39

43-07

40-78

24-9

16-94

45-62

31-18

2-18

20-06

19-31

42-31

14-31

14-12

15-18

28-87

6-18

13-06

32-75

23-87

29-56

15-18

7-62

15-31

15-25 36-93

11-93

70-0

77-0

66-0

66-0

78-0

50-0

38-0

41-0

62-0

72-0

1070

1346

790

923

1468

535

239

294

1131

1238

23-5

30-6

9-7

21-8

9-0

9-4

1-7

1-2

7-0

22-1

*** *** **** *** **** ** **** ***** *** * *** *** ** ** ** *** * ** * * *

?

****

462

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kraussiana. A s s u m i n g t h a t these losses d u e to indigestibility w e r e e q u a l to 7 0 % of t h e total losses, further allowances w e r e m a d e for u r i n a r y losses of 1 0 % , a n d those d u e to S D A of 2 0 % . Finally, d e d u c t i o n s for such t o t a l losses w e r e m a d e from t h e gross a v a i l a b l e values for e a c h p l a n t s a m p l e . T h e resulting estimates of net a v a i l a b l e m e t a b o l i z a b l e e n e r g y a n d p r o t e i n a r e listed in T a b l e I I I .

2.4. Theoretical Nutritional Requirements and Estimates of Possible Intakes of the Study Animals Since d a t a a r e available o n b o t h t h e energy a n d p r o t e i n values of all t h e m a i n food items of t h e study gorillas, a n d also of m a n y m i n o r food items, it is possible to speculate o n t h e daily intakes of t h e study a n i m a l s a n d r e l a t e this to their theoretical n u t r i t i o n a l r e q u i r e m e n t s . T h e total w e t w e i g h t i n t a k e of food p e r d a y was e s t i m a t e d for various age/sex classes of t h e s t u d y a n i m a l s . Based o n calculations m a d e from t h e daily d u n g o u t p u t , t h e w e t w e i g h t intakes for a n a d u l t silverback m a l e a n d a n a d u l t female gorilla w e r e e s t i m a t e d to b e 30 a n d 18 kg p e r d a y respectively (Goodall, 1974). Calculations of daily intakes based o n direct observations of a n i m a l s feeding o n single food items, such as b a m b o o shoots a n d Myrianthus fruits, for p r o l o n g e d periods of t i m e i n d i c a t e d t h a t t h e estimations based o n d u n g o u t p u t were realistic ones. Since it w o u l d obviously b e impossible to s i m u l a t e t h e a c t u a l daily " m e n u " of t h e study a n i m a l s t h e e n e r g y a n d p r o t e i n values for t w o very simplified versions h a v e b e e n w o r k e d o u t as s h o w n in T a b l e I V . I t c a n b e seen t h a t e q u a l p r o p o r t i o n s of t h r e e m a i n food items, Basella (leaves) a n d Urera (bark a n d leaves), w o u l d yield some 45 530 k j a n d 560 g p r o t e i n / 3 0 kg wet weights. A similar a m o u n t of b a m b o o shoots will yield slightly less energy i.e. 44 220 k j b u t d o u b l e t h e a m o u n t of p r o t e i n a t 1 1 0 4 g . T h u s , these a r e possible net available values for a n a d u l t w e i g h i n g some 200 kg. T h e relationship b e t w e e n b o d y w e i g h t a n d basal m e t a b o l i c r a t e ( B M R ) is discussed b y B r o d y (1945) w h o suggests r

B M R = 70^ °-

75

k c a l / d a y ( W = w t in kg).

I n a s t u d y of 120 species of zoo a n i m a l s , E v a n s a n d Miller (1968) f o u n d t h a t t h e v o l u n t a r y calorie intakes w e r e almost exactly twice t h a t r e q u i r e d for basal m e t a b o l i s m . E x a m i n a t i o n of T a b l e I V shows t h a t t h e e s t i m a t e d n e t a v a i l a b l e energy for b o t h a 200 kg a n i m a l a n d a 100 kg a n i m a l (equivalent to a n a d u l t m a l e a n d a n a d u l t female gorilla respectively) a r e almost t h r e e times t h e theoretically p r e d i c t e d v a l u e for t h e i r basal m e t a b o l i c r a t e . R e g a r d i n g p r o t e i n levels, Bilby (1968) refers to a figure of 1 g p r o t e i n p e r kg b o d y w e i g h t p e r d a y b e i n g a useful

15. GORILLA

GORILLA

463

BERINGE/

TABLE IV Simplified

menus for

Food item (A) 2 0 0 k g a n i m a l

(Ä) = 200 kg animal and (Β) =100 values of each

W e t weight of i n t a k e (in kg)

Dry weight of i n t a k e (in kg)

kg animal,

Available water ( i n litres)

to show the

Net available energy (in kj)

nutritive

Net available protein ( i n g)

(i) T h r e e t y p i c a l m a i n f o o d i t e m s e a t e n i n s e c o n d a r y forest a r e a s 10-0

1-0

9-0

13 9 1 0

300

Urera (leaves)

10-0

1-5

8-5

16 6 2 0

180

Urera ( b a r k )

10-0

3-0

7-0

15 0 0 0

90

Totals

30-0

5-5

24-5

45 530

570

Basella

(leaves)

(ii) B a m b o o s h o o t s —- e a t e n a l m o s t e x c l u s i v e l y w h e n i n s e a s o n B a m b o o (shoots)

20-0

2-0

18-0

29 480

736

B a m b o o (shoots)

30-0

3-0

27-0

44 220

1104

*cf. T h e o r e t i c a l l y p r e d i c t e d v a l u e s for a 2 0 0 k g a n i m a l / d a y = ( 1 5 5 6 0 B M R ) ( 2 0 0 )

(B)

100 k g a n i m a l

Basella

(leaves)

(i) T h r e e t y p i c a l m a i n f o o d i t e m s e a t e n i n s e c o n d a r y forest a r e a s 6-0

0-6

5-4

8346

180

Urera (leaves)

6-0

0-9

5-1

9972

108

Urera ( b a r k )

6-0

1-8

4-2

9000

108

18-0

3-3

14-7

27 318

396

Totals

(ii) B a m b o o s h o o t s — e a t e n a l m o s t
18-0

1-8

16-2

26 352

662

*cf. T h e o r e t i c a l l y p r e d i c t e d v a l u e s for a 100 k g a n i m a l / d a y = ( 9 2 5 2 B M R ) ( 1 0 0 ) Compare " G u y " — 2 2 0 kg a d u l t m a l e gorilla in L o n d o n Z o o r e c e i v e s / d a y ( N . B . gross v a l u e s )

28 060

222

A 2 0 0 k g p i g r e c e i v e s / d a y ( N . B . gross v a l u e s )

30 206

117

guide to the protein requirements of man. It can be seen from Table IV that the figures for net available protein for both 100 and 200 kg animals are between three and six times greater than this theoretical value. It must be stressed here that the gorillas could do even better than these simplified estimates by eating more leaves of Basella alba or other similarly highly digestible items with high gross nutritive values. Examples of alternative food items like this would be the leaves of Piper

464

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G.

GOODALL

capense a n d Achyrospermum omicranthum, b o t h of w h i c h a r e listed as " p r e f e r r e d " food items, i.e. found only in some areas b u t usually e a t e n w h e n e v e r found. I n a d d i t i o n all gorillas w e r e seen to choose m a i n l y t h e u p p e r leaves a n d g r o w i n g tips of almost all food items. T h u s they w o u l d get h i g h e r values t h a n i n d i c a t e d from t h e p l a n t samples w h i c h w e r e analysed since I was frequently forced to collect older leaves from some p l a n t s in o r d e r to h a v e sufficient for analysis. All a n i m a l s h a v e a d d i t i o n a l energy r e q u i r e m e n t s to those for B M R i.e. g r o w t h = 2 - 3 χ B M R ( A b r a m s , 1968) e.g. m o v e m e n t a n d l a c t a t i o n [ t h o u g h l a c t a t i n g females m a y well increase their food i n t a k e ( A b r a m s , 1968)]. I t w o u l d a p p e a r from t h e analyses t h a t t h e h e r b i v o r o u s diets of t h e K a h u z i gorillas p r o v i d e a m p l e energy a n d p r o t e i n to m e e t these r e q u i r e m e n t s . I n p a r t i c u l a r , t h e a n n u a l n e w shoots of b a m b o o h a v e high p r o t e i n levels. Since these are e a t e n in vast a m o u n t s d u r i n g S e p t e m b e r / O c t o b e r it is possible t h a t t h e gorillas build u p reserve energy a n d fat levels from this item. I n t u r n this m a y influence t h e periodicity of b i r t h s — b u t u n f o r t u n a t e l y n o evidence is available to confirm or d e n y this possibility. O n e further aspect of t h e n u t r i t i o n a l h a b i t s of free-living gorillas is t h a t t h e y a p p a r e n t l y r a r e l y d r i n k free-standing w a t e r . I t c a n be seen from t h e d a t a p r e s e n t e d in T a b l e I V t h a t a n a d u l t silverback m a l e gets b e t w e e n 24 a n d 27 litres of w a t e r p e r d a y from 30 kg of food a n d a n a d u l t female gets 15 to 16 litres from 18 kg of food. Analysis of t h e d u n g showed t h a t it c o n t a i n e d a p p r o x i m a t e l y 8 0 % w a t e r (possibly 9 0 % w h e n they w e r e feeding o n y o u n g b a m b o o shoots). Therefore, if w e assume for a n a d u l t m a l e t h a t a p p r o x i m a t e l y 4 litres are lost p e r d a y in t h e d u n g , a further 4 litres in u r i n e losses a n d 2 litres in e v a p o r a t i o n losses, h e still has some 14-17 litres of w a t e r available for m e t a b o l i s m . I n these circumstances it is h a r d l y surprising t h a t t h e y only rarely d r i n k free-standing w a t e r .

3. FEEDING A N D R A N G I N G

BEHAVIOUR

3.1. Home Range Size and Day Journey Length D u r i n g t h e s e v e n - m o n t h p e r i o d ( A p r i l - O c t o b e r inclusive) t h e m a i n s t u d y g r o u p w e r e found to visit 256 q u a d r a t s w i t h i n t h e s t u d y a r e a . A further 32 q u a d r a t s , w h i c h h a d n o t b e e n visited b u t w e r e s u r r o u n d e d by o t h e r q u a d r a t s w h i c h h a d , w e r e i n c l u d e d in t h e final h o m e r a n g e total 2 of 288 q u a d r a t s — o r 22-6 k m . H o w e v e r , in t h e hilly t e r r a i n of this region a correction factor of χ 1-5 was a p p l i e d (Goodall, 1974, p . 191), 2 2 giving a final v a l u e of 34 k m . T h i s is smaller t h a n t h e 4 0 - 5 0 k m

15.

GORILLA

GORILLA

BERINGEI

465

a

suggested b y Casimir a n d B u t e n a n d t ( 1 9 7 3 ) for a n e i g h b o u r i n g g r o u p in t h e M t K a h u z i region, b u t c o n s i d e r a b l y l a r g e r t h a n t h e h o m e r a n g e estimates for gorillas in o t h e r regions, such as those given b y Schaller b ( 1 9 6 3 ) , G o o d a l l (1971), J o n e s a n d S a b a t e r Pi (1971) a n d Fossey (1974). D u r i n g t h e s e v e n - m o n t h s t u d y p e r i o d t h e trails left b y foraging a n d w a n d e r i n g a n i m a l s took every c o n c e i v a b l e form, consisting of s t r a i g h t lines, zig-zags, curves a n d even ellipses a n d circles. I n c o n s e q u e n c e t h e straight-line i n t e r n e s t distances w e r e often c o n s i d e r a b l y less t h a n t h e a c t u a l d a y j o u r n e y of t h e g r o u p . E a c h d a y ' s trail was different. Occasionally t h e g r o u p followed old e l e p h a n t tracks, a n d even m o r e r e c e n t p o a c h e r ' s p a t h s , t h r o u g h t h e forest. S o m e p a t h s w e r e used r e p e a t e d l y — e s p e c i a l l y w h e n t h e g r o u p m o v e d from o n e a r e a to a n o t h e r . H o w e v e r , t h e g r o u p sometimes s u d d e n l y c h a n g e d direction, b r a n c h e d off a g o o d trail, a n d m a d e t h e i r o w n w a y t h r o u g h t h e forest. E a c h d a y j o u r n e y was p a c e d , or p a c e d a n d e s t i m a t e d , over t h e w h o l e s e v e n - m o n t h p e r i o d . W h e r e v e r possible t h e m a i n trail left b y t h e large silverback was followed, a n d all distances r e c o r d e d a r e in r e l a t i o n to t h e n i g h t nest of this a n i m a l . T h e total r a n g e of d a y j o u r n e y s r e c o r d e d for t h e m a i n s t u d y g r o u p a t T s h i b i n d a w a s from 200 paces t o 4850 p a c e s ( m e a n 1511, s.d. 8 3 7 ) . E a c h p a c e c a n b e t a k e n as a p p r o x i m a t e l y e q u a l to 0-7 m — t h u s giving a r a n g e of 140 m - 3 - 4 k m . A l t h o u g h h e a v y r a i n t e m p o r a r i l y h a l t e d m o v e m e n t s , t h e r e was n o direct c o r r e l a t i o n b e t w e e n d a y j o u r n e y l e n g t h a n d rainfall. M o n t h l y s u m m a r i e s of all d a y j o u r n e y s a r e s h o w n in Fig. 2 w h e r e c o n s i d e r a b l e v a r i a t i o n c a n b e seen in t h e d a t a b o t h w i t h i n and between months. T h e s e d a y j o u r n e y s w e r e c o m p a r e d o n a m o n t h - t o - m o n t h basis (t-test) see T a b l e V . I t w a s found t h a t : (a) d a y j o u r n e y s w e r e low d u r i n g A p r i l , M a y a n d J u n e w i t h m e a n values n o t significantly different b e t w e e n t h e t h r e e m o n t h s ; (b) t h e r e was a significant increase in m e a n d a y j o u r n e y from J u n e J u l y ( Ρ = < 0 · 0 0 1 ) , w h i c h was further increased d u r i n g A u g u s t a n d September; a

W h e n calculating t h e h o m e r a n g e a r e a of Gasimir's s t u d y g r o u p I believe t h a t these a u t h o r s h a v e erroneously i n c l u d e d d a t a ( O c t . 1 9 7 1 - F e b . 1972) o n t h e m o v e m e n t s of another g r o u p whose h o m e r a n g e lay to t h e n o r t h . T h i s was t h e g r o u p w h i c h b e c a m e m y " m a i n s t u d y g r o u p " in A p r i l 1972. By c a l c u l a t i n g r a n g e size a n d usage almost entirely from trail signs, w h i c h were often u n c o n f i r m e d b y visual sightings, t h e y a r e p e r p e t u a t i n g t h e m i s t a k e n belief t h a t t h e h o m e r a n g e s of t h e t w o n e i g h b o u r i n g g r o u p s a r e c o m p l e t e l y i n t e r c h a n g e a b l e . T h i s is n o t so. H o w e v e r , t h e trails of t h e t w o g r o u p s d i d cross o n several occasions—hence t h e u n w a r y tracker could easily follow t h e " w r o n g " g r o u p unless visual identifications w e r e m a d e . b I believe t h a t Schaller m a d e his calculations using a n i n c o r r e c t scale for t h e aerial p h o t o ­ g r a p h s from w h i c h h e d r e w his m a p s . F o r further details see G o o d a l l , 1974, p . 230.

466

A L A N G. G O O D A L L FT

KM

IOOOO-,

25

8000

Υ2 6000 I mile — Ι·5

4000

1/2 mile — 2000 Η 0-5

April May June July Aug. Sept. Oct. Kahuzi totals Study months FIG. 2. D a y j o u r n e y s ( m a i n s t u d y g r o u p ) .

(c) there was a significant decrease in mean day journey length from September to October (P = < 0 · 0 0 1 ) . However, the October mean was still higher than that in June (P= < O 0 0 1 ) . 3.2. Differential Utilization of Home Range The main study group visited and utilized different areas within their home range at differing rates and times during the study period. The 256 quadrats which were entered within the home range received a total of 805 visits. The frequency and location of these visits are shown

1 5 . GORILLA

GORILLA

467

BERING El

TABLE V Comparisons of mean monthly internest distances for main study group, showing the distribution of "d" and its significance value if any April April May June July Aug. Sept. Oct.

May

June

July

Aug.

Sept.

Oct.

0-78

0-76 1-37

3-62* 5-16* 2·7|

6-0* 6-69* 4-47* 1-07

5-92* 6-46* 4-74* 1-75 0-89

4-08* 5-64* 3 12t 0-41 1-94 2-59f

Significance values i n d i c a t e d a r e : * P < O 0 0 1 ; f P < 0-01.

in Fig. 3 a n d t h e d i s t r i b u t i o n of visits p e r q u a d r a t in Fig. 4. I t was found t h a t : (a) 32 q u a d r a t s inside t h e b o u n d a r i e s of t h e h o m e r a n g e w e r e never visited d u r i n g t h e s t u d y p e r i o d . (b) 5 0 % of all visits w e r e r e c o r d e d from a total of only 57 q u a d r a t s . T e n q u a d r a t s a c c o u n t e d for 134 e n t r i e s — o n e q u a d r a t a l o n e b e i n g e n t e r e d 15 times. T h e s t u d y a r e a was a mosaic of n i n e of t h e 11 v e g e t a t i o n types p r e s e n t in t h e K a h u z i a r e a (see T a b l e H a ) . T h e h o m e r a n g e a r e a i n c l u d e d 6 8 - 7 % of s e c o n d a r y v e g e t a t i o n types a n d 3 1 - 3 % of p r i m a r y v e g e t a t i o n types. U n t i l this a r e a was g r a n t e d N a t i o n a l P a r k status (in N o v e m b e r 1970) extensive p a t c h e s of forest w e r e subjected to "slash a n d b u r n " types of a g r i c u l t u r a l usage. T h e y w e r e later also used for g r a z i n g cattle (Deschryver, pers. c o m m . ) . A r e a s a b a n d o n e d after a few years of cultivation t h u s formed pockets of s e c o n d a r y r e g e n e r a t i n g forest of v a r y i n g age, species composition a n d p h y s i o g n o m y . T h e s e w e r e p a r t i c u l a r l y extensive in t h e eastern section (near t h e p r e s e n t p a r k b o u n d a r y ) . M o r e westerly areas c o n t a i n e d p r i m a r y forest v e g e t a t i o n types, some of w h i c h , especially Arundinaria alpina a n d Cyperus latifoliusy w e r e very localized (see G o o d a l l , 1974 for v e g e t a t i o n m a p s ) . T h e n u m b e r s of visits p a i d to e a c h v e g e t a t i o n t y p e a r e s h o w n in T a b l e l i b . A c o m p a r i s o n of t h e resources of t h e q u a d r a t s w h i c h w e r e n e v e r visited a n d w h i c h w e r e frequently visited reveals t h a t , of t h e 32 q u a d r a t s w h i c h w e r e n o t visited d u r i n g t h e s t u d y p e r i o d : 17 c o n t a i n e d l a r g e areas of Cyperus s w a m p or a m i x t u r e of s w a m p a n d forest v e g e t a t i o n ; t h r e e c o n t a i n e d entirely or m a i n l y p r i m a r y m o n t a n e forest v e g e t a t i o n ;

468

ALAN G.

GOODALL

No. of visits/quadrat

FIG. 3 . N u m b e r s of visits t o q u a d r a t s in h o m e r a n g e d u r i n g seven consecutive m o n t h s .

Ο

1

2

3

4

5

6

7

8

9

10

N u m b e r o f v i s i t s p e r quadrat FIG. 4 . H o m e r a n g e u t i l i z a t i o n .

11

12

13

14

15

15. GORILLA

GORILLA

469

BERINGEI

five c o n t a i n e d very large areas of y o u n g Hagenia trees a n d o p e n , scrub-like v e g e t a t i o n ; seven c o n t a i n e d s e c o n d a r y v e g e t a t i o n types. T h e first 17 q u a d r a t s a n d t h e following t h r e e of t h e a b o v e list a r e situated either in t h e c e n t r e or n e a r t h e b o r d e r s of t h e very large Cyperus s w a m p in t h e n o r t h - e a s t e r n section of t h e h o m e r a n g e . T h e five q u a d r a t s c o n t a i n i n g t h e y o u n g Hagenia trees w e r e very p o o r feeding areas a n d w e r e exceptionally d e h y d r a t e d d u r i n g t h e d r y season. T h e final seven q u a d r a t s all c o n t a i n e d s e c o n d a r y v e g e t a t i o n w i t h a n a b u n d a n c e of available food, b u t all w e r e in t h e south-eastern section of t h e h o m e r a n g e , across t h e t a r m a c r o a d from t h e m a i n p a r t of t h e r a n g e a n d in a n a r e a frequently visited b y a n o t h e r gorilla g r o u p . D u r i n g t h e p e r i o d of investigation t h e m a i n s t u d y g r o u p d i d n o t s p e n d m u c h t i m e in this section of their h o m e r a n g e — d e s p i t e t h e fact t h a t a b u n d a n t s e c o n d a r y g r o w t h was a v a i l a b l e d u r i n g t h e r a i n y seasons. T e n q u a d r a t s received t e n or m o r e entries. Six w e r e in a n a r e a of b a m b o o forest/swamp a n d swamp/forest m i x t u r e , t w o w e r e in a n a r e a c o n t a i n i n g s w a m p a n d several large Myrianthus tree " c r o s s r o a d s " of m a n y previous trails of t h e m a i n s t u d y g r o u p , while t h e r e m a i n i n g t w o c o n t a i n e d m a i n l y l u x u r i a n t s e c o n d a r y forest g r o w t h a n d also m o r e Myrianthus tree " c r o s s r o a d s " .

3.3. Monthly Variation of Quadrat Visits T h e s u m m a r i e s of all m o n t h l y q u a d r a t visits a r e c o n t a i n e d in T a b l e V I . A c h i - s q u a r e d test showed t h a t t h e d a t a w e r e n o t h o m o g e n e o u s

TABLE VI Monthly

totals of visits and revisits per quadrat study group

N o . of visits per quadrat 1 2 3 4 5 6 7 8

Apr.

in their home range by the

T o t a l s / m o n t h p a i d to each category May June Sept. Oct. Aug. July

30 11

50 9 2

53 8 2 1

58 32 2 4 2

61 25 15 4 2

38 26 7 7 3 1 1

41

61

64

98

107

83

21 18 5 2 2 0 1 1 50

main

Totals 311 129 33 18 9 1 2 1 504

470

ALAN

G.

GOODALL

[P= < O 0 0 1 ) . E x a m i n a t i o n of these d a t a shows t h e following m o n t h l y trends. (a) T h e r e was a g r a d u a l increase in t h e n u m b e r of total visits p e r m o n t h (from 52 in A p r i l ) , u p to a m a x i m u m of 183 in A u g u s t . (b) Similarly t h e r e was a p a r a l l e l increase seen in t h e total n u m b e r of different q u a d r a t s visited p e r m o n t h (from 41 in April) u p to a m a x i m u m of 107 in August. (c) Both of these t r e n d s w e r e reversed d u r i n g S e p t e m b e r a n d especially O c t o b e r ; d u r i n g this m o n t h t h e total n u m b e r of different q u a d r a t s visited p e r m o n t h [(b) a b o v e ] fell to 50. (d) T h e r e was a differential distribution of visits a n d revisits p e r q u a d r a t p e r m o n t h as follows: April, M a y , J u n e :

9 7 % of all visits w e r e t w o or less p e r q u a d r a t ; only 3 % of all visits/revisits w e r e t h r e e or m o r e . June/July: 8 5 % of all visits w e r e t w o or less p e r q u a d r a t ; 14· 1 % of all visits/revisits w e r e t h r e e or m o r e . S e p t e m b e r / O c t o b e r : 7 7 - 5 % of all visits w e r e t w o or less p e r q u a d r a t ; while 22*5% of all visits/revisits w e r e n o w t h r e e or m o r e p e r q u a d r a t , u p to a m a x i m u m of eight visits p e r q u a d r a t . (e) t h e r a t e of re-crossing previous trails was highest in O c t o b e r . T h u s t h e p a t t e r n s of h o m e r a n g e utilization r e c o r d e d for t h e m a i n study g r o u p a t T s h i b i n d a - K a h u z i c a n b e s u m m a r i z e d as follows. T h e d a y j o u r n e y s d u r i n g t h e wet season m o n t h s of A p r i l , M a y a n d early J u n e w e r e n o t extensive a n d w e r e m a i n l y restricted to localized areas of o p e n s e c o n d a r y r e g e n e r a t i n g forests. H o w e v e r , w i t h t h e a d v e n t of t h e d r y season in m i d - J u n e , a n d t h e c o r r e s p o n d i n g decrease in b o t h t h e a b u n d a n c e a n d r e g e n e r a t i o n of food p l a n t s (especially vines) in t h e exposed s e c o n d a r y forest areas, t h e m a i n g r o u p was observed to increase significantly t h e l e n g t h of t h e d a y j o u r n e y s , t h e total n u m b e r of q u a d r a t s visited ( a n d h e n c e m o n t h l y h o m e r a n g e size) a n d t h e n u m b e r of visits a n d revisits to various q u a d r a t s . T h i s b e h a v i o u r was also correlated w i t h t h e seasonal a p p e a r a n c e of t h e fruits of Myrianthus holstii w h i c h w e r e e a t e n b y t h e study a n i m a l s in vast a m o u n t s d u r i n g this t i m e . As t h e fruiting season of this species c a m e to a n e n d in early S e p t e m b e r , t h e n e w vegetative shoots of b a m b o o (Arundinaria alpina) w e r e a p p e a r i n g a b o v e t h e g r o u n d in increasing n u m b e r s in t h e localized stands of b a m b o o forest. T h e b e h a v i o u r of t h e gorillas showed significant decreases in d a y j o u r n e y l e n g t h a n d m o n t h l y r a n g e size, a n d increases in t h e revisiting of localized areas of b a m b o o a t this t i m e . L a r g e a m o u n t s of t h e fruits of Syzygium guinense a n d t h e basal p a r t s of Cyperus latifolius

1 5 . GORILLA

GORILLA

BERINGEI

471

w e r e e a t e n n e a r t h e e n d of O c t o b e r from t h e localized c o n c e n t r a t i o n s of these species w h i c h o c c u r r e d in or a t t h e sides of t h e s w a m p s a n d n e a r t h e b a m b o o forest. T h e nesting h a b i t s of t h e gorillas w e r e n o t found to influence t h e i r m i g r a t i o n s , for t h e gorillas g e n e r a l l y n e s t e d w h e r e v e r t h e y h a p p e n e d t o b e feeding n e a r nightfall a n d used w h a t e v e r v e g e t a t i o n was a v a i l a b l e in t h e i m m e d i a t e vicinity to c o n s t r u c t nests—unless t h e y w e r e in e i t h e r Cyperus s w a m p or p u r e s t a n d s of b a m b o o , in w h i c h case t h e y w o u l d t r a v e l to t h e m i x e d forest interface to nest. T h u s t h e r e w e r e n o p e r m a n e n t or even r e g u l a r l y used nest-site locations, n o r w e r e a n y p a r t i c u l a r types of v e g e t a t i o n used exclusively for t h e c o n s t r u c t i o n of i n d i v i d u a l nests ( G o o d a l l , 1974). By N o v e m b e r t h e o p e n s e c o n d a r y forest a r e a s h a d a n a b u n d a n c e of n e w g r o w t h after t h e l a t e rains a n d a l m o s t all t h e traces of h e a v y feeding b y t h e m a i n s t u d y g r o u p earlier in t h e y e a r h a d d i s a p p e a r e d . A c c o r d i n g to A d r i e n D e s c h r y v e r (pers. c o m m . ) , C o n s e r v a t o r of t h e K a h u z i - B i e g a N a t i o n a l P a r k , t h e m a i n s t u d y g r o u p m o v e d b a c k to these areas b y D e c e m b e r , t h u s r e p e a t i n g t h e a n n u a l cycle w h i c h , a c c o r d i n g to t h e m o s t e x p e r i e n c e d p y g m y t r a c k e r , h a s b e e n t h e s a m e p a t t e r n d u r i n g t h e 15 years or m o r e h e has b e e n h u n t i n g gorillas in this a r e a .

4. D I S C U S S I O N 4.1. Dietetic Diversity and the Selection of Food Items Lists of gorilla food p l a n t s s u p p l i e d b y Schaller (1963) a n d G o o d a l l (1971) i n d i c a t e a food r e p e r t o i r e of s o m e 40 different species for t h e gorillas of t h e V i r u n g a volcanoes region. T h e gorillas of t h e K a h u z i region a t e a m u c h w i d e r variety. T h e t o t a l of 104 food p l a n t s r e c o r d e d b y G o o d a l l (1974) in K a h u z i is four m o r e t h a n t h e c o m b i n e d t o t a l listed b y Schaller (1963) for all six g e o g r a p h i c regions visited b y E m l e n a n d himself. I n view of t h e far g r e a t e r diversity of forest types, a n d therefore of different p l a n t species, w h i c h a r e p r e s e n t in t h e K a h u z i r e g i o n in c o m p a r i s o n w i t h t h e V i r u n g a region, these findings a r e in a g r e e m e n t w i t h t h e p r e d i c t i o n of Levins a n d M a c A r t h u r (1969), i.e. t h a t p o l y p h a g y is m o r e likely: (a) t h e g r e a t e r t h e p r o b a b i l i t y of failure to find a k n o w n suitable plant; (b) t h e g r e a t e r t h e p r o p o r t i o n of u n k n o w n , suitable p l a n t s ; (c) t h e lower t h e p r o p o r t i o n of u n k n o w n , u n s u i t a b l e p l a n t s . V i s u a l sightings r e v e a l e d t h a t t h e K a h u z i gorillas selected t h e i r diet carefully. S o m e of t h e r a m p a n t h e r b s a n d vines g r e w in t a n g l e d masses, w i t h t h e i r stems a n d leaves c o m p l e t e l y m i x e d t o g e t h e r — y e t , o u t of such

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tangles, feeding a n i m a l s w e r e observed r e g u l a r l y to pick o u t only t h e leaves of k n o w n food p l a n t s a n d to reject others either w i t h their fingers or lips. S u c h careful selectivity was n o t confined to interspecific choice for it was also exercised w i t h i n t h e various species. I n some food p l a n t s leaves w e r e e a t e n , while in others only t h e b a r k or stem w a s e a t e n . I n e x t r e m e cases only some leaves (usually t h e y o u n g e r ones) or even p a r t s of these leaves, or t h e g r o w i n g tips of t h e stems, w e r e e a t e n while others w e r e ignored. O n m a n y occasions t h e c a m b i u m layer was seen to b e s c r a p e d o u t from b a r k of species such as Myrianthus or Scheffiera. F r o m Hagenia abyssinica t h e small side b r a n c h e s w e r e split o p e n a n d t h e i n n e r p i t h was excised a n d e a t e n . I n a t t e m p t i n g to e x a m i n e t h e selectivity of food items b y m o u n t a i n gorillas, I h a v e d r a w n u p a list of factors w h i c h could possibly influence t h e food selectivity of free-living m o u n t a i n gorillas as follows. (1) Abundance and availability of t h e various p l a n t species—including t h e influences of seasonality a n d interspecific c o m p e t i t i o n . (2) Nutritive value in t e r m s of energy, p r o t e i n , v i t a m i n s , t r a c e elements, m i n e r a l s a n d w a t e r . (3) Taste (? d u e to presence of p l a n t s e c o n d a r y c o m p o u n d s , F r e e l a n d a n d j a n z e n , 1974). (4) Smell or odour. (5) Size, shape and texture. (6) Preparation necessary to r e n d e r it r e a d y for ingestion. (7) Physical action in t h e digestive system of t h e feeder. (8) L o c a l or g r o u p "tradition". (9) Individual (personal) preference. E a c h of t h e a b o v e factors has b e e n discussed in detail b y G o o d a l l (1974). W h i l s t limitations of space a v a i l a b l e h e r e d o n o t p e r m i t a further full discussion, o n e factor—nutritive v a l u e — s e e m s likely to be of p a r a m o u n t i m p o r t a n c e . S o m e of t h e m a i n food items in t h e diet of t h e m a i n study g r o u p w e r e of h i g h n u t r i t i v e v a l u e , others w e r e n o t . H o w e v e r , o t h e r highly n u t r i t i o u s a n d easily available p l a n t s w e r e very rarely e a t e n — o r n o t a t all. T h i s raises a d o u b t as to w h e t h e r , in t e r m s of n u t r i t i v e v a l u e or " n e t e n e r g y / u n i t feeding t i m e " (Schoener, 1971) t h e m a i n study g r o u p w a s feeding optimally. A n o t h e r w a y of looking a t this is to ask w h y , in such a forest h a b i t a t , b e selective at all ? W o u l d unselective feeding b e m o r e efficient ? G i v e n t h e realistic possibility t h a t a t least some of t h e w i d e v a r i e t y of p l a n t s in t h e m o n t a n e forest a r e poisonous, t h e n t h e a d v a n t a g e s of some degree of selectivity a r e obvious. H o w e v e r , to j u d g e food p l a n t s , or p o t e n t i a l food p l a n t s , b y a single criterion (even o n e as a p p a r e n t l y i m p o r t a n t as n u t r i t i v e value) m a y well b e misleading. F o r e x a m p l e , as

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m e n t i o n e d earlier, t h e n e t a v a i l a b l e (metabolizable) e n e r g y from t h e food items is g r e a t l y r e d u c e d b y h i g h e r levels of indigestibility, t h u s t h e h i g h e r d e g r e e of digestibility of most leaves m a k e s t h e m b e t t e r v a l u e for e n e r g y (weight for weight) t h a n most of t h e b a r k t h a t was e a t e n . T h e gross a v a i l a b l e e n e r g y of t h e b a r k of Scheßera sp. of 1880 k j / 1 0 0 g (dry m a t t e r ) is r e d u c e d to a n e t a v a i l a b l e (metabolizable) e n e r g y v a l u e of a p p r o x i m a t e l y only 300 k j / 1 0 0 g — b e c a u s e of indigestibility a n d o t h e r losses. W h e n o n e considers t h a t t h e gorillas usually h a v e to c l i m b to r e a c h this vine, a n d t h e n e x p e n d c o n s i d e r a b l e e n e r g y in r e m o v i n g t h e b a r k , it is n o t difficult to i m a g i n e t h a t t h e y a r e a c t u a l l y losing e n e r g y w h e n t h e y feed o n this a n d similar food items. A d d to this t h e very low initial gross a v a i l a b l e p r o t e i n (7-62 g/100 g d r y m a t t e r ) , w h i c h could b e r e d u c e d by losses to a p p r o x i m a t e l y 3 g/100 g (dry m a t t e r ) , o n e w o n d e r s w h y t h e gorillas b o t h e r a t a l l — w h a t does Schefflera, a n d o t h e r food items w i t h similarly low n e t e n e r g y a n d p r o t e i n values, h a v e to offer ? I n t h e seasons w h e n large a m o u n t s of b a r k w e r e e a t e n , t h e d u n g was seen to consist of discrete lobes c o n t a i n i n g leaf, fruit a n d stem fragments, w i t h m a n y lobes b e i n g j o i n e d t o g e t h e r like a string of b e a d s , b y l o n g b a r k fibres. D r T . L a w r e n c e (pers. c o m m . ) suggests t h a t a l a r g e i n t a k e of b a r k could h a v e a " g u t full" effect i.e. n o t p r o v i d i n g s u b s t a n t i a l a m o u n t s of n u t r i t i v e v a l u e b u t a v o i d i n g i n d i c a t i o n s of h u n g e r w h i c h m a y c o m e from a n otherwise e m p t y s t o m a c h . H o w e v e r , w h e n gorillas w e r e seen to feed o n b a r k t h e y could easily, a n d in most cases m o r e easily, h a v e fed o n l e a v e s — t h u s o b t a i n i n g g r e a t e r n e t n u t r i t i v e values p e r energy o u t p u t . I t w o u l d a p p e a r therefore t h a t b a r k s of various p l a n t species a r e selected b y feeding gorillas, despite initial low n u t r i t i v e v a l u e a n d s u b s e q u e n t indigestibility losses, a n d it m a y b e t h a t t h e i r physical a c t i o n in t h e g u t , i.e. in h e l p i n g to m o v e t h e food along, is a p r i m a r y reason for this selection. T h e relative roles p l a y e d b y all t h e a b o v e factors, a n d others as yet u n k n o w n , in influencing t h e final expression of this selectivity a r e far too c o m p l e x to a t t e m p t a n y sort of o r d e r of i m p o r t a n c e . R a t h e r t h e y s h o u l d b e viewed as influencing food selectivity in a d y n a m i c , multifactorial w a y — b o t h in t h e daily lives of i n d i v i d u a l gorillas a n d t h e i r social g r o u p s , a n d in t h e longer t e r m d u r i n g t h e evolution of t h e g e n u s . T h e final expression of t h e diet of p a r t i c u l a r gorilla g r o u p s therefore is a result of this d y n a m i c i n t e r a c t i o n a n d n o t t h e result of u n i t o r y factors such as n e t e n e r g y g a i n p e r u n i t t i m e , as i m p l i e d b y Fossey (1974, p . 580), (see R o y a m a , 1970). I n a d d i t i o n , such feeding strategies m u s t b e viewed as p a r t of w h a t P a r k e r (1974) calls overall " t i m e i n v e s t m e n t s t r a t e g y " . T h u s these a r e likely to v a r y from region to region a n d even between individual animals.

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T h e strictly v e g e t a r i a n diet of gorillas has recently b e e n d o u b t e d o n theoretical g r o u n d s ( T . - W . Fiennes, 1972). Analysis of t h e diet of various p r i m a t e species in zoos (Bilby, 1968) showed t h a t a 220-kg a d u l t m a l e gorilla received 5057 kcal (21 130 kj) a n d 177 g p r o t e i n p e r d a y in winter, a n d 6713 kcal (28 060 kj) a n d 222 g of p r o t e i n p e r d a y in s u m m e r . M u c h of t h e p r o t e i n was in a n i m a l f o r m — m i l k , eggs a n d m e a t . I t m u s t b e stressed t h a t these figures a r e for gross a v a i l a b l e v a l u e , therefore various losses w o u l d occur d u r i n g digestion as o u t l i n e d a b o v e . I n a d d i t i o n t h e r e was considerable w a s t a g e of food by t h e c a p t i v e g o r i l l a — u p to 5 0 % in s u m m e r . T h e K a h u z i gorillas a r e far m o r e active t h a n their c a p t i v e c o u n t e r p a r t a n d will obviously r e q u i r e a h i g h e r energy i n t a k e . T h e analyses of t h e K a h u z i p l a n t samples show t h a t t h e gorillas of this region c a n get sufficient energy a n d p r o t e i n from a p u r e l y v e g e t a r i a n diet. W h i l e it is possible t h a t gorillas in some areas of p o o r forage could s u p p l e m e n t their diet w i t h a n i m a l p r o t e i n (especially w h e r e b a m b o o is absent) this m a y b e a n unnecessary, or even a c c i d e n t a l , a d d i t i o n to their i n t a k e . I suggest t h a t most gorillas o b t a i n all of their n u t r i t i o n a l r e q u i r e m e n t s simply by e a t i n g m o r e a n d m o r e vegetable m a t t e r . I n their daily lives this is usually c o n s u m e d in t w o or t h r e e extensive feeding sessions p e r d a y . Since n o t all g r o u p m e m b e r s are feeding o n exactly t h e s a m e diet it is possible t h a t some a r e feeding s u b - o p t i m a l l y . H o w e v e r , given t h e w i d e variety of possible food items a v a i l a b l e in t h e K a h u z i a r e a I feel t h a t this is unlikely since " p o l y m o r p h i c o p t i m a " a r e p r o b a b l y feasible. I n a d d i t i o n i n d i v i d u a l differences in feeding h a b i t s could r e d u c e i n t r a ­ specific c o m p e t i t i o n for some of t h e a v a i l a b l e resources.

4.2. Regional Variations and the Evolution of Gorilla Diets I t is interesting to speculate o n t h e possible p a t h of t h e l o n g - t e r m p a t t e r n s of gorilla feeding h a b i t s . T h e b o u n d a r i e s of forest areas h a v e in t h e past b e e n in a d y n a m i c state of m o v e m e n t u n d e r t h e influence of various factors. D u r i n g t h e Pleistocene, t h e m a i n factors influencing these m o v e m e n t s , especially of a l t i t u d i n a l v e g e t a t i o n zones, h a v e b e e n climatic ones, M o r e a u (1966), L a n g d a l e - B r o w n et al. (1964). As a result of such m o v e m e n t s of forest, o n e c a n n o w find identical species or closely r e l a t e d e n d e m i c species of m a n y p l a n t s in widely s e p a r a t e d areas, especially m o n t a n e a r e a , L a n g d a l e - B r o w n et al. (1964). W h i l e t h e b o u n d a r i e s of t h e various forest types h a v e m o v e d d u e to c h a n g i n g conditions, so t h e forests in p a r t i c u l a r areas h a v e u n d e r g o n e various c h a n g e s in floristic composition or d i s a p p e a r e d completely. A n i m a l p o p u l a t i o n s living in such areas could either follow t h e

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shifting forest, a n d t h e r e b y live in t h e s a m e or similar conditions, or a d a p t their feeding h a b i t s to a c h a n g i n g flora in o n e a r e a — o r b o t h . O n e w o u l d expect a n i m a l s w h i c h w e r e a b l e to a c c o m p l i s h b o t h a l t e r n a t i v e s to h a v e a w i d e p r e s e n t d a y d i s t r i b u t i o n a n d to be e a t i n g a w i d e r a n g e of food items. T h e s e a r e exactly t h e feeding p a t t e r n s seen a m o n g e x t a n t gorilla p o p u l a t i o n s (Schaller, 1 9 6 3 ; S a b a t e r Pi, 1972; G o o d a l l , 1974) in t h e i r p r e s e n t d a y disjunctive d i s t r i b u t i o n . F u r t h e r , since c h a n g e s in floristic composition in t h e geological past h a v e t a k e n p l a c e very slowly, o n e w o u l d expect to see only g r a d u a l c h a n g e s in t h e diets of gorillas living in areas farther a n d farther a p a r t . C o m p a r i s o n s of t h e lists of all k n o w n food items in t h e diets of gorillas (taken from t h e l i t e r a t u r e m e n t i o n e d a b o v e a n d from d a t a collected d u r i n g this study) show t h a t this is t r u e only to a c e r t a i n e x t e n t . T h e r e a r e a p p a r e n t differences in feeding h a b i t s b e t w e e n gorilla p o p u l a t i o n s i n h a b i t i n g different r e g i o n s — w h i c h h a v e b e e n e x p l a i n e d as " c u l t u r a l " differences b y Schaller (1963). M y findings in t h e K a h u z i region h a v e t e n d e d to confirm Schaller's explanation. T h e r e f o r e w e n o w h a v e to explain h o w these similarities a n d differences could h a v e arisen. B o t h similarities a n d differences m a y b e d u e to t h e i n t e r a c t i o n of c u l t u r a l t r a d i t i o n in feeding h a b i t s of p a r t i c u l a r g r o u p s w i t h c h a n g e s in t h e floristic c o m p o s i t i o n at a local level. T h u s t h e feeding h a b i t s of e a c h g r o u p h a v e c h a n g e d a n d a r e still c h a n g i n g , w i t h i n t h e confines set b y n u t r i t i o n a l needs, to suit t h e local c o n d i t i o n of t h e forest w h i c h t h e g r o u p i n h a b i t s .

4.3. Home Range Utilization Schaller (1963) states Except for bamboo, gorilla habitats throughout the range show no conspicuous seasonal differences in the abundance of forage. . . . The only generalisation regarding movement which can safely be made are that gorillas (at Kabara—Virunga volcanoes) travel continuously within the boundaries of their home range at irregular intervals. A m u c h longer s t u d y o n t h e n e a r b y M t Visoke indicates t h a t the presence or absence of other gorilla groups and/or individuals appeared to be the strongest determination of the range patterns of the study group, though food distribution may have played some role in movements between vegetation zones or parts of the range. (Fossey, 1974). I n t h e m o r e h e t e r o g e n o u s a n d seasonal forest areas of K a h u z i definite p a t t e r n s of h o m e r a n g e utilization b y t h e s t u d y g r o u p a p p a r e n t w i t h i n several m o n t h s . R e l i a b l e local sources i n d i c a t e such p a t t e r n s h a v e b e e n r e p e a t e d for over 20 y e a r s — a t least t h e life

quite were that span

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GOODALL

of m a n y g r o u p m e m b e r s . G r o u p a v o i d a n c e was also observed in t h e K a h u z i study a r e a . O n o n e occasion t h e m a i n g r o u p m o v e d directly from o n e side of its h o m e r a n g e to t h e o t h e r d u r i n g t e n days of direct travel after a c o n t a c t w i t h a lone silverback. I n a d d i t i o n t h e r e a p p e a r e d to be a far lower degree of h o m e r a n g e o v e r l a p in t h e K a h u z i a r e a t h a n Schaller (1963) observed in t h e V i r u n g a volcanoes. T h u s while t h e presence or absence of o t h e r gorillas m a y b e o n e p r o x i m a t e cause of some gorilla m o v e m e n t s in K a h u z i , it is suggested t h a t different p a t t e r n s of food distribution a r e t h e most i m p o r t a n t factors. O b v i o u s l y food w h i c h is available in discrete, discontinuous a m o u n t s will d e m a n d a different strategy for its o p t i m a l exploitation t h a n food w h i c h is w i d e s p r e a d a n d c o n t i n u o u s . T h e r e will be a n o p t i m a l gain p e r u n i t t i m e spent feeding o n p a r t i c u l a r food items w h i c h is directly p r o p o r t i o n a l to t h e " s e a r c h t i m e " necessary to find t h e m ( R o y a m a , 1970a a n d P a r k e r , pers. c o m m . ) . S u c h different strategies of m o v e m e n t a n d r a n g e utilization by t h e m a i n s t u d y g r o u p a r e evident from t h e d a t a p r e s e n t e d a b o v e . T h e s e also serve p a r t l y to explain w h y t h e r e a p p e a r s to b e so m u c h w a s t a g e of food in areas of high food density, e.g. b a m b o o . I n a d d i t i o n , either directly or indirectly as a result of climatic factors, food distribution a p p e a r e d to b e t h e m a i n u l t i m a t e factor influencing gorilla m o v e m e n t s t h r o u g h o u t t h e year. F o r these w e r e n o t r a n d o m l y distributed m o v e m e n t s b u t obvious migrations as p a r t of a well established p a t t e r n of r a n g e utilization. T h e s e m i g r a t i o n s , a n d t h e fact t h a t some p a t h s w e r e used r e p e a t e d l y , i n d i c a t e d t h a t some of t h e g r o u p m e m b e r s w e r e familiar w i t h t h e spatial distribution of their m a i n food items. T h e visits t h e y p a i d to t h e b a m b o o zones, a n d their digging for y o u n g shoots before they a p p e a r e d a b o v e t h e g r o u n d , i n d i c a t e t h a t t h e gorillas also a n t i c i p a t e d some of t h e seasonal changes. O v e r l y i n g t h e climatic a n d resulting vegetational factors a r e t h e m o r e r e c e n t a n t h r o p o g e n i c influences, m a i n l y as a result of t h e creation of extensive areas of s e c o n d a r y r e g e n e r a t i n g forest. I n K a h u z i these areas p r o v i d e d t h e largest a m o u n t of gorilla foods d u r i n g m u c h of t h e year—especially d u r i n g t h e r a i n s — a n d their expansion has p r o b a b l y greatly influenced gorilla m i g r a t i o n s in r e c e n t times. T h e a p p a r e n t n o n - t e r r i t o r i a l n a t u r e of gorillas has b e e n t h e subject of considerable a c a d e m i c d e b a t e . I n o n e a r e a a t K a b a r a in t h e V i r u n g a volcanoes, Schaller (1963) found six gorilla g r o u p s utilizing t h e s a m e resources. H e c o n c l u d e d t h a t territorial b e h a v i o u r is a b s e n t in gorillas. H o w e v e r , territoriality m a y o c c u r in t h e absence of active defence (Fretwell, 1972). I n K a h u z i , g r o u p a v o i d a n c e a n d t h e low d e g r e e of h o m e r a n g e o v e r l a p suggest t h a t some form of territoriality does occur.

15. GORILLA

GORILLA

BERINGE/

477

T h i s m a y b e effected m a i n l y as a result of l o n g - r a n g e , i n t e r g r o u p vocalizations a n d chest-beat displays (hoot series a n d chest-beat, w i t h or w i t h o u t g r o u n d t h u m p s ) b y t h e a d u l t m a l e silverbacks of t h e various g r o u p s . W h a t w e see in t h e V i r u n g a volcanoes, w h e r e t h e r e is a n a b u n d a n c e of food all y e a r r o u n d , is t h e compression of t h e h o m e r a n g e , a c c o m p a n i e d b y extensive overlaps, such t h a t a p p a r e n t l y all t h a t is left is t h e defence of t h e g r o u p i n t e g r i t y — w i t h occasional g r o u p a v o i d a n c e . T h u s t h e differences, b o t h in a b s o l u t e h o m e r a n g e size a n d p a t t e r n s of h o m e r a n g e utilization, w h i c h w e r e observed b e t w e e n t h e gorillas of t h e K a h u z i a n d V i r u n g a regions a r e , to a g r e a t degree, reflections of differences in t h e a b u n d a n c e a n d availability of local food resources in each region. T h e t w o p r i m a r y factors influencing food a b u n d a n c e a n d availability a p p e a r e d to be (a) c l i m a t i c — p a r t i c u l a r l y seasonal v a r i a t i o n s in rainfall, a n d (b) t h e c r e a t i o n of o p e n s e c o n d a r y forest conditions b y m a n . O t h e r influences, such as t h e gorilla p o p u l a t i o n size a n d its d i s t r i b u t i o n t h r o u g h o u t t h e region, local pressures from h u n t i n g , h a r a s s m e n t a n d h a b i t a t destruction, " c u l t u r e " a n d " t r a d i t i o n s " w i t h i n p a r t i c u l a r gorilla g r o u p s p r o b a b l y influence t h e final expression of h o m e r a n g e size a n d its utilization b y t h e a n i m a l s of all regions. Since it is extremely likely t h a t all these influences v a r y widely, b o t h b e t w e e n a n d w i t h i n t h e various g e o g r a p h i c regions i n h a b i t e d by all subspecies of gorilla, it is likely t h a t t h e b e h a v i o u r of t h e gorillas themselves will also v a r y u n d e r t h e different ecological influences.

4.4. Conservation T h e biggest d a n g e r to t h e c o n t i n u e d existence of gorillas in t h e V i r u n g a volcanoes region is t h e d e s t r u c t i o n of t h e i r h a b i t a t b y m a n . T h e situation in t h e K a h u z i region, a n d p r o b a b l y t h r o u g h o u t eastern Z a i r e , is m o r e p a r a d o x i c a l . T h e p a t t e r n of a g r i c u l t u r a l p r a c t i c e in these areas h a s c r e a t e d mosaics of s e c o n d a r y r e g e n e r a t i n g forests, of differing ages, a m o n g vast areas of p r i m a r y forest. T h e feeding b e h a v i o u r of gorillas, evolved d u r i n g extensive forest c h a n g e s d u r i n g t h e geological past, t o g e t h e r w i t h t h e i r p e c u l i a r a n a t o m y , h a v e e n a b l e d t h e m to exploit these areas most successfully. U n f o r t u n a t e l y this often brings t h e m i n t o direct conflict w i t h villagers w h o m a y still h a v e crops in a p p a r e n t l y a b a n d o n e d fields. T h u s gorillas a r e accused of a c t u a l l y r a i d i n g t h e crops a n d a r e killed e.g. in some p a r t s of t h e U t u region. As a c o n s e q u e n c e of t h e increase in areas of s e c o n d a r y r e g e n e r a t i n g forest r e c e n t l y c r e a t e d b y m a n , it is possible t h a t t h e gorillas of some regions a r e u n d e r g o i n g b o t h a p o p u l a t i o n increase a n d a r a n g e e x p a n s i o n . U n f o r t u n a t e l y t h e r e is too little d a t a a v a i l a b l e as yet o n t h e

478

A L A N G. G O O D A L L

extent of d e a t h s a n d c a p t u r e s . H o w e v e r , it is obvious from field research t h a t t h e future survival of t h e eastern gorillas ( a n d p r o b a b l y t h e western gorillas) does n o t d e p e n d o n their b e h a v i o u r a l a n d ecological efficiency b u t u p o n t h e p a t t e r n s of l a n d usage w h i c h a r e a d o p t e d b y t h e people of countries like R w a n d a , U g a n d a a n d Z a i r e . I n this respect it is most e n c o u r a g i n g to h e a r t h a t President M b u t o Sese Seko of Z a i r e plans to e x t e n d their N a t i o n a l P a r k s to cover 1 2 - 1 5 % of their c o u n t r y . S o m e of these n e w p a r k areas could b e selected w i t h r e g a r d to t h e d i s t r i b u t i o n of gorillas. I t w o u l d be b o t h unrealistic a n d unwise to claim m a n y of these areas as " i n v i o l a t e " gorilla reserves. Since it is n o w clear t h a t gorillas thrive in a w i d e v a r i e t y of forest h a b i t a t s it is this variety w h i c h should be m a i n t a i n e d . T h i s will call for t h e careful management of gorillas a n d their h a b i t a t s . T h u s while controlled " n e g l e c t " m a y be t h e best policy in some areas, it m a y be wise in o t h e r areas to create s e c o n d a r y / p r i m a r y forest mosaics as in K a h u z i . S u c h m a n a g e m e n t t e c h n i q u e s w o u l d n o t only ensure viable gorilla p o p u l a t i o n s a n d a n e x p a n d i n g tourist industry, t h e y could also greatly r e d u c e t h e conflicts of interest s u r r o u n d i n g t h e choice of l a n d usage by giving t h e local people t h e felled t i m b e r — w h i c h t h e y desperately n e e d to build houses a n d for use as cooking fuel. H o w e v e r , it m u s t be m a d e clear t h a t t h e wholesale creation of s e c o n d a r y forest is n o t b e i n g a d v o c a t e d — m u c h m o r e basic scientific g r o u n d w o r k r e m a i n s to be d o n e r e g a r d i n g gorilla d i s t r i b u t i o n a n d ecology before realistic m a n a g e m e n t plans c a n be f o r m u l a t e d .

5.

S U M M A R Y

As p a r t of a c o m p a r a t i v e research p r o g r a m m e to investigate t h e b e h a v i o u r a n d ecology of m o u n t a i n gorillas in different p a r t s of their r a n g e , a p a r t i a l l y h a b i t u a t e d g r o u p of 20 a n i m a l s was studied for seven m o n t h s in t h e m i x e d m o n t a n e forests of T s h i b i n d a - K a h u z i (Zaire) (Section 1.0). T h e y w e r e found to eat a w i d e r r a n g e of food items t h a n t h e m o u n t a i n gorillas of the V i r u n g a volcanoes region, a n d to c l i m b expertly a n d frequently in o r d e r to exploit t h e m a n y v e g e t a t i o n resources available in different forest s t r a t a (Sections 2 . 1 , 2.2). M a n y of t h e food plants w e r e analysed for their gross a v a i l a b l e n u t r i t i v e values. Allowances w e r e m a d e for various losses from these gross values a n d estimates of n e t available or " m e t a b o l i z a b l e " values a r e given. T h e s e estimates w e r e used to calculate t h e n u t r i t i v e values of simplified gorilla " m e n u s " , from w h i c h it was c o n c l u d e d t h a t t h e s t u d y a n i m a l s could o b t a i n a m p l e e n e r g y a n d p r o t e i n for all their r e q u i r e m e n t s w i t h o u t e a t i n g a d d i t i o n a l a n i m a l m a t t e r (Section 2.4).

1 5 . GORILLA

GORILLA

BERINGEI

479

Eleven v e g e t a t i o n types w e r e recognized in t h e s t u d y a r e a a n d t h e relative p e r c e n t a g e cover of these was c a l c u l a t e d from v e g e t a t i o n m a p s p r e p a r e d from aerial survey p h o t o g r a p h s (scale 1 : 28 000) (Section 1.2). T h e s t u d y a r e a was d i v i d e d i n t o q u a d r a t s e q u i v a l e n t to 280 χ 280 m o n t h e g r o u n d . T h e h o m e r a n g e of t h e m a i n s t u d y g r o u p , c a l c u l a t e d o n a q u a d r a t basis a n d allowing for hilly t e r r a i n , was e s t i m a t e d to b e s o m e 2 34 k m a n d is far l a r g e r t h a n previous estimates for gorillas in o t h e r regions. T h e d a y j o u r n e y s , q u a d r a t visits a n d r a n g e utilization a r e a n a l y s e d a n d discussed. I t is c o n c l u d e d t h a t differences in t e m p o r a l a n d seasonal d i s t r i b u t i o n of food resources, via climatic factors, a r e t h e m o s t i m p o r t a n t factors influencing gorilla m o v e m e n t s a n d a r e p r o b a b l y responsible for regional differences in ecology of gorillas (Sections 3.0, 4.3). I t is suggested t h a t , o w i n g to t h e e x p a n s i o n of s e c o n d a r y r e ­ g e n e r a t i n g forest areas caused b y m a n , gorillas could be i n c r e a s i n g t h e i r n u m b e r s a n d possibly e x p a n d i n g t h e i r r a n g e . Suggestions a r e m a d e for t h e f o r m u l a t i o n of a " m a n a g e m e n t p l a n " for gorilla reserves in east Z a i r e , w h i c h w o u l d entail t h e artificial m a i n t e n a n c e a n d possible increase of s e c o n d a r y forest mosaics inside areas of p r i m a r y forest. T h i s could p r o v i d e o p t i m u m gorilla h a b i t a t a n d s u p p l y m u c h - n e e d e d t i m b e r to t h e local p o p u l a t i o n o n a c o n t i n u i n g basis (Section 4.4).

ACKNOWLEDGEMENTS T h i s research w o u l d n o t h a v e b e e n possible w i t h o u t t h e h e l p , e n c o u r a g e ­ m e n t a n d advice of m a n y p e o p l e . I w o u l d therefore like to t a k e this o p p o r t u n i t y of expressing m y most sincere t h a n k s to t h e following. T h e G o v e r n m e n t s a n d p a r k officials of t h e R e p u b l i c s of R w a n d a a n d Z a i r e ; t h e D i r e c t o r - G e n e r a l a n d staff of I N C N a n d I R S A C — Z a i r e ; t h e Science R e s e a r c h C o u n c i l ; t h e W o r l d Wildlife F u n d ; t h e F a u n a P r e s e r v a t i o n Society; Liverpool U n i v e r s i t y ; t h e U n i v e r s i t y of G h e n t ; t h e late D r L. S. B. L e a k e y ; Miss D i a n Fossey a n d h e r staff a t c a m p M o s h i ( M t V i s o k e ) ; Professor A. J . C a i n ; Professor R . A. H i n d e ; Drs Peter a n d Irene K u n k e l ; Adrien Deschryver; Citoyen M u s h e n z i ; C i t o y e n J . K a l a m o ; P a t r i c e W a z i - W a z i ; all m e m b e r s of " M i s s i o n Scientifique Beige des V o l c a n s d e l'Afrique C e n t r a l e : 1 9 7 1 - 1 9 7 2 " , especially M i l e C h r i s t i n e M a r i u s ; M r s I r e n e L e p p i n g t o n ; M r E. D j o l e t o - N a t t e y ; M r s L. T u r t l e ; D r T . L a w r e n c e ; D r R . G. P e a r s o n ; D r G. A. P a r k e r ; D r J . B i s h o p ; D r S. B r a d l e y ; D r R . W h i t e ; D r F . a n d M m e E. D o n d e y n e ; D r H . J . a n d F r a u Η . S c h l i c h t e ; M r s Μ . Priestley a n d finally, most of all, to m y wife M a r g a r e t .