close

Вход

Забыли?

вход по аккаунту

?

Western lowland gorilla diet and resource availability New evidence cross-site comparisons and reflections on indirect sampling methods.

код для вставкиСкачать
American Journal of Primatology 58:91–116 (2002)
RESEARCH ARTICLE
Western Lowland Gorilla Diet and Resource Availability:
New Evidence, Cross-Site Comparisons, and Reflections
on Indirect Sampling Methods
DIANE M. DORAN1,2, ALASTAIR McNEILAGE2,3, DAVID GREER1,2, CAROLYN
BOCIAN1,2, PATRICK MEHLMAN1,2, and NATASHA SHAH4
1
Department of Anthropology, SUNY at Stony Brook, Stony Brook, New York
2
Mondika Research Center, Central African Republic and Republic of Congo
3
Wildlife Conservation Society, Bronx, New York
4
Interdepartmental Doctoral Program in Anthropological Sciences, SUNY at Stony Brook,
Stony Brook, New York
We describe the resource availability and diet of western lowland gorillas
(Gorilla gorilla gorilla) from a new study site in the Central African
Republic and Republic of Congo based on 3 years of study. The results,
based on 715 fecal samples and 617 days of feeding trails, were similar to
those reported from three other sites, in spite of differences in herb and
fruit availability. Staple foods (consumed year-round) included highquality herbs (Haumania), swamp herbs (when present), and a minimal
diversity of fruit. A variety of fruits (average of 3.5 species per day and 10
per month) were selectively consumed; gorillas ignored some common
fruits and incorporated rare fruits to a degree higher than predicted
based on availability. During periods of fruit abundance, fruit constituted
most of the diet. When succulent fruits were unavailable, gorillas used
low-quality herbs (i.e., low-protein), bark, and more fibrous fruits as
fallback foods. Fibrous fruit species, such as Duboscia macrocarpa and
Klainedoxa gabonensis, were particularly important to gorillas at
Mondika and other sites as fallbacks. The densities of these two species
are similar across sites for which data are available, in spite of major
differences in forest structure, suggesting they may be key species in
determining gorilla density. No sex difference in diet was detected. Such
little variation in western lowland gorilla diet across sites and between
sexes was unexpected and may partly reflect limitations of indirect
sampling. Am. J. Primatol. 58:91–116, 2002.
r 2002 Wiley-Liss, Inc.
Contract grant sponsor: National Science Foundation; Contract grant numbers: SBR-9422438;
SBR-9729126; Contract grant sponsor: State University of New York at Stony Brook.
n
Correspondence to: Diane Doran, Department of Anthropology, SUNY at Stony Brook, Stony
Brook, NY 11794. E-mail: ddoran@notes.cc.sunysb.edu
Received 23 January 2002; revision accepted 23 September 2002
DOI 10.1002/ajp.10053
Published online in Wiley InterScience (www.interscience.wiley.com).
r
2002 Wiley-Liss, Inc.
92 / Doran et al.
Key words: Gorilla; foraging strategy; food availability; fecal samples;
feeding traces
INTRODUCTION
At all sites studied to date, western lowland gorillas (Gorilla gorilla gorilla)
have greater dietary breadth and eat more fruit than do mountain gorillas (G. g.
beringei) [reviewed in Watts, 1996; Doran & McNeilage, 1998, 2001]. Western
lowland gorillas eat herbs and fruit throughout the year, with the relative
proportion and diversity of food categories shifting with fluctuating fruit
availability [Williamson, 1989; Tutin et al., 1991; Nishihara, 1995; Remis, 1997;
Goldsmith, 1999]. Gorillas eat herbaceous food throughout the year, but increase
their intake of low-quality herbs when succulent fruit is scarce. At most sites,
western lowland gorillas eat fruit daily (Z95% of fecal samples contained fruit, an
average of Z3 species per day) and include many species (Z70 overall and 10–15
per month) in their diet (reviewed in Doran and McNeilage [2001], but see
Calvert [1985]). The incorporation of substantial quantities and varieties of fruit
in the diet should have consequences for western lowland gorilla ranging and
social behavior, relative to that of mountain gorillas [Doran & McNeilage, 2001].
However, our understanding of western lowland gorilla behavior remains elusive
due to the absence of habituated groups. Our current understanding of western
lowland gorilla diet is based on indirect sampling, fecal samples, and trail sign
data, primarily from four studies at three sites.
Western lowland gorilla diets appear similar across sites, but interpretations
of foraging strategy differ. Williamson [1989] noted that at Lopé, Gabon, gorillas
were selective in their fruit choice, and incorporated fruit in their diet when
vegetative plant parts were simultaneously available. The amount of stem-fiber,
leaf, and bark in the diet were influenced by the availability of fruit in the
environment, rather than by their own availability. She described gorilla dietary
strategy as one of selection of succulent fruits. Fruits were complemented
nutritionally by the inclusion of other food types when their availability was
reduced.
Subsequently, Kuroda and colleagues [1996] described gorillas at Ndoki,
Republic of Congo, as less selective and persistent in their fruit-eating (compared
to chimpanzees), and noted that high-quality herbs (such as Haumania shoots
and Hydrocharis roots, which are available and consumed year-round) were the
staple components of the diet. They suggested that in areas with access to
superabundant, high-quality swamp vegetation (or, presumably, extensive tracts
of Haumania), gorillas have few incentives to seek out fruit when its abundance is
low, and select fruit only as an alternative food choice [Nishihara, 1995; Kuroda
et al., 1996].
Remis’ [1997] and Goldsmith’s [1999] findings support Williamson’s [1989]
interpretation. The gorillas at Bai Hokou, Central African Republic, were
persistent fruit-eaters; they traveled farther to obtain it when it was available,
but were able to subsist entirely on fibrous foods when necessary because of their
large body size. Swamp herbs, a key feature of the Ndoki site, were absent from
the Bai Hokou and Lopé study sites.
Differing opinions concerning western gorilla foraging strategy persist,
although no study has explicitly examined how changes in distribution and
availability of resources across sites influence diet. In this study, we document the
resource availability and diet of western lowland gorillas from a fourth site, the
Lowland Gorilla Diet at Mondika / 93
Mondika Research Center, Central African Republic and Republic of Congo,
where, as at Ndoki, swamp herbs are available and used regularly. We compare
these results to previous studies to examine whether: 1) overall increased herb
availability or reliance on superabundant herbs in swamps leads to a decreased
reliance on fruit, as proposed by Kuroda et al. [1996]; and 2) higher fruit species
diversity is reflected in greater dietary breadth, as predicted by a strategy of
dietary opportunism. Since gorillas are extremely sexually dimorphic in body size
(male: 170.4 kg, female: 71.5 kg [Smith & Jungers, 1997]), and large body size is a
strategy to cope with a more generalized, lower-quality diet [reviewed in Lambert,
1999], we test whether larger male body size is reflected in increased folivory,
relative to females. Finally, since we used indirect data to examine diet, as in
previous studies, we consider how sampling method may influence results. We
report findings from two indirect measures of diet–fecal samples and trail signs–
and comment on the strengths and limitations of each.
METHODS
Study Site and Sampling Methods
The Mondika study site (50 km2) straddles the boundaries of the Central
African Republic (Dzanga-Ndoki National Park) and Republic of Congo (021 210
85900 N, 0161 160 46500 E). The habitat consists of mixed-species tropical lowland
forest, monodominant Gilbertiodendron dewevrei (Caesalpiniaceae) forest, and
swamp forest. Mean annual rainfall averaged 1,301 mm during the study (July
1995 to June 1998; SD = 207) and 1,415 mm over the first 5 years (July 1995 to
June 2000; SD = 219.1). Rainfall is seasonal, and the study period was
characterized by the typical 2–3-month annual dry season, with o50 mm of rain
per month (Fig. 1a). Mean minimum and maximum daily temperatures were 21.0
7 0.8 and 28.2 7 1.7 (n = 5 years). The area is free of human disturbance, has
never been logged, and is rich (10 diurnal species) in primate fauna.
Measures of Resource Availability
Phenology.
Phenological data on fruit, leaf, and flower production were collected
beginning October 1996. We sampled important gorilla food trees (see below for
definition), rather than overall habitat productivity, to ensure adequate sampling
of rare species. We recorded the presence or absence of ripe fruit from 10
individuals (when possible) of 20 tree and three liana species (totaling 197–216
tree and 20–32 liana individuals). We report fruit availability as the number of
individuals with ripe fruit per month.
Fruit tree identification, density, and diameter at breast height (DBH).
Densities of tree species (stems/ha) were determined by counting and
identifying stems of all individuals (of DBH Z10 cm) along 28 (200 m 10 m)
transects, placed randomly within each 1-km2 grid of the study area (total of 5.6 ha
sampled). The size of the gorilla food tree species (large (>80 cm DBH), medium
(50–80 cm DBH), small (20–50 cm DBH), and very small (o20 cm)) was based on
mean DBH from phenology trees, rather than transects, because phenology trees
were more representative of the large fruiting trees gorillas visit, vs. those on
transects that included all stages of growth. To assess monthly food tree size, we
identified dominant fruit food species each month (those that were present
94 / Doran et al.
(a) Rainfall (Jul 95 - Jun 98)
300
millimeters
250
200
150
100
50
1995/96
1996/97
JUN
MAY
APR
MAR
FEB
JAN
DEC
NOV
OCT
SEP
AUG
JUL
0
1997/98
(b) Phenology (Oct 96 - Sep 98)
Oct-96
Nov-96
Dec-96
Jan-97
Feb-97
Mar-97
Apr-97
May-97
Jun-97
Jul-97
Aug-97
Sep-97
Oct-97
Nov-97
Dec-97
Jan-98
Feb-98
Mar-98
Apr-98
May-98
Jun-98
Jul-98
Aug-98
Sep-98
80
70
60
50
40
30
20
10
0
%INDIVIDUALS WITH RIPEFRUIT
%SPECIES WITH RIPE FRUIT
Fig. 1. Variation in (a) rainfall and (b) phenological patterns at Mondika. Phenological data are
based on the presence or absence of ripe gorilla fruit in 197–216 trees and 20–32 lianas (individuals)
of 23 species. No phenological data are available for June 1998.
in >65% of fecal samples) and calculated the percentage of dominant foods from
each tree size category.
Herb densities.
We determined the densities of all important herb species (occurring in 45%
of food trails) by counting and identifying stems rooted within 136 4-m2 circular
plots (i.e., of radius 1.13 m), located randomly within each 250 250 grid of the
study area. Habitat type (i.e., Gilbertiodendron forest or mixed forest) was noted
for each plot. To ensure adequate sample size, we plotted cumulative means of
each species against numbers of plots sampled, and continued to add additional
plots until changes in mean were negligible [Grieg-Smith, 1983].
Lowland Gorilla Diet at Mondika / 95
Measures of Diet
We followed fresh trails of gorilla groups and recorded indirect indicators of
diet (trail signs and fecal samples) from July 1995 through June 1998. We
collected samples from all areas of the study site, which was used by a minimum
of nine genetically distinct gorilla groups with overlapping home ranges (Bradley,
personal communication). Groups were not individually recognizable, so samples
were most probably collected from all groups, but were not ascribable to any
particular group.
Fecal analyses.
We collected fecal samples from fresh (o24 hr old) gorilla trail and nesting
sites and assigned sex on the basis of bolus size (silverback dung can usually be
distinguished from smaller individuals (females and blackbacks) because dung
diameter is proportional to body size) [reviewed in Tutin & Fernandez, 1993].
Although adult female and blackback males are similar in body size, and thus
their fecal samples are indistinguishable, the majority of these samples from
group nest sites can be assumed to be from females, given that long-term
demographic studies of western gorillas indicate that adult females account for a
far larger proportion of adult-sized individuals in mixed-sex groups than do
blackbacks (proportion of adult-sized individuals: Maya (n = 588 adult-sized
individuals: blackbacks = 4%, females = 96% [Magliocca et al., 1999]; Mbeli (n =
52 individuals in mixed-sex groups: blackbacks = 20%, females = 80% [Parnell,
2002]).
We analyzed one fecal sample (of approximately 300 g) from one adult male
and one female-sized gorilla per group per day, when possible. We dropped
individual months from the analyses when there were less than three of either the
male or female samples. As our ability to track gorillas improved, the number of
nest sites (and fecal samples) encountered per month increased. On average we
analyzed 23 samples per month (SD = 9.5, range = 8–44). Sample size (when >3)
had little effect on our measure of the amount of fruit in diet per month: no
significant correlation existed between the sample size and 1) the mean number of
fruit species per fecal sample, 2) the total number of fruit species consumed per
month, or 3) the average fruit or fiber score (Spearman rank correlation: n = 62
months; mean number of fruit species r = –0.2, t = –1.7, P = 0.09: total fruits r =
0.2, t = 1.7, P = 0.09; fruit score r = –0.14, t = –1.1, P = 0.27). Goldsmith [1996]
also found no effect of fecal sample size on mean number of fruit species per
sample, or on total fruits per month. She did not test for effect on fiber score.
Fecal samples were weighed, dissociated in water, and rinsed through a 1mm screen. Wet samples were divided into component parts, including fruit
(identified predominantly by seeds and occasionally by pulp), fiber (remnants of
leaf and stem), and insects. The fiber score (0–100%) was defined as the
proportion (by volume) of fiber, relative to fruit (including seeds, skins, and pulp)
in a sample. The fiber score and its inverse, the fruit score (100% - fiber score),
were used to track relative amounts of fiber (or fruit) in the diet throughout the
year. We assessed fruit content by three measures: 1) quantity of fruit (fruit
score), 2) number of different species present per sample, and 3) number of
different fruit species occurring in all samples per month (total). We estimated
abundance scores to define important fruits based on a scale of 1–4 (few, rare,
common, and abundant, respectively). The fruits of most species could be
identified to the species level. Exceptions occurred with some genera, such as
Aframomum, Landolphia, Dialium, Drypetes, and Ficus, which had more than
96 / Doran et al.
one species with indistinguishable seeds. For these, we recorded the genus
present in fecal analysis, following Nishihara [1995].
Trail signs.
While following the trail of a group, we recorded the presence of each food
item the first time we encountered it each day (n = 617 feeding trails of >200 m).
Food items were recognizable by the characteristic manner by which each food
type had been processed, and by the particular plant parts discarded [Williamson,
1989]. Each month we tallied the percentage of days on which each food item
appeared. We limited trail sign data to months (n = 31) where n 4 8 (n = 519
days of trails). We found a significant relationship between the number of herb/
leaf species encountered and trail sample size (r = 0.5, P = 0.004, n = 31 months),
but unlike Goldsmith [1996], we found no significant correlation between the
number of fruit species encountered each month and trail sample size (r = 0.1,
P =0.6, n = 31 months).
Important and reliable fruits.
We defined fruits as ‘‘important’’ if they were present in fecal samples in at
least 50% of months (long duration) in either large (type I) or small (type III)
quantities, or if they were used in large quantities over a shorter duration (type
II). ‘‘Large quantity’’ is defined as occurring in at least 50% of fecal samples
during at least a single month, or a mean abundance fecal score Z2. Additionally,
if fruits appeared in Z5% of feeding trails, we classified them as important (type
IV). Fruits were considered ‘‘reliable’’ (or ‘‘highly reliable’’) if they occurred in
>20% of fecal samples in the same month for 2 or 3 consecutive years.
Food list.
The food list includes plant items present in Z1% of overall food trails or
fecal samples, or Z10% of food trails or fecal samples in any single month.
Statistics
The results of correlations are reported as ‘‘r’’ and are based on two-tailed
Pearson correlation tests, unless otherwise indicated.
RESULTS
Resource Availability and Seasonality
Herb densities.
Overall densities of herbs commonly consumed by gorillas were less than one
stem per m2 in both Gilbertiodendron and mixed forest (Table I). Marantaceae
constituted the majority of stems in both forest types. Haumania danckelmaniana, the herb gorillas ate most frequently, was the most abundant species at the
study site. However, its stem densities cannot be directly equated to food
availability because gorillas eat only growing shoots, not mature plants (personal
observation, Nishihara et al., 1995). Only 5.5% of Haumania stems recorded in
the sample plots were growth shoots, effectively reducing the availability of this
resource to 0.018 shoots per m2. Herbs followed a clumped distribution, both
overall and at the species level. Herb stems were found in only 61% and 45% of
plots in mixed and Gilbertiodendron forest, respectively, with individual species
found in fewer plots. Overall mean herb density was 0.78 stems per m2 in mixed
Marantaceae
Marantaceae
Marantaceae
Zingiberaceae
Commelinaceae
Marantaceae
Marantaceae
Marantaceae
Zingiberaceae
Commelinaceae
Family
0.27
0.29
0.08
0.07
0.52
0.58
0.24
0.06
0.23
0.27
0.10
0.06
0.19
0.04
0.08
795%
confidence
interval
0.48
0.03
0.60
0.78
0.33
0.07
0.20
0.07
0.11
Mean
(stems/m2)
0.77
0.83
0.23
0.70
0.17
1.20
1.37
0.51
0.29
0.95
0.20
0.43
SD
0–2.8
0–2.8
0–0.8
0–2.2
0–1
0–8.8
0–10
0–2.8
0–2.5
0–6
0–1
0–3
Range
(stems/m2)
42
45
9
42
3
54
61
46
11
6
16
11
% of plots
in which
found
–
–
3.04
3.99
4.00
–
–
3.14
4.83
18.54
2.13
6.66
Variance:
mean ratio
Standard deviations (SD), ranges, and percentage occurrence are given as measures of the variation in herb density in different plots. The variance to mean ratio, where greater than
one (P o 0.001 in each case), indicates a clumped distribution [Grieg-Smith, 1983]
All Marantaceae
All THV
Gilbertiodendron forest (n = 33 plots)
Haumania danckelmaniana
Sarcophrynium spp.
Megaphrynium macrostachyum/trichogynum
Aframomum spp.
Palisota ambigua
All Marantaceae
All herbs
Mixed forest (n =103 plots)
Haumania danckelmaniana
Sarcophrynium spp.
Megaphrynium macrostachyum/trichogynum
Aframomum sp.
Palisota ambigua
Species
TABLE I. Stem Densities of Herbaceous Plants in Forest Types
Lowland Gorilla Diet at Mondika / 97
98 / Doran et al.
forest, although stem densities varied considerably (0–10 stems/m2). Standard
deviations were correspondingly high. Variance-to-mean ratios (coefficient of
dispersion) were greater than one for all species, indicating significantly clumped
distributions (significance testing procedure from Grieg-Smith [1983]).
Tree densities.
Average stem density for all trees (of Z10 cm DBH) was 419.5 trees/ha, with
a total of 2,349 trees of 269 species enumerated (5.6 ha). The average DBH of the
2,349 trees was 22.7 7 17.6 cm (range = 13–175, based on transect sampling).
Species commonly indicative of recent disturbance or human presence, Musanga
cercropoides and Elaeis guineensis, were absent. The forest was diverse, with the
most common species, Gilbertiodendron dewevrei (Caesalpiniaceae), accounting
for only 6% of trees sampled (Table II). Gorillas selectively consumed fruit
species; they ate fruit from only 26% of possible tree species. Trees of important
fruit species used in large quantities for long time periods (type I trees) were rare,
accounting for r 0.5% of trees. Trees of other important gorilla fruit species were
among the 25 most common tree species, including Anonidium mannii
(Annonaceae), Polyalthia suaveolens (Annonaceae), Dialium spp. (Caesalpiniaceae), Diospyros ituriensis (Ebenaceae), Angylocalyx pynaertii (Papilionaceae),
TABLE II. Relative Availability, Shown in Decreasing Order, of the 25 Most Common Tree
Species at Mondika
Family
Caesalpinaceae
Ulmaceae
Euphorbiaceae
Ebenaceae
Olacaceae
Annonaceae
Unk
Annonaceae
Caesalpinaceae
Sapindaceae
Meliaceae
Meliaceae
Sterculiaceae
Rubiaceae
Flacourtiaceae
Meliaceae
Sterculiaceae
Euphorbiaceae
Ebenaceae
Papilionaceae
Euphorbiaceae
Sapindaceae
Ebenaceae
Tiliaceae
Moraceae
Sterculiaceae
Species
Density
% stems
Gilbertiodendron dewevrei
Celtis mildbraedii/tessmannii/zenkeri*
Dichostemma glaucescens
Diospyros bipendensis
Strombosia nigropunctata/pustulata
Anonidium mannii
Unk
Polyalthia (Greenwayodendron) suaveolens
Dialium spp.
Pancovia harmsiana/pedicellaris
Guarea vel sp aff thompsonnii/laurentii
Carapa procera
Nesogordonia papaverifera
Pausinystalia macroceras
Oncoba (Caloncoba) mannii
Trichilia prieuriana and other spp.
Sterculia (Eribroma) oblonga
Macaranga barteri/monandra/spinosa
Diospyros ituriensis
Angylocalyx pynaertii
Grossera macrantha
Pancovia laurentii
Diospyros canaliculata
Desplatsia spp.
Myrianthus arboreus
Cola lateritia
27
21
19
16
12
11
11
10
10
8
8
8
6
6
6
6
6
5
5
5
5
5
5
4
4
4
6
5
5
4
3
3
3
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
A total of 2,349 trees of 269 species were enumerated in 28 (200 10 m) transects. Density is the number of stems
per hectare. Percent stems indicates the relative proportion of each species. ‘‘Important’’ gorilla fruit (bold type)
and leaf (*) species are indicated.
Lowland Gorilla Diet at Mondika / 99
Pancovia laurentii (Sapindaceae), and Myrianthus arboreus (Moraceae). Celtis
(Ulmaceae), an important leaf species, was the second most common tree.
Seasonality in resource availability.
Fruit production varied across months and years (Fig. 1b). An average of 9%
of individuals (range = 2–21%) and 38% of species (range = 14–68%) bore ripe
fruit per month (n = 23). Generally, fruit availability increased in June or July,
peaked in August or September, and was low from November to March. The
number of individuals bearing ripe fruit was positively correlated with rainfall,
unlike the number of species (individuals r = 0.54, P = 0.009: species r = 0.34,
P = 0.12: n = 22 months).
Overall Diet Composition
Food list.
The gorilla plant food list consisted of 127 plant food items (of 100 species),
including 70 fruit, 33 leaf, 14 stem, two flower, and eight bark species (Table III).
Gorillas also consumed ants, termites, and soil.
Composition of the diet based on fecal samples.
Fifty-six fruit and one leaf species were identified from fecal samples
(n=715; Table III). Gorillas fed on fruit year-round, with an average of 3.5 species
per day, 10 species per month, and an average fruit score of 37% (Table IV). Fruit
was present in 99.8% of male and 100% of female fecal samples. The nonfruit
component of diet (herbs/leaves), present in 99.7% of samples and accounting for
61% of samples by volume (fiber score), dominated the majority of samples. We
could not reliably assess herb/leaf species from fecal samples. No sex difference
existed in the degree of frugivory or amount of fiber in diet (Table IV).
Important fruit species.
We classified 24 fruit species as important based on fecal samples, including
five type I (present in large quantity for 4 50% of months), three type III (present
in over 50% of months but in smaller quantities), and the majority (type II) that
were used intensively for shorter periods of time (Table V). We considered an
additional species (Irvingia excelsa) important because it appeared on more than
5% of the feeding trails. Important gorilla fruit species were from trees of various
sizes, including three large (mean >80 cm DBH), four medium (mean >50–
80 cm), 10 small (mean >20–50 cm), and eight very small (DBH of mean o20 cm,
or from herbs) (Table V).
Composition of the diet based on trail signs.
We identified 81 items of 62 plant and two insect species in Z1% of gorilla
trails (n = 619; Table III). Trail signs indicated that gorillas ate an average of 10
fruit species (Table IV) and 15 herb/leaf species per month, including a total of 33
leaf, 14 stem, and eight bark species that were not detected in fecal samples
(Table IV). The 24 most frequently encountered food items occurred on Z5%
of trails, and could be considered important foods. These included seven species
of herb stem, pith, or shoot; nine species of fruit (including six tree and three
herb species); six species of leaves; one species of bark (Celtis spp.); and termites
(Table VI). The four most frequently encountered items were herbs
(Haumania danckelmaniana (Marantaceae), Aframomum limbatum (Zingiberaceae),
Ingooka
Indolu
Modiali
Indoya/Indolya
Mobei
Mbeko
Pota
Motunga
Beti
Ivua
Landa
Ndembo
Bosindja
Pembe
Mosebe
Etokoloko
Gombo
Mongenje
Ekoule-akoumba
Gao
Baba
Mondumba
Etebele
Mokombe
Mbaso
Mbanda
Bemba
Antrocaryon klaineanum/micraster
Trichoscypha acuminata
Anonidium mannii
Artabotrys sp.?
Hexalobus crispiflorus
Greenwayodendron (Polyalthia) suaveolens
Uvariastrum germainii/ pierreanum
Aponcynaceae sp.
Funtumia elastica
Landolphia sp 1
Landolphia sp 2
Landolphia sp 3
Pleiocarpa pycnantha
Tabernaemontana penduliflora
Tabernaemontana crassa
Pycnobotrya nitida
Anchomanes difformis
Laccosperma secundiflora
Santira trimera
Copaifera mildbraedii
Detarium macrocarpum
Dialium sp.
Dialium sp.
Erythrophleum ivorense
Gilbertiodendron dewevrei
Anacardiaceae
Annonaceae
Apocynaceae
Arecaceae
Burseraceae
Caesalpiniaceae
Local name
Thomandersia hensii
Whitfieldia elongata?
Species
Acanthaceae
Family
d
d,t
t
t
t,d
d
t,d
t
t
t,d
t,d
t,d
d,t
d
t,d
t,d
d
t,d
t,d
F
t
t
t
t
t
t
t
t
L
T
T
T
S
T
T
F
T
T
T
T
B
1
2F,2L
1
1
2
13
1
2
2
2
3
1
1
2
6
1
1
14
10
%TR
4
5
5
1
2
8
1
5
6
1
%DM
4
7
2
3
7
1
1
%DF
TABLE III. Gorilla Plant Foods Recorded in Either a Minimum of 1% of Fecal (d) or Trail Samples (t) Between June 1995 and July 1998
100 / Doran et al.
Doto
Mangabo
Mondiki
Lembe
Babangu
Mulombo
Embandja
Mongamba
Kpaya
Tembo
Embundubundo
Mokosa
Isoko
Ita ti isoku
–
Batalo
Koko
Payo
Bokoko
Mobangui
Tonga
Boboko
Basele
Poposo
Mbili
Ngungu
Kaya
Mbonge
Palisota ambigua and other spp.
Palisota brachythyrsa/tholloni
Dioscorea sp.?
Diospyros
Diospyros
Diospyros
Diospyros
Dichostemma glaucescens
Drypetes diopa & others
Drypetes spp.
Keayodendron brideloides
Manniophytum fulvum
Oncoba (Caloncoba) glauca
Oncoba (Caloncoba) welwitschii
Unknown sp. 2
Unknown sp. 3
Oncoba (Caloncoba) crepiniana
Gnetum africanum
Irvingia excelsa
Klainedoxa gabonensis
Strychnos sp.
Helixanthera subalata
Ataenidia conferta
Haumania danckelmaniana
Hypselodelphis scadens?
Maranthochola congolensis
Megaphrynium macrostachyum/trichogynum
Sarcophrynium schweinfurthii/brachystachys
Trachyphyrnium braunianum
Commelinaceae
Dioscoreaceae
Ebenaceae
Euphorbiaceae
Flacourtiaceae
Gnetaceae
Irvingiaceae
Loganiaceae
Loranthaceae
Maranthaceae
crassiflora
ituriensis
mannii
bipindensis
Mokanja
Parinari excelsa or Maranthes glabra
Chrysobalanaceae
t,d
d
t,d
d
t,d
t,d
D
t,d
D
D
D
t,d
t,d
t,d
t,d
t,d
t
t,d
t
t
t
t
t
t
t
t
t
t
t
t
T
T
T
T
T
T
T
T
7F,29L 16S
3L,4S
1
1
6F,79S
2
1
5
11
10
1F,1L
1
2
1
1
2
43
4L,9S
1
22
1
8
1
30
5
4
4
3
19
1
10
4
32
1
4
2
4
1
1
1
3
Lowland Gorilla Diet at Mondika / 101
Mola
Ludyasumbu
Etokobola
–
Ebamba
Ecombolo
Ngumu
Ngumu/Dobo
Ngata
Vousa
Embungu
Mokana
Manjombe
Inganda
Embema
Molinda
Ngumanguma
Mobobo
Mosei
Esekerende
Motokodi
Ingoyo
Banga
Ingadje
Bambu
Albizia sp.?
Tetrapleura tetraptera
Ficus natalensis?
Ficus spp.
Myrianthus arboreus
Treculia Africana
Strombosia nigropunctata/postulata
Panda oleosa
Angylocalyx pynaertii
Milletita sp.?
Pterocarpus soyauxii
Unknown sp. 7
Barteria dewevrei/fistulosa
Colletocoema dewevrei
Nauclea diderrichii
Chytranthus gilletii
Chytranthus macrobotrys
Pancovia laurentii
Autranella congolensis
Chrysophyllum beguei or pruniforme
Chrysophyllum (Gambeya) lacourtiana
Mimosaceae
Moraceae
Olacaceae
Pandaceae
Papilionaceae
Passifloraceae
Rubiaceae
Sapindaceae
Sapotaceae
Local name
Dioscoreophyllum cumminsii
Triclisia dictyophylla
Tiliacora sp.
unknown sp. 6
Species
Menispermaceae
Family
TABLE III. Continued
t
d
t,d
t,d
t
t,d
d
t,d
t,d
t,d
t
t,d
d
t,d
d,t
t,d
d,t
t,d
d
d
F
t
t
t
t
t,d
t
t
L
S
F
11
T
T
B
14
1
1
3
1
1
1F,6L
1
1
1
4
1
6
1
1
%TR
16
7
1
1
3
7
16
2
7
24
2
1
%DF
7
1
1
3
5
20
2
7
27
1
1
%DM
102 / Doran et al.
Bongbo
Liamba
Nguluma
Boukou
Kakala
Ngombe
Mogweagwea
Esandja
Esandja-mbongo
Mojambi
Njombo
Njokoko
Cola chlamydantha
Desplatsia spp.
Duboscia macrocarpa
Grewia oligoneura
Celtis adolfi-friderici
Celtis mildbraedii/ tessmannii/zenkeri
Vitex doniana or welwitschii
Rinorea spp.
Rinorea spp.
Cissus spp.
Aframomum limbatum and other spp.
Aframomum subsericium
Unknown sp. 21
Sterculiaceae
Tiliaceae
Ulmaceae
Verbinaceae
Violaceae
Vitaceae
Zingiberaceae
Unknown
t,d
t
d
d
d
t,d
t,d
d
t
t
t
t
t
t
t
t
t
t
t
t
T
T
2
9F,58S
1F,5S
10L,7B
4F,1L
19
1
1
1
31
1
1
5
74
3
1
29
7
5
75
2
1
Parts consumed include fruit (F), leaf (L), stem (S), flower (F), and bark (B). Percentage of trails (%TR) and fecal samples (%DF) = females; %DM = males) in which the item was
found are indicated. A question mark indicates that identification remains to be confirmed. Local names are in the BaAka dialect.
Ikumbi
Mondamandama
Koloka
Mongenja
Mokenjenje
Mondonge A,B
Chrysophyllum (Gambeya) perpulchrum
Mannilkara mabokeensis
Synsepalum longecuneatum
Chrysophyllum bukokense
Lowland Gorilla Diet at Mondika / 103
315
10.274.4
(range=3–21)
3.671.6
(range=1.6–8.3)
37.3714.7%
(range=10–82%)
9.573.2
(range = 4–16)
NA
400
12.975.6
(range=4–24)
3.571.5
(range=2–8.2)
38.0714.3%
(range=15–70)
10.273.0
(range=6–18)
NA
Fecal: male
NS (Mann Whitney U=464,
P=0.816, n =31 mo)
NS (Mann Whitney U = 469,
P=0.871
NS (Mann Whitney U=400,
P=0.257
Sex difference?
9.676.8
(range=1–30)
15.076.9
(range=4–31)
NA
617
19.178.8
(range=8–18)
NA
Trail signs: group
Fecal data include three measures of frugivory including mean number of fruit species per sample, fruit score, and different number of fruit species consumed per month (7one
standard deviation). Trail sign data include the mean number of herb/leaf species and the total number of fruit species consumed per month.
Mean no. of herb/leaf species per mo
Total number of fruits/month (n = 31 mo)
Fruit score (n = 31 mo)
No. Fruit species/sample (n = 31 mo)
Sample size
Mean no. of samples/mo
Fecal: female
TABLE IV. Comparison of Results of Gorilla Diet Based on Female and Male Fecal Samples and Trail Signs
104 / Doran et al.
Apocynaceae
Annonaceae
Ebenaceae
Caesalpiniaceae
Annonaceae
Passifloraceae
Marantaceae
Euphorbiaceae
Ebenaceae
Loganiaceae
Flacourtiaceae
Sapindaceae
Apocynaceae
Annonaceae
Papilionaceae
Verbinaceae
Moraceae
Sapotaceae
Moraceae
Type II. Short duration, large quantity
Landolphia spp
Anonidium mannii
Diospyros ituriensis
Dialium spp. (pachyphylum and zenkeri)
Hexalobus crispiflorus
Barteria dewevrei/fistulosa
Haumania danckelmaniana
Drypetes spp. (diopa and others)
Diospryos mannii
Strychnos sp.
Oncoba (Caloncoba) welwitschii
Pancovia laurentii
Tabernaemontana spp. (penduliflora and crassa)
Greenwayodendron (Polyalthia) suaveolens
Angylocalyx pynaetrii
Vitex doniana or welwitschii
Type III. Long duration, small quantity
Myrianthus arboreus
Chrysophyllum (Gambeya) lacourtiana
Ficus spp.
Tree, big
Tree, small
Tree, big
Strangler
Liana
Tree, small
Shrub
Tree, small/very small
Tree, med
Tree, small
Herb
Small, tree or shrub
Tree, small
Liana
Tree, small
Tree, small
Tree, small
Tree, small
Tree, med
Tree, small
Tree, med
Tree, med
Tree, big
Herb
Herb
Life Form
0.7
(22.5)
(23.2)
and 12.1)
(23.9)
123.7 (52.9)
36.3 (31.3)
82.7 (23.8)
21.6
36.3
(12.0
38.1
51.2
37.8
2.5
4.8
0.9 and 1.25
10.2
5.2
2.1
4.1
2.3
20.6
49.3 (26.9)
(13.8)
(11.6 and 19.6)
67.6 (24.7)
(17.2)
–
69.3 (36.3)
52.4 (39.2)
111.1 (38.1)
DBH
0.4 and 3.0
1.2
10.9
5.2
1.25 and 0.53
1.1
2.1
2.1
1.6
0.7
Density (No./ha)
Definitions are modified after Nishihara [1995]. Long duration fruits were present in fecal samples during 50% of months. Large quantity occurred in 4than 50% of samples in at
least a single month or had a mean abundance fecal score of 2. Sizes of trees include: big (480 cm), med (50–80 cm), small (20–50 cm), and very small (o20 cm). DBH in
parentheses is from transects.
Type IV. Additional fruits that are present in 45% of feeding trails
Irvingia excelsa
Sapotaceae
Tiliaceae
Mimosaceae
Irvingiaceae
Zingiberaceae
Marantaceae
Family
Type I, Long duration, large quantity
Duboscia macrocarpa
Tetrapleura tetraptera
Klainedoxa gabonensis
Aframomum spp.
Megaphrynium macrostachyum
Species
TABLE V. Important Fruits in the Diet
Lowland Gorilla Diet at Mondika / 105
106 / Doran et al.
TABLE VI. Important (Appearing on 5% Trails) Gorilla Foods Based on Trail Sign Data
Family
Species
Haumania danckelmaniana
Aframomum limbatum and other spp.
Palistota ambigua and other spp.
Megaphrynium macrostachyum
Duboscia macrocarpa
Cubitermes sp.
Marantaceae
Megaphrynium macrostachyum
Acanthaceae
Thomandersia hensii
Arecaceae
Laccosperma secundiflora
Sapotaceae
Chrysophyllum (Gambeya) lacourtiana
Irvingiaceae
Klainedoxa gabonensis
Ulmaceae
Celtis mildbraedii/tessmannii/zenkeri
Gnetacae
Gnetum africanum
Acanthaceae
Whitfieldia elongata?
Zingiberaceae Aframomum limbatum and others
Commelinaceae Palisota brachythyrsa/tholloni
Ulmaceae
Celtis mildbraedii/tessmannii/zenkeri
Marantaceae
Megaphrynium macrostachyum/
trichogynum
Marantaceae
Haumania danckelmaniana
Papilionaceae Angylocalyx pynaertii
Mimosaceae
Tetrapleura tetraptera
Annonaceae
Anonidium mannii
Irvingiaceae
Irvingia excelsa
Zingiberaceae Aframomum subsericium
Marantaceae
Zingiberaceae
Commelinaceae
Marantaceae
Tiliaceae
Local
name
Life
form
Part %TR
Basele
Njombo
Doto
Ngungu
Nguluma
Kusu
Ngungu
Ingoka
Gao
Bambu
Bokoko
Ngombe
Koko
Indolu
Njombo
Mangabo
Ngombe
Ngungu
Herb
Herb
Herb
Herb
Tree, med
Insect
Herb
Shrub
Herb
Tree, big
Tree, big
Tree, big
Vine
Shrub
Herb
Herb
Tree, big
Herb
sh
st
st
sh
fr
st
lf
st
fr
fr
lf
lf
lf
fr
st
bk
fr
79
58
43
29
19
19
16
14
13
11
11
10
10
10
9
9
7
7
Basele
Manjombe
Ecombolo
Mobei
Payo
Njokoko
Herb
Tree,
Tree,
Tree,
Tree,
Herb
fr
lf
fr
fr
fr
st
6
6
6
6
5
5
med
med
small
big
Percent of days on which each food was encountered (%TR), life form, and plant parts eaten including stem (st),
shoot (sh), leaf (lf), fruit (fr), and bark (bk) are indicated.
Palisota ambigua (Commelinaceae), and Megaphrynium macrostachyum (Marantaceae). Two species of herbs, Haumania danckelmaniana and Aframomum
limbatum, were eaten on more than 50% of days. Four species of leaf, including
Thomandersia hensii (Acanthaceae), Celtis mildbraedii (Ulmaceae), Gnetum
africanum (Gnetaceae), and Whitfieldia elongata (Acanthaceae), were consumed
on more than 10% of days. None of these herb or leaves from woody plants were
identified as important on the basis of fecal sampling. Gorillas ate termites on
19% of days.
Three species of fruit were present on more than 10% of days. These include
Duboscia macrocarpa (19%), Klainedoxa gabonensis (11%), and Chrysophyllum
(Gambeya) lacourtiana (11%). Of the nine species of fruit classified as important
on the basis of feeding trails, all but one, Irvingia excelsa, was identified as
important on the basis of fecal samples. Sixteen additional species of important
fruits (based on fecal samples) were not observed on 4 5% of feeding trails, but
appeared in 4 10% of trails during at least 1 month.
Seasonal Influence on Diet
Fecal samples.
Fruit and herbs were present in the gorilla diet throughout the year, with
their proportions fluctuating relative to changes in fruit availability (Fig. 2). As
fruit availability increased, the mean number of fruit species per sample
Lowland Gorilla Diet at Mondika / 107
(a) Number of fruit species per sample
10
8
6
4
2
NOV
DEC
DEC
1997
OCT
SEP
AUG
JUL
1996
NOV
1995
JUN
MAY
APR
MAR
FEB
JAN
0
1998
(b) Total fruit species per month
25
20
15
10
5
95/96
96/97
OCT
SEP
AUG
JUL
JUN
MAY
APR
MAR
FEB
JAN
0
97/98
(c) Fiber score per sample
60
40
1995
1996
1997
DEC
NOV
OCT
SEPT
AUG
JUL
JUN
MAY
APR
MAR
FEB
20
0
JAN
% fiber
100
80
1998
Fig. 2. Monthly and annual variation in male gorilla diet (fecal samples), including (a) mean number
of fruit species per fecal sample per month, (b) total number of different fruit species consumed
each month, and (c) relative fiber score. Although only the male pattern is shown, the female
pattern is not significantly different.
increased and fiber score decreased for both males and females [r (fruit): males =
0.67, P = 0.001; females =0.60, P = 0.004: r (fiber score): males = 0.69, P o
0.001; females = 0.69, P o 0.001: n = 21 months). Fiber score and the number of
fruit species per fecal sample were inversely correlated (r: male = –0.68, P o
0.001, female = –0.46, P = 0.04; n = 21 months). The total number of different
fruit species appearing in fecal samples each month was neither significantly
positively correlated with fruit availability (male r = 0.41, P = 0.06; female r =
0.29, P = 0.2) nor negatively correlated with fiber score (male r = –0.32, P = 0.14;
female r = –0.15, P = 0.5; n = 22 months), suggesting that a minimum variety of
108 / Doran et al.
fruit is maintained in the diet throughout the year, even when fruit availability is
minimal.
Fruit availability and the number of reliable fruits per month were not
significantly correlated (r = 0.54, P = 0.09, n = 11 months). On average, five
species of reliable fruit, including three from trees (Duboscia macrocarpa,
Tetrapleura tetraptera, and Desplatsia dewevrei) and two herbs (Aframomum spp.
and Megaphrynium macrostachyum), were particularly important during the
period of lowest fruit availability (October–February).
Seasonal variation existed in the size of food trees that dominate gorilla diet
each month (Fig. 3). Gorillas commonly used medium-sized trees throughout the
year (with a slight drop in September–October). Small trees were used most
frequently during the major fruiting season (June–September), with seven of 10
important small species (Table V) among those most commonly available (Table
II). Large trees, used immediately prior to (March–June), and after (September–
October), the major fruiting season, are rare in the environment and require
longer distances to travel between them.
Trail signs.
Seasonal differences in diet determined from trail signs broadly resembled
those from fecal sampling; as fruit availability increased, herb/leaf diversity in
diet decreased (r = –0.57, P = 0.007; n = 21 months; Fig. 4a). However, this was
not uniformly true across all herb species. Protein-rich Haumania danckelmaniana shoots [Nishihara, 1995] were a dietary staple; they were eaten on average
79% of days each month (Fig. 4b; SD = 13, range = 55–94), and their use was not
correlated with fruit availability (Fig. 4b; r = 0.11, P = 0.63, n = 21 months).
Other food items were used (to varying extents) as fallback foods, as indicated by
an inverse relationship with fruit availability (Fig. 4c; n = 21 months in all cases).
These included the pith of the second most frequently encountered herb,
Aframomum limbatum (r = –0.72, P 4 0.001), the four most important leaf
species, significantly in three of the four cases and approaching significance in the
fourth (T. hensii r = –0.61, P = 0.003; C. mildbraedii/tessmannii/zenkeri r =
–0.49, P = 0.02; G. africanum r = –0.46, P = 0.03; W. elongata r = –0.40, P = 0.07)
and bark (r = –0.77, P o 0.0001). Termite use increased with rainfall (r = 0.4,
P = 0.01, n = 36 months) but was not significantly correlated with fruit availability (r = 0.13, P = 0.58, n = 21 months).
DISCUSSION
Site Differences in Resource Availability
Herb availability.
Overall herb densities (of important gorilla foods) at Mondika were
comparable to those at Bai Hokou, but lower than those at Lopé and Ndoki
(Table VII). In particular, the density of Haumania spp., arguably the most
important herb across sites, was two to three times lower at Mondika (0.33 stems/
m2) and Bai Hokou (0.44 stems/m2 [Goldsmith, 1996]) relative to Lopé (0.90
stems/m2 [White et al., 1995]). Reduced herb densities should result in smaller
group size or longer daily path lengths, if other factors are equal [Doran &
McNeilage, 2001]. In addition to herbs in terra firma forest, swamp herbs were
present (but not measured) at Mondika and Ndoki, but absent from Bai Hokou
and Lopé. The four sites, ranked in descending order of herb density, are Ndoki
Lowland Gorilla Diet at Mondika / 109
95/96
96/97
97/98
May
Apr
Mar
Feb
Jan
Dec
Nov
Oct
Sep
Aug
Jul
Jun
% of fecal samples
(a) Large Trees (> 80 cm DBH)
120
100
80
60
40
20
0
98
97/98
May
Apr
Mar
Feb
Jan
Dec
96/97
98
95/96
96/97
97/98
May
Apr
Mar
Feb
Jan
Dec
Nov
Oct
Sep
Jul
Aug
(c) Small Trees (20 - 50 cm DBH)
120
100
80
60
40
20
0
Jun
% of fecal samples
95/96
Nov
Oct
Sep
Aug
Jul
Jun
% of fecal samples
(b) Medium trees (50 - 80 cm DBH)
120
100
80
60
40
20
0
98
Fig. 3. Monthly variation in the relative size of fruit trees used. Proportion of dominant food species
that were from (a) large (>80 cm), (b) medium (50–80 cm), and (c) small (20–50 cm) trees each
month are indicated.
(with high herb density plus presence of swamp vegetation), followed by Lopé,
Mondika, and Bai Hokou (low density and absence of swamp herbs).
Fruit availability.
Comparable data on fruit availability are limited to those from Mondika and
Lopé. The forest at Mondika was more diverse in its species composition than
Lopé, with higher tree density (stems/ha: Mondika = 419.5; Lopé = 384.5
(a) Fruit and herb consumption (trails)
40
30
20
10
Fruit
May-98
Feb-98
Nov-97
Aug-97
May-97
Feb-97
Nov-96
Aug-96
May-96
Feb-96
Nov-95
0
Aug-95
Number of species
110 / Doran et al.
Leaf/Herb
(b) Monthly herb consumption (trails)
% of trails
100
80
60
40
20
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Haumania
Aframomum
% of trails
(c) Monthly leaf consumption (trails)
45
40
35
30
25
20
15
10
5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Fig. 4. Monthly variability in diet (trail signs), including (a) total number of fruit and herb/leaf
species encountered each month, (b) frequency of Haumania danckelmaniana and Aframomum sp.,
and (c) frequency of the four most important leaf species (Thomandersia hensii, Celtis mildbraedii/
tessmannii/zenkeri, Gnetum africanum, and Whitfieldia elongata).
[Williamson, 1989]), a greater number of species contributing to make up 50% of
the population (Mondika = >20: Lopé = 6 [Williamson, 1989]), and a lower
proportion of forest formed by the 10 most common trees (Mondika = 32%: Lopé
= 61% [Williamson, 1989]). At Lopé, one species (Cola lizae) accounted for a
quarter of all trees, and was an important food (found in 16.7% of fecal samples)
[Williamson, 1989]; the most common important fruit tree species at Mondika
accounted for only 3% of trees (Table III). In spite of considerable differences in
forest structure, habitat (Lopé includes seven habitat types compared to three at
Mondika), and species composition, densities of two key fallback species of fruit
Lowland Gorilla Diet at Mondika / 111
Table VII. Resource Availability and Western Lowland Gorilla Diet at 4 Sites
Sites
Ndoki
Lopé
Mondika
Bai Hokoua Bai Hokoub
Length of study
Average rainfall
Number of dry season months
Fecal sample size
Fecal samples per month
Resource availability
Herb density
Gorilla diet: staple foods
Herbs, Haumania sp.
Swamp Herbs
Fruit
No. species overall
Average no. species/mo
Species/d
Average fruit score
Fallback foods
Duboscia
Low quality herbs
Bark
Leaves
Seasonally important
Klainedoxa
Tetrapleura
Herb fruits
1 yr
1,430
3
522
42 (9–53)
3 yr
1,536
3
180*
5 (5)
3 yr
1,415
3
715
21 (6–45)**
27 mo
1,401
3
859
35 (3–102)
19 mo
1,952
3
528
31 (6–56)
2.25
1.87
0.78
0.82
0.82
+
+
+
–
+
+
+
–
+
–
115
91
70
77
17(8–36) 14.2 (7–20) 9–10 (4–16) 13 (5–28)
3
3.0
3.5
3.2
48%
39%
15.7
3.5
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
–
+
+
+
+
+
+
–
+
+
+
+
+
+
+
+
+
+
Sites are listed in decreasing rank order of herb availability. Staple foods are used throughout the year. Fallback
foods are used more frequently when succulent fruits are not available, although generally available throughout
the year. Fruit is included as a staple food, because a minimum diversity is maintained in diet throughout year
(although no single species is used throughout the year) in spite of major changes in availability. Diet data are
from Nishihara [1995] (Ndoki); Tutin et al. [1991]; Rogers & Williamson [1988]; (Lopé); this study (Mondika);
Remis [1997] (Bai Hokoua); Goldsmith, 1996 (Bai Hokoub). Resource availability data are from Malenky et al.
[1994]; White et al. [1995]; this study, and Goldsmith [1996]. *Sample size for Lopé is from five random samples
per month. **Sample size for Mondika is from combined male and female samples.
trees were similar and relatively rare at both sites (Mondika vs. Lopé (stems/ha):
Duboscia macrocarpa (2.1 vs. 2.2), and Klainedoxa gabonensis (0.7 vs.0.7)).
Gorilla Diet at Mondika
Rainfall, length of dry season, and fruit availability were seasonal at
Mondika, and similar to other western lowland gorillas sites (Table VII). Previous
researchers reported gorilla diet on a monthly basis, but distinguished between
two different periods of fruit availability: rainy vs. dry season [Remis, 1997;
Goldsmith, 1999], or good fruit and bad fruit months [Tutin, 1996]. We
distinguish three annual time periods at Mondika, which differ in the size,
density, and distribution of available gorilla foods in a manner we expect is
associated with predictable differences in ranging and/or group cohesion.
During the first period (November–February), which included but was not
restricted to the dry season, fruit availability was low (Fig. 1). Nonfruit foods,
including low-quality herbs (e.g., Aframomum spp. and Palisota), a wide variety
of leaves, and Celtis bark, dominated the diet. These resources were commonly
available, and those in trees (particularly Celtis sp.) occurred in patches large
112 / Doran et al.
enough to permit many gorillas to feed. In spite of reduced fruit availability,
gorillas continued to feed on five species of reliable fruit. These were also
commonly available, either in herb patches (Aframomum spp. and Megaphrynium
macrostachyum) or in trees that were relatively common (intermediate in density
= 4 o x o 1 stem/ha) and 450 cm DBH in size (Tetrapleura tetraptera and
Duboscia macrocarpa), or small and commonly distributed (Desplatsia dewevrei,
four stems/ha). During this period, gorillas ate two species of succulent fruit
(Pancovia laurentii and Angylocalyx pynaertii) that were among the most
common species in the study site. However, their fruiting patterns were
synchronous (within species), brief and unpredictable, and less likely to influence
ranging. The first annual period was thus characterized by low fruit availability
combined with relatively evenly distributed and commonly available resources.
Important fallback tree species (Tetrapleura tetraptera and Duboscia macrocarpa)
were large enough to permit all individuals in a group to feed (average nest group
size = 7.4, SD = 3.7, n = 514 nest sites [Mehlman & Doran, 2002]). During this
period, daily path lengths should be at their lowest and groups should be
relatively cohesive.
The second period (April–June) was characterized by increased rainfall and
fruit availability. Gorillas responded by eating a greater quantity and variety of
fruit, and reduced variety of leaf species. Both protein-rich (Haumania
danckelmaniana) and lower-quality herb consumption was high. Termite
consumption increased as a function of rainfall. Gorillas used three reliable
fibrous (vs. succulent) fruit species, including Duboscia macrocarpa, the large and
rare Klainedoxa gabonesis, and the small and common Myrianthus arboreus.
Additionally, Chrysophyllum (Gambeya) lacourtiana, a succulent fruit found in
large and rare trees, was eaten frequently when available, although its
availability was less predictable between years than the other species. Thus
during the second period, reliance on commonly available tree leaves decreased,
and as a result fewer preferred foods were available in a given area. Additionally,
key fallback fruit trees were large and rare, and as a result daily path length
increased, but groups could remain cohesive.
During the third period (July–September), fruit availability peaked. Fruit
consumption was at its highest, and herb (with one exception), leaf, and bark
consumption was at its lowest. Haumania danckelmaniana consumption
remained high, consistent with Kuroda et al.’s [1996] finding that protein-rich
herbs were ingested throughout the year, unlike lower-quality herb use, which
diminished with increasing fruit availability. During this period, succulent fruit
was readily available, although patches were too small to permit all group
individuals to feed, and too widely spaced to allow individuals to feed in separate
patches and retain short-range vocal contact. We expect the number of trees
visited and the daily path length to increase, or groups to be less cohesive–
a pattern not predicted during the second period.
Thus, throughout the year gorilla diet consisted of fluctuating proportions of
herbs, leaves, and fruit. Fruit remained a consistent component in diet yearround at Mondika, even in periods of low fruit availability. Gorillas ate an average
of 3.5 fruit species per day and were selective in their fruit choice, ignoring some
common fruits and incorporating rare fruits to a degree higher than predicted on
availability. Although ranging data are not available from the same time period of
this study, more recent data, including those from all other sites, indicate that
gorillas travel farther to add fruit to their diet when it is available, rather than
subsist on lower-quality forage [Doran & Greer, 2002; Goldsmith, 1999; Remis,
1997; Tutin, 1996]. This was not the case for herb consumption [Doran & Greer,
Lowland Gorilla Diet at Mondika / 113
2002; Goldsmith, 1999], except on occasions when gorillas traveled long distances
to reach swamps to feed on aquatic herbs (unpublished results).
Our results did not indicate a sex difference in diet. Large male body size was
not associated with greater folivory; males and females did not differ in daily or
monthly fruit consumption or fiber content. Similarly, Remis [1997] did not
detect a sex difference in the mean monthly number of fruits consumed, although
she reported that females ate more species of fruit per day than did males during
one poor fruit season. Given that sex differences have been reported in other ape
species, with less body size dimorphism [e.g., Galdikas & Teleki, 1981; McGrew,
1992], perhaps indirect methods are simply not fine-grained enough to detect
subtle differences in diet. We discuss this topic further below.
Gorilla Diet Across Sites
In spite of differences in resource availability across sites, gorilla diet, as
currently measured, was remarkably similar (Table VII). Reduced herb density at
Mondika (relative to Ndoki and Lopé) was not associated with increased reliance
on fruit, as proposed by Kuroda et al. [1996], nor was a greater number of
available fruit species (at Mondika relative to Lopé) coincident with a greater
number of fruits recorded in the diet (Table VI).
At all sites, fruit and herbs were important components of diet throughout
the year, fruit production was highly seasonal. The quantity and quality of fruit
and fiber, and the number of fruit species in the diet fluctuated in response to
changes in fruit availability. Gorillas ate an average of three and maximum of
12 species of fruit per day, and an average of 10–17 and maximum of 36 species
per month. Greatest monthly fruit diversity in the diet occurred at Ndoki, the
site of greatest herb availability. Average fiber scores were also equivalent
across sites, although less directly comparable across sites due to differences in
methods.
Gorillas shared key components of diet across sites. Staple foods, eaten yearround, included high-quality herbs (Haumania), swamp herbs when present, and
(we would argue) a minimal diversity of fruit. Fallback foods (those present yearround but incorporated into the diet when more preferred foods are absent)
included leaves, bark, low-quality herbs (but see Goldsmith [1996]), and two
species of fruit (Duboscia macrocarpa and Klainedoxa gabonensis). Densities of
these two species were similar across sites for which data are available, in spite of
major differences in forest structure, suggesting that these may influence gorilla
density.
Our findings are consistent with Williamson’s [1989] interpretation that
western lowland gorillas pursue succulent fruits at some cost, while incorporating
high-quality herbs as a staple and complementary dietary component. Costs to
frugivory entail greater day ranges and possible constraints on group size (which
can be buffered somewhat in areas with higher herb densities). However, such
little variation in diet across sites is surprising, given differences in resource
availability, such as a twofold difference in herb density. This may partly reflect
the fact that researchers have thus far sampled primarily a somewhat restricted
range of habitat types (but see Sabater Pi [1977] and Calvert [1985]). Further
sampling of gorilla diet from areas of much higher (as at Lossi) or lower (as at
Petit Loango) herb density may provide more evidence of greater cross-site
variation in gorilla diet.
114 / Doran et al.
Fecal Samples and Trail Signs: Pros and Cons
The findings of no sex difference and little cross-site variation in diet may
reflect the limitations of indirect sampling methods. Fecal samples and trail sign
data can greatly inform our understanding of diet composition and the relative
importance of different components during the year, as indicated in this and
previous studies [Williamson, 1989; Tutin et al., 1991; Nishihara, 1995; Remis,
1997; Goldsmith, 1999]. However, both methods can potentially bias the results.
Fecal samples provided the most effective means of assessing diet during the
early stage of our study, because: 1) they are easy to collect (relative to following a
trail for an entire day), 2) they represent a diet ‘‘snapshot’’ for one individual (vs.
the entire group), and 3) results are not dependent upon length of trail followed
and are less dependent upon monthly sample size (compared to trail signs). Fecal
samples provided a good assessment of the diversity and frequency of fruit
consumption, because seeds of most fruits were swallowed whole, passed through
the gut intact, and were identifiable to species, as reviewed previously [Tutin &
Fernandez, 1993].
However, quantitative measures of fruit consumption were, at best, poor
approximations, as noted previously [Tutin & Fernandez, 1985]. Although
relative changes in fiber content could be monitored, we could not identify
species of leaf, pith, or bark macroscopically from fecal samples, and therefore
failed to detect the variety of leaf, stem, and bark species in the diet. This
apparently was less of a problem in previous studies [e.g., Tutin et al., 1991;
Remis, 1997; Goldsmith, 1996].
Feeding-trail data added important additional information on the diversity
and frequency of herb and leaf consumption to fecal samples: we identified 33 leaf,
14 stem, and eight bark species that were undetected from fecal samples (Table
III). Fewer studies have conducted extensive trail sampling (but see Goldsmith
[1996]), because it is difficult to track gorillas (the average daily path length of
western gorillas exceeds 2 km/day at all sites). Additionally, sample size appears
to influence results to a greater degree than in fecal sampling. Trail sample size,
unlike fecal sample size, was correlated with the number of species detected in
this study as well as previously [Goldsmith, 1996]. When shorter trails are
followed, foods occurring in highest density, such as herbs, tend to be
oversampled. Because trail signs cannot be ascribed to particular individuals,
results tend to overestimate individual daily dietary breadth. This was true in our
study, as we detected more fruit species (5.4 vs. 3.8) on a daily basis from trail
signs vs. fecal samples. However, this difference averaged out, since we found no
difference between the monthly number of fruit species detected by either
method. The degree to which oversampling individuals or undersampling group
diet occurs should vary throughout the year, and lead to seasonal biases in data.
For example, when group cohesion is high, trails of a greater number of
individuals are encountered and thus more food items are detected, relative to
when interindividual spacing is greater. All trail sign feeding remains can be
quantified to estimate quantity of leaf and herb (but not fruit) in the diet
[Goldsmith, 1996], but only if habituation is not a priority, since it is timeconsuming. However, these results should be interpreted with caution because it
is nearly impossible to know how many individuals the trail represents.
For these reasons, and to minimize sampling effort, we chose to sample only
the presence or absence of each food on trails each day, and to collect fecal
samples from only two individuals (male and female) per day vs. the entire group
(as was done in previous studies). In spite of these differences in sampling
Lowland Gorilla Diet at Mondika / 115
protocol, the results are nearly identical across sites. This leads us to conclude
that a basic knowledge of diet can be obtained with smaller sampling effort on a
daily basis than is currently used, and that more extensive sampling does not
provide results that are more accurate or finer in detail enough to warrant the
additional effort. We conclude that indirect sampling of diet provides a broad
estimate of diet, but is not sufficiently informative to capture the range of
interindividual and intersite variation in diet predicted.
ACKNOWLEDGMENTS
We gratefully acknowledge the Ministries of Eaux et Forets and Recherches
Scientifiques in Central African Republic, and the Ministry of l’Enseignement
Primaire, Secondaire et Superieur Charge de la Recherche Scientifique in
Republic of Congo for permission to carry out research at the Mondika Research
Center. We thank the late M. Urbain Ngatoua, former National Director of the
Dzanga-Ndoki National Park; Allard Blom and Lisa Steel of WWF, and Bryan
Curran and Fiona Maisels of WCS for continued support and logistical assistance;
David Harris for assistance in identifying botanical specimens; Terry Brncic for
collecting much of the tree transect data; and Richard Malenky for many
‘‘fruitful’’ discussions on ape foraging behavior. The manuscript was greatly
improved as a result of comments from Richard Malenky, Sharon Pochron, and
four anonymous reviewers. Finally, the work at Mondika would not have been
possible without the efforts of many people in the field, including Cleve Hicks,
Shannon Crowley, and Patrice Mongo, and the skilled tracking and botanical
lessons of many BaAka field colleagues, especially Mangombe, Mokonjo, Ndima,
Mamandele, Bakombo, and Mbokola.
REFERENCES
Calvert JJ. 1985. Food selection by western
gorillas (G.g. gorilla) in relation to food
chemistry. Oecologia 65:236–246.
Doran DM, McNeilage A. 1998. Gorilla ecology
and behavior. Evol Anthropol 6:120–131.
Doran DM, McNeilage A. 2001. Subspecific
variation in gorilla behavior: the influence
of ecological and social factors. In: Robbins
MM, Sicotte P, Stewart KJ, editors. Mountain gorillas: three decades of research at
Karisoke. Cambridge: Cambridge University Press. p 123–149.
Doran DM, Greer D. 2002. The influence of
swamp use and fruit consumption on
western lowland gorilla (Gorilla gorilla
gorilla) ranging behavior at Mondika Research Center. Am J Phys Anthropol 34:
64–65.
Galdikas B, Teleki G. 1981. Variations in
subsistence activities of female and male
pongids: new perspectives on the origins of
hominid labor division. Curr Anthropol
22:241–256.
Goldsmith ML. 1996. Ecological influences on
the ranging and grouping behavior of
western lowland gorillas at Bai Hohou,
Central African Republic. Ph.D. thesis,
SUNY at Stony Brook, Stony Brook, New
York.
Goldsmith ML. 1999. Ecological constraints on
the foraging effort of western gorillas
(Gorilla gorilla gorilla) at Bai Hokou,
Central African Republic. Int J Primatol
20:1–23.
Grieg-Smith P. 1983. Quantitative plant ecology. Berkeley: University of California
Press. 399 p.
Kuroda S, Nishihara T, Suzuki S, Oko RA.
1996. Sympatric chimpanzees and gorillas
in the Ndoki Forest, Congo. In: McGrew
WC, Marchant LF, Nishida T, editors. Great
ape societies. Cambridge: Cambridge University Press. p 71–81.
Lambert JE. 1999. Primate digestion: interactions among anatomy, physiology, and feeding ecology. Evol Anthropol 7:8–20.
Magliocca F, Querouil S, Gautier-Hion A.
1999. Population structure and group composition of western lowland gorillas in
north-western Republic of Congo. Am J
Primatol 48:1–14.
Malenky RK, Kuroda S, Vineberg E, Wrangham RW. 1994. The significance of terrestrial herbaceous foods for bonobos,
116 / Doran et al.
chimpanzees and gorillas. In: Wrangham
RW, McGrew WC, deWaal FBM, Heltne P,
editors. Chimpanzee cultures. Cambridge,
MA: Harvard University Press. p 93–108.
McGrew WC. 1992. Chimpanzee material
culture. Cambridge: Cambridge University
Press.
Mehlman PT, Doran DM. 2002. When do
western lowland gorillas (Gorilla gorilla
gorilla) build nests? Factors influencing
gorilla nest construction at Mondika Research Center. Int J Primatol 23:1257–1285.
Nishihara T. 1995. Feeding ecology of western
lowland gorillas in the Nouabale-Ndoki
National Park, Congo. Primates 36:151–
168.
Parnell RJ. 2002. Group size and structure in
western lowland gorillas (Gorilla gorilla
gorilla) at Mbeli Bai, Republic of Congo.
Am J Primatol 56:193–206
Remis MJ. 1997. Western lowland gorillas
(Gorilla gorilla gorilla) as seasonal frugivores: use of variable resources. Am J
Primatol 43:87–109.
Rogers ME, Williamson EA, Tutin CEG,
Fernandez M. 1988. Effects of the dry
season on gorilla diet in Gabon. Primate
Rep 22:25–33.
Sabater Pi J. 1977. Contribution to the study
of alimentation of lowland gorillas in the
natural state, in Rio Muni, Republic of
Equatorial Guinea (West Africa). Primates
18:183–204.
Smith R, Jungers WL. 1997. Body mass in
comparative primatology. J Hum Evol
32:523–559.
Tutin CEG, Fernandez M. 1985. Foods consumed by sympatric populations of Gorilla
g. gorilla and Pan t. troglodytes in Gabon:
some preliminary data. Int J Primatol
6:27–43.
Tutin CEG, Fernandez M, Rogers ME, Williamson EA, McGrew WC. 1991. Foraging
profiles of sympatric lowland gorillas and
chimpanzees in the Lope Reserve, Gabon.
Phil Trans R Soc Lond B 334:179–186.
Tutin CEG, Fernandez M. 1993. Composition
of the diet of chimpanzees and comparisons
with that of sympatric lowland gorillas in
the Lope Reserve, Gabon. Am J Primatol
30:195–211.
Tutin CEG. 1996. Ranging and social structure of lowland gorillas in the Lope Reserve,
Gabon. In: McGrew WC, LF Marchant LF,
Nishida T, editors. Great ape societies.
Cambridge: Cambridge University Press. p
58–70.
Watts DP. 1996. Comparative socio-ecology of
gorillas. In: McGrew WC, LF Marchant LF,
Nishida T, editors. Great ape societies.
Cambridge: Cambridge University Press.
p 16–28.
White LJT, Rogers ME, Tutin CEG, Williamson E, Fernandez M. 1995. Herbaceous
vegetation in different forest types in the
Lope Forest Reserve, Gabon: implications
for keystone food availability. Afr J Ecol
33:124–141.
Williamson EA. 1989. Behavioural ecology of
western lowland gorillas in Gabon. Ph.D.
dissertation, University of Stirling, Stirling,
UK.
Документ
Категория
Без категории
Просмотров
9
Размер файла
172 Кб
Теги
site, reflection, diet, method, lowlands, gorillas, cross, comparison, new, resource, evidence, availability, sampling, western, indirect
1/--страниц
Пожаловаться на содержимое документа