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Nutritional Aspects of Western Lowland Gorilla (Gorilla gorilla gorilla) Diet During Seasons of Fruit Scarcity at Bai Hokou, Central African Republic

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Traditionally, gorillas were classified as folivores, yet 15 years of data on western lowland gorillas (Gorilla gorilla gorilla) show their diet to contain large quantities of foliage and fruit, and to vary both seasonally and annually. The consumption of fruit by gorillas at Bai Hokou, Central African Republic, is correlated with rainfall and ripe fruit availability (Remis, 1997a). We investigated the nutritional and chemical content of gorilla foods consumed at Bai Hokou during two seasons of fruit scarcity as measured by phenological observations and compared our findings with the nutrient content of gorilla foods at other African sites. We conclude that during lean times, Bai Hokou gorillas consumed fruits with higher levels of fiber and secondary compounds than those of other populations of western lowland or mountain gorillas. Conversely, leaves consumed by Bai Hokou gorillas were relatively low in fiber and tannins. Bai Hokou gorillas appeared to meet their nutritional needs by eating a combination of fruit and foliage. They ate fruits comparatively high in secondary compounds and fiber when necessary. While gorillas are selective feeders, wherever and whenever preferred foods are scarce, their large body size and digestive anatomy enable them to consume and process a broader repertoire of foods than smaller bodied-apes.
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International Journal of Primatology [ijop] PP104-298611 October 1, 2001 10:23 Style file version Nov. 19th, 1999
International Journal of Primatology, Vol. 22, No. 5, 2001
Nutritional Aspects of Western Lowland Gorilla
(Gorilla gorilla gorilla) Diet during Seasons
of Fruit Scarcity at Bai Hokou, Central
African Republic
M. J. Remis,
1,5
E. S. Dierenfeld,
2
C. B. Mowry,
3
and R. W. Carroll
4
Received May 4, 2000; revision September 1, 2000; accepted October 1, 2000
Traditionally, gorillas were classified as folivores, yet 15 years of data on west-
ern lowland gorillas (Gorilla gorilla gorilla) show their diet to contain large
quantities of foliage and fruit, and to vary both seasonally and annually. The
consumption of fruit by gorillas at Bai Hokou, Central African Republic,
is correlated with rainfall and ripe fruit availability (Remis, 1997a). We in-
vestigated the nutritional and chemical content of gorilla foods consumed at
Bai Hokou during two seasons of fruit scarcity as measured by phenological
observations and compared our findings with the nutrient content of gorilla
foods at other African sites. We conclude that during lean times, Bai Hokou
gorillas consumed fruits with higher levels of fiber and secondary compounds
than those of other populations of western lowland or mountain gorillas. Con-
versely, leaves consumed by Bai Hokou gorillas were relatively low in fiber
and tannins. Bai Hokou gorillas appeared to meet their nutritional needs by
eating a combination of fruit and foliage. They ate fruits comparatively high in
secondary compounds and fiber when necessary. While gorillas are selective
feeders, wherever and whenever preferred foods are scarce, their large body
1
Department of Sociology/Anthropology, Purdue University, West Lafayette, IN.
2
Department of Wildlife Nutrition, Wildlife Conservation Society, New York.
3
Department of Biology, Berry College, Mt. Berry, Georgia.
4
World Wildlife Fund-US, Washington D.C. 20037.
5
All correspondence should be addressed to Dr. Melissa Remis, Department of Sociology
and Anthropology, Purdue University, West Lafayette, IN 47907-1365; Fax: 765-496-1476,
e-mail: remism@sri.soc.purdue.edu.
807
0164-0291/01/1000-0807$19.50/0
C
°
2001 Plenum Publishing Corporation
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808 Remis, Dierenfeld, Mowry, and Carroll
size and digestive anatomy enable them to consume and process a broader
repertoire of foods than smaller bodied-apes.
KEY WORDS: fiber; phenolics; feeding ecology; African apes.
INTRODUCTION
Gorillas are the largest living primates and possess colons that contain
a large number of cellulose digesting ciliates (Collet et al., 1984). Otherwise,
gorillas have a generalized ape morphology that seems adapted for frugivory
(Kay, 1975; Chivers and Hladik, 1980; Martin et al., 1985). Gorillas have tra-
ditionally been viewed as terrestrial folivores, largely a consequence of their
size (Schaller, 1963; Fossey and Harcourt, 1977) and theoretical considera-
tions of the effects of body size on ecology (Schoener, 1971; Demment and
van Soest, 1985; Perrin, 1994). They appear to be able to retain low qual-
ity foods in the gut in order to maximize absorption of nutrients (Remis,
2000). Our perceptions of gorillas have been reinforced by study of the
terrestrial mountain gorilla, whose diet is herbivorous, not very diverse or
seasonally variable (Watts, 1996), and whose teeth are adapted for shearing
leaves (Groves, 1986; Uchida, 1998). A growing body of field data on western
lowland gorillas, however, challenges the notion that gorillas are folivores
(Tutin et al., 1991; Remis, 1994, 1997a, b). More than 15 years of fieldwork on
western lowland gorillas suggests that diets vary seasonally but their dietary
preferences are similar to those of sympatric frugivorous chimpanzees.
Western lowland gorillas consume ripe fleshy fruits whenever available
(Williamson et al., 1990). In these periods, their foraging strategies (Tutin
et al., 1993a), ranging behavior and grouping patterns may converge with
those of frugivorous chimpanzees (Remis, 1997b; Goldsmith, 1996). During
months of fleshy fruit availability, western lowland gorilla diet consists of
50% and 90% fruit (Tutin et al., 1997; Remis, 1999). Despite the impor-
tance of fruit in their diet, diet shifts seasonally at all lowland gorilla sites.
Consequently, they might be best referred to as seasonal frugivores (Tutin
et al., 1991; Remis, 1997a). During periods of fleshy fruit scarcity, fibrous
fruits become staple fallback foods and consumption of herbs, leaves and
bark increases (Rogers et al., 1990). Leaf flush frequently occurs during non-
fruiting seasons, and gorillas take advantage of high quality young leaves
(Tutin and Fernandez, 1993b).
Variation in dietary selectivity and food preferences among primates
likely relates to niche separation (Ganzhorn, 1989; Wrangham et al., 1998),
which may reflect differences in digestive strategies for metabolizing nutri-
ents and secondary compounds in plants (Freeland and Janzen, 1974; Rhodes
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Nutritional Aspects of Western Lowland Gorilla Diet 809
and Cates, 1976; Glander, 1981). Folivorous primates often consume foods
that are relatively high in digestion inhibitors, such as lignin and carbon-
based secondary plant compounds, e.g., tannins, and also relatively higher
in protein (Hladik, 1978; Waterman et al., 1983; Waterman and Kool, 1994;
Mowry et al., 1996) compared with foods not eaten.
Gorillas lack the morphological specializations associated with forest-
omach fermentation and microbial adaptations to tannins (Oates et al., 1977;
Kay and Davies, 1994). However, they have an enlarged hindgut (Chivers
and Hladik, 1980), associated with colocecal fermentation (Bauchop, 1978;
Parra, 1978; Milton and Demment, 1988). Accordingly, relative to other
hominoids, they are probably well equipped to digest fiber and may tolerate
tannin-rich foods (Rogers et al., 1992; Cork and Foley, 1992; Simmen et al.,
1999).
Despite these features, the dietary choices of gorillas may be more
similar to those of other ape frugivores than of folivores. Primate frugi-
vores often choose mature or ripe fruits over immature or unripe fruits
or other plant parts (Gautier-Hion et al., 1985); apes exemplify this trend
(Wrangham et al., 1998). Frugivores typically act as seed dispersers rather
than seed destroyers (Garber and Lambert, 1998). Unripe fruits may be
avoided if they have low energy or protein content or higher concentrations
of secondary compounds than ripe fruits (Hulme, 1971; McKey, 1979; Janzen,
1983). Many plant species have considerable quantities of condensed tannins
in unripe fruit flesh (Swain, 1979), which cause astringency and may inter-
fere with digestion by reducing protein availability in the gut (cf. Mole and
Waterman, 1987; Waterman and Mole, 1994). During ripening, fruits usu-
ally undergo color change, tannins lose their astringency and palatability
increases (Barnell and Barnell, 1945; Harborne, 1988, 1991).
Gorillas are selective eaters, despite their large size and ability to con-
sume a tough, low quality diet (Strait, 1997). They often discard the outer
bark of vines, stem layer of herbs and proximal, distal and midrib sections
of leaves (Casimir, 1975). Mountain gorillas are primarily herbivorous and
choose vegetation that is generally higher in protein content and lower in
acid detergent fiber and condensed tannins compared with lowland gorilla
foods (Goodall, 1977; Waterman et al., 1983; Rogers et al., 1990; Plumptre,
1995; Popovich et al., 1997). Chemical and nutrient analysis of the diet of
the more frugivorous western lowland gorillas at Lope
0
shows that many
fleshy fruits consumed by gorillas are fructose rich, low in crude protein and
fat. Some fruits, especially those species used in times of scarcity by both
chimpanzees and gorillas, have been described as high in fiber and phenolics
(Rogers et al., 1990; Wrangham et al., 1991; Popovich et al., 1997). Rela-
tive consumption of tannins is variable; it is not yet clear whether gorillas
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810 Remis, Dierenfeld, Mowry, and Carroll
consume more tannins than chimpanzees do (Rogers et al., 1990; Tutin and
Fernandez, 1993a; Wrangham et al. 1998,). To date, we lack an adequate
picture of the variability of ape diet between and within sites and species.
The nutritional and chemical analyses of primate foods permit a better
understanding of species-specific ecological niches and provide a means to
compare closely related taxa. We investigated the chemical content of foods
eaten by western lowland gorillas at Bai Hokou, Central African Republic.
Analysis of diet during scarcity of preferred fruits permits us to examine how
the anatomical specializations of gorillas may impact dietary selectivity and
distinguish them from other species. Our specific objectives were 1) chemi-
cally to analyze foods eaten by Bai Hokou gorillas during seasons of scarcity,
2) to determine the influence of plant chemistry on Bai Hokou gorilla feed-
ing ecology during scarcity, and 3) to compare the seasonal nutrient and
other phytochemical profile of the Bai Hokou gorilla diet to the averages
from other gorilla populations. The analyses do not yet represent a com-
plete picture of Bai Hokou gorilla diet, but they will complement existing
data to facilitate a better understanding of the seasonal dietary variability
and flexibility reported for gorillas across their range.
METHODS
Field Data
Ecological monitoring and studies of gorilla foraging ecology have been
conducted at Bai Hokou, Central African Republic since 1984 by Remis
(1994, 1997a), Goldsmith (1996) and Carroll (1997). Rainfall averages
1365 mm annually, with a single 3-mo dry season, generally December-
March each year. The heaviest rainfall occurs in September-October, usu-
ally a period of high fruit availability, with ripe fruit most abundant August-
October.
We combined the nutritional analysis of gorilla plant foods collected
on gorilla feeding trails and during observations at Bai Hokou by Carroll
during late-dry season, March 1989 (leaves and fibrous fruits), with those
collected by Remis on a wider variety of fruits consumed by the same gorilla
groups during the fruiting or wet season June-August, 1998. This combined
data set permits analysis of temporal variations in dietary patterns.
Basic monthly phenological patterns (presence or absence of ripe, un-
ripe or fallen fruit, and new and mature leaves on trees >10 cm diameter
at breast height) of 973 marked trees of 152 species along 19 km of cut
north-south transects have been recorded at Bai Hokou when researchers
were present (Remis, 1997a). The phenological data represent monthly
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Nutritional Aspects of Western Lowland Gorilla Diet 811
community-wide fruiting patterns rather than the availability of specific
gorilla foods. The data represent the following months: August 1988–
May 1989; March 1990–October 1992, August 1993–January 1995, June–
December 1995, August 1997, and August 1998. During August 1998, we
noted the phenological patterns of only 720 of the 973 marked trees.
We have collected information on gorilla diet of unhabituated and semi-
habituated study groups. As at other gorilla study sites, we used a combi-
nation of direct observations (1685 1-min interval samples of undisturbed
focal subjects feeding), and indirect methods including trail and macroscopic
fecal analysis (Remis, 1994, 1997a; Carroll, 1997). In addition, we recorded
feeding visits once per animal per feeding patch (usually a tree) on a par-
ticular date (n = 155). Observational dietary data at Bai Hokou are biased
in favor of the 9-mo wet seasons (n = 143 feeding visits; 1527 feeding min),
with less data collected during the 3-mo dry seasons (n = 12 feeding visits;
158 feeding min). During research at Bai Hokou, we collected fecal samples
opportunistically and examine them macroscopically to determine the num-
ber of fruit species eaten. During fecal analysis, we combined the relative
abundance score of monocot herbs (0–4 scale) and other foliage (leaf and
bark; 0–4 scale) in each fecal sample to yield a total foliage score (0–8 scale)
(Tutin and Fernandez, 1993c; Remis, 1997a). It was not possible to accurately
detail the relative proportions of plant species eaten in gorilla diet by either
observations or fecal analysis, but we collected samples of all fruits observed
or known to be eaten by the gorillas.
During 1989 and 1998, we collected foods partially consumed and dis-
carded by gorillas during observations or on feeding trails in association
with gorilla knuckle-prints, nests or dung. We dried leaves and stored them
in plastic containers with desiccant and fixed fruit samples in ethanol. In
addition, we manually collected representative food samples from 3–5 in-
dividual plants per species. We attempted to obtain 50–100 g (wet weight)
of material for subsequent chemical assays. We determined water content
in the field by weighing samples and air-dried leaves in an open-air kitchen
until constant weight was achieved. We weighed fruit samples to the nearest
0.1 g and stored them in 70% ethanol for transport. During the 1998 data
collection, we also assessed ripeness of fruits eaten, avoided and discarded
during observations and along feeding trails, based on color, odor and soft-
ness. Seed-eating plays a very minor role in gorilla fruit consumption; Bai
Hokou gorillas consume few seeds that do not emerge intact in their feces.
We characterized fruits as fleshy if they had a juicy (succulent) pulp or dry if
they had a dry fibrous fruit pulp or dehiscent pod. We further defined impor-
tant fruits as those most prevalent on a yearly basis in gorilla fecal samples
(> 1% of fecal samples), on feeding trails and during observation over >5
years of field research (Remis, 1999).
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812 Remis, Dierenfeld, Mowry, and Carroll
Identification of Bai Hokou gorilla plant foods was conducted by
Dr. David Harris, Royal Botanical Gardens, Edinburgh Scotland and Dr.
J. Michael Fay, Wildlife Conservation Society, New York, USA. Plant parts
that we collected in 1989 and 1998 include ripe and unripe fruits, seeds,
leaves, stems and vines. We analyzed the 1998 samples for chemical com-
position and other phytochemicals shortly following collection. Dierenfeld
had analyzed samples collected by Carroll in 1989 (Dierenfeld unpublished
data; Popovich et al., 1997), but we further analyzed them for phenolics and
alkaloids.
Laboratory Data Analyses
Before nutrient analysis, we evaporated ethanol preservative at room
temperature under a laboratory fume hood and collected all residue upon
drying to a constant weight. We then ground samples using a laboratory mill,
and kept them at room temperature until analysis. We calculated crude pro-
tein (CP) content as total nitrogen X 6.25 using a macro-Kjeldahl method
with a Cu catalyst (AOAC, 1996). We conducted analyses of plant cell wall
constituents—(neutral detergent fiber (NDF), acid detergent fiber (ADF),
and sulfuric acid lignin (Ls))—on all plant samples via the methods of Van
Soest et al. (1991). We evaluated water soluble carbohydrates, predomi-
nantly simple sugars (SS), in fruits via a phenol/sulfuric acid colorimetric
assay (DuBois et al., 1956) as modified by Strickland and Parsons (1972).
We assayed macromineral concentrations (Ca, K, Mg, Na, P) and trace ele-
ments (Cu, Cr, Fe, Mn, Zn) by inductively-coupled plasma argon emission
spectroscopy (Stahr, 1991).
We prepared fruit pulp and seed samples for secondary compound anal-
ysis by evaporating the ethanol preservative and drying fruits in an exhaust
oven at 30
C, and ground samples with any corresponding residue to a
powder (20-mesh screen) in a Wiley mill. Leaf samples had been previously
collected, dried and milled (Popovich et al., 1997). Although some loss of
extractable phenolics may have occurred since leaf collection, the tannin
analysis method is appropriate for comparative purposes (Waterman and
Mole, 1994). We prepared extractions for hydrolyzable and condensed tan-
nin assays using 50% methanol (Bate-Smith, 1977, 1981; Hagerman and
Butler, 1991). We measured hydrolyzable tannins (HT–ellagitannins) under
an N
2
environment using an acetic acid-sodium nitrate procedure (Bate-
Smith, 1972, 1977). We estimated condensed tannins (CT) as proantho-
cyanidins using a butanol-HCl technique (Bate-Smith, 1975, 1981). Our
procedures followed Shure and Wilson (1993) and Dudt and Shure (1994).
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Nutritional Aspects of Western Lowland Gorilla Diet 813
We used tannic acid and quebracho tannin as standards for hydrolyzable
and condensed tannins, respectively (Waterman and Mole, 1994). Hydrolyz-
able tannins are expressed as mg per dry weight of hexahydroxydiphenyl-
glucose equivalents (mg/g HHDP) and condensed tannins as percentage dry
mass quebracho tannin equivalents (%QTE). Condensed tannin values are
not expressed as percent dry weight of a given sample. Some condensed tan-
nin values actually exceed 100%, indicating the particular sample is more
reactive than an equal amount of quebracho tannin. We measured protein
precipitation via a modified version of the radial diffusion (RD) method
for determining tannin in plant extracts (Hagerman, 1987). Modifications
included using a smaller concentration of BSA protein and staining the
agarose gels with Prussian blue reagent. We measured the stained gels on a
Macintosh computer using the public domain NIH Image program (deve-
loped at the U.S. National Institutes of Health and available on the Internet
at http://rsb.info.nih.gov/nih-image/). Standards and units for the RD assay
are the same as those for the CT assay. A significant positive correlation
exists between the CT and RD assays (R = 0.685, p < 0.0001).
We determined the presence or absence of alkaloids in plant samples
using Iodoplatinate and Dragendorffs reagents. We extracted 100 mg of
milled sample in 10 ml of 95% ethanol at room temperature for 24–36 hours.
We then dried the extract completely, and added equal parts water and
petroleum ether. After two distinct layers formed, we spotted several drops
of the bottom (aqueous) layer on two pieces of filter paper. We then sprayed
one filter paper with Dragendorffs reagent and the other with Iodoplatinate
reagent. We recorded the degree of color change (0–3) for each reagent spray,
indicating the presence and relative amount of alkaloid in each sample, and
we generated a single value from the mean.
Statistical Analysis
We used Mann-Whitney U tests to examine nutrient and other phy-
tochemical differences between medians for each of the following: a) food
categories (fruits, leaves), b) ripe versus unripe fruit, c) fruits collected in dif-
ferent years (1989 versus 1998), d) dry versus fleshy fruits, and e) important
versus less important fruits. We generated Pearson product-moment corre-
lations between each of the nutrient and other phytochemical measurements
for all categories of food. We then investigated the levels of significance for
pairs of variables that appeared to be correlated by running a regression
analysis (Rosner, 1990). We used DataDesk for Macintosh for all statistical
analysis (Data Description, 1997).
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814 Remis, Dierenfeld, Mowry, and Carroll
RESULTS
Food Availability
Long-term community-wide phenological data reveal two major sea-
sons of high and low ripe fruit availability correlated with seasonal patterns
of rainfall at Bai Hokou (Remis, 1997a; Goldsmith, 1996; Carroll, 1997).
Leaf flush usually occurs at the end of the dry season. The data reveal con-
siderable yearly variation and the occurrence of good and poor fruit years as
well as seasons. Nevertheless, the numbers of species that contain ripe fruit
generally peaks during the rainy months of July and August. During the dry
season, unripe and fibrous fruits are present, but ripe fleshy fruit is scarce.
During 1988–89, however, fewer species on phenology trails produced fruit
than in subsequent good fruit months or years (Fig. 1).
Data collection in 1998 also occurred during a poor fruiting season,
relative to other years. In August 1998, only 7% of marked species contained
fruit, with only 4% having ripe fruit (n = 720). [In August of other years:
1988, 6%; 1990, 37%; 1991, 21%; 1992, 36%; 1994, 30%; 1995, 35% and
1997, 72% of species bore fruit] (Fig. 1; Remis, 1997a, 1999). While we
were not able to quantify total monthly rainfall during 1997 and 1998, the
1998 dry season was unusually long, the rains and fruiting season markedly
late, and fruit productivity poor compared with previous years (Chiopelleta
and Kpanou unpub). Moreover, leaf flush typically occurs in the dry season,
but in 1998 it occurred in August, when 25% of trees bore young leaves. Thus,
overall food availability during the study period resembled late dry season
months more than previous peak fruiting seasons. Accordingly, in both 1989
and 1998, we had collected gorilla plant food samples during seasonal or
interannual periods of low fruit availability. Figure 1 shows the monthly
patterns of variation in fruit availability and rainfall over the 10-year period,
June, 1988–August, 1998, with June of each year marked on the x-axis for
reference.
Gorilla Diet at Bai Hokou
Proportions of fruit, herbs, leaves and bark in western lowland gorilla
diet vary in response to rainfall and ripe fruit availability (Remis 1997a; Tutin
et al., 1997). When fruit abundance is high at Bai Hokou, consumption of
foliage declines, but during periods of low rainfall, when fruits are scarce,
consumption of fleshy fruits is reduced. Figure 2 shows seasonal variation
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Nutritional Aspects of Western Lowland Gorilla Diet 815
Fig. 1. Phenological patterns of fruit availability at Bai Hokou, 1988–1998. Fruit availability was
measured monthly on marked trees at Bai Hokou by one of two observers, Jean Bosco Kpanou
or Etienne Ndolongbe from August 1988–May 1989 (Carroll, 1997), March 1990–January 1991,
March 1991–October 1992 (Remis, 1994), October 1993–January 1995 (Goldsmith, 1996), June
1995–December 1995, August 1997, August 1998 (Remis, 1999 and unpublished). Data are
reported here as number of tree species containing ripe fruit in each month (n = 152 species,
total of 973 individuals). In August 1998 only 148 species, 720 trees were monitored.
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816 Remis, Dierenfeld, Mowry, and Carroll
Fig. 2. Gorilla feeding observations by Remis at Bai Hokou (combined data
from 4 wet seasons, 2 dry seasons (1685 one-minute observations, 155 feeding
visits, 1990–1995).
in the consumption of fruit and foliage by Bai Hokou gorillas during ob-
servations by Remis, averaged over 4 wet seasons (143 feeding visits) and 2
dry seasons (12 feeding visits). The data were collected during focal-animal
samples on undisturbed gorillas feeding at Bai Hokou (n = 1685 1-minute
intervals). A small sample of gorilla fecal samples (n = 9) collected during
the 1998 study period showed that while gorillas ate fruit (mean number
of fruit species per fecal sample is 3), foliage made up a greater proportion
of fecal samples (mean foliage abundance score = 5.9, on a scale of 0–8),
than it had in 4 previous good fruit wet seasons (foliage scores averaged 3.7,
n = 905 samples). In previous dry season fecal samples, foliage scores av-
eraged 5.9 (n = 199 samples) (Remis, 1997a; Remis, 1999). More extensive
fecal analysis and trail data from other field seasons support observations
and indications of dietary switching from fruit to leaves during periods of
low ripe fruit availability (Remis, 1997a; Goldsmith, 1996; Carroll, 1997).
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Nutritional Aspects of Western Lowland Gorilla Diet 817
Nutritional and Phytochemical Content of Gorilla Foods
We analyzed a total of 68 plant samples for the presence of nutri-
ents and other phytochemicals. At least 20 different plant families and
35 species are represented, though some samples are currently known only
by their local BaAka names. Overall, Bai Hokou gorilla foods analyzed
from these periods of scarcity are relatively high in mean crude protein
(12.68%), fiber (mean NDF = 61.63%, mean ADF = 46.55%), and lignin
(mean Ls = 24.4%). Compared to foods from other gorilla study sites (see
Table IV), Bai Hokou foods are also moderately high (all values listed as
% dry weight) in soluble sugars (12.31% dry weight), and tannin: mean
HT = 3.13 mg/g HHDP, mean CT = 7.58%QTE, mean RD = 2.11% QTE
(Appendix A). Minerals were present at the following unweighted mean
levels (all values reported on a dry matter basis): Ca = 0.18%, K = 1.21%,
Mg = 0.14%, Na = 0.01%, P = 0.13%, Cu = 11.12 mg/kg, Cr = 0.85 mg/kg,
Fe = 79.34 mg/kg, Mn = 135.17 mg/kg, Zn = 18.40 mg/kg (Appendix B). Bai
Hokou fruits are lower in sugars and higher in fiber than one might expect
during a good fruit season (Rogers et al., 1990). Nevertheless, the negative
correlation between fiber and SS suggests that the gorillas were choosing
relatively sweet, low fiber fruit (NDF/SS, R =−0.761, p = 0.0006; ADF/SS,
R =−0.623, p = 0.01). Crude protein in leaves, but not fruits, is negatively
correlated with fiber (for foliage, CP/NDF R =−0.406, p = 0.05; CP/ADF
R =−0.516, p = 0.005). Table I shows Pearson correlation values for fruit
and foliage.
Secondary plant compounds and individual mineral constituents are
not consistently correlated with any other nutrient component of the foods
evaluated. Fruits used as food resources are low in Ca, Mg, P, Na and Fe com-
pared to nutrient recommendations for nonhuman primates (NRC, 1978).
While the Ca:P ratio is within suggested ranges (1:1 to 2:1), none of the fruits
contained adequate Ca (0.55%). Even selective feeding could not meet re-
quirements for Ca. Native fruits, similar to domestic fruits, are high in potas-
sium compared with all other macrominerals.
Nutritional Comparisons between Plant Parts
Foliage vs. Fruits
Foliage—(herbs and leaves of woody species, including vines)—is higher
than fruits in ash and crude protein (p < 0.0001 for both ash and CP) (Tables
II and III). There is no significant difference in the ash (p = 0.8579) or crude
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Table I. Pearson correlation statistics for nutrient and other phytochemical assays of fruit and foliage
All Foilage
HT CT RD Ash NDF ADF HC Ls CP
HT 1
CT 0.698
∗∗∗
1
RD 0.582
∗∗
0.854
∗∗∗
1
Ash 0.025 0.26 0.332 1
NDF 0.205 0.053 0.036 0.134 1
ADF 0.159 0.164 0.142 0.29 0.95
∗∗∗∗
1
HC 0.079 0.372 0.344 0.541
∗∗
0.24 0.53
∗∗
1
Ls 0.146 0.284 0.315 0.425 0.724
∗∗∗
0.853
∗∗∗
0.688
∗∗
1
CP 0.228 0.206 0.204 0.681
∗∗∗
0.406
0.516
∗∗
0.504
0.402 1
All Fruit
HT CT RD Ash NDF ADF HC Ls CP SS Ca K Mg Na P Cu Cr Fe Mn Zn
HT 1
CT 0.199 1
RD 0.331 0.814
∗∗∗
1
Ash 0.255 0.403
0.19 1
NDF 0.025 0.251 0.551
∗∗
0.078 1
ADF 0.015 0.179 0.417 0.121 0.911
∗∗∗
1
HC 0.027 0.179 0.255 0.472
∗∗
0.124 0.295 1
Ls 0.225 0.043 0.244 0.173 0.674
∗∗∗
0.832
∗∗∗
0.441
1
CP 0.108 0.039 0.067 0.335 0.264 0.238 0.039 0.156 1
SS 0.047 0.171 0.437 0.022 0.761
∗∗
0.623
∗∗
0.122 0.555
0.358 1
Ca 0.394 0.118 0.225 0.044 0.153 0.214 0.116 0.066 0.021 0.222 1
K 0.146 0.251 0.035 0.713
∗∗∗
0.001 0.166 0.295 0.163 0.329 0.481 0.226 1
Mg 0.241 0.513
0.264 0.687
∗∗∗
0.192 0.046 0.252 0.022 0.179 0.01 0.442 0.685
∗∗
1
Na 0.253 0.051 0.066 0.303 0.133 0.271 0.252 0.484
0.286 0.351 0.2 0.392 0.22 1
P 0.47
0.559
0.497
0.294 0.009 0.04 0.055 0.008 0.493
0.045 0.048 0.3 0.543
∗∗
0.226 1
Cu 0.166 0.556
0.182 0.494
0.008 0.01 0.033 0.25 0.365 0.166 0.128 0.392 0.359 0.13 0.395 1
Cr 0.021 0.293 0.219 0.157 0.305 0.161 0.369 0.009 0.253 0.77 0.431 0.347 0.255 0.212 0.117 0.071 1
Fe 0.099 0.103 0.237 0.082 0.179 0.161 0.024 0.053 0.247 0.318 0.186 0.215 0.056 0.25 0.033 0.109 0.059 1
Mn 0.108 0.42 0.616
∗∗
0.031 0.322 0.162 0.271 0.066 0.191 0.133 0.419 0.12 0.154 0.134 0.181 0.036 0.644 0.263 1
Zn 0.396 0.372 0.266 0.41 0.058 0.03 0.046 0.119 0.203 0.201 0.017 0.266 0.391 0.157 0.41 0.095 0.383 0.327 0.244 1
Abbreviations as in Appendix I and II.
*p = 0.05.
**p = 0.01.
***p = 0.001.
**p = 0.0001.
818
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Nutritional Aspects of Western Lowland Gorilla Diet 819
Table II. Descriptive statistics on the composition of plant parts consumed at
Bai Hokou, Central African Republic
All fruit N Mean Range Unripe fruit N Mean Range
HT 31 3.91 0.24–24.12 HT 7 2.78 0.24–5.79
CT 31 11.96 0.01–95.74 CT 7 22.62 0.01–95.74
RD 27 3.70 0.00–12.17 RD 8 3.96 0.00–9.48
Ash 30 3.82 1.82–7.58 Ash 8 3.28 1.82–6.80
NDF 30 59.27 19.84–94.16 NDF 8 55.82 46.15–68.39
ADF 30 45.30 10.94–83.83 ADF 8 40.52 10.94–56.32
HC 30 13.97 2.84–36.11 HC 8 15.30 2.84–36.11
Ls 30 24.18 3.00–49.18 Ls 8 25.00 3.64–41.92
CP 30 8.72 3.05–15.62 CP 8 9.25 7.11–11.15
SS 16 12.73 1.43–48.92 SS 6 8.80 3.93–13.93
Ca 22 0.18 0.01–0.38 Ca 7 0.18 0.10–0.28
K 22 1.21 0.04–2.97 K 7 1.07 0.44–1.89
Mg 22 0.14 0.02–0.25 Mg 7 0.13 0.02–0.25
Na 22 0.01 0.00–0.02 Na 7 0.01 0.00–0.02
P 22 0.12 0.07–0.18 P 7 0.12 0.07–0.15
Cu 22 11.20 1.74–26.07 Cu 7 8.82 1.74–26.07
Cr 8 0.90 0.25–1.48 Cr 3 0.84 0.52–1.13
Fe 22 80.47 29.33–167.84 Fe 7 90.06 30.98–129.80
Mn 22 135.18 19.54–332.07 Mn 7 118.27 60.15–207.91
Zn 22 18.40 8.44–40.70 Zn 7 19.40 8.65–40.70
Ripe fruit N Mean Range All foliage N Mean Range
HT 24 4.24 0.44–24.12 HT 23 2.29 0.30–6.79
CT 24 8.84 0.17–49.26 CT 23 3.13 0.11–30.71
RD 19 3.59 0.00–12.17 RD 24 2.83 0.00–34.11
Ash 22 4.01 2.44–7.58 Ash 24 7.50 2.66–14.47
NDF 22 60.52 19.84–94.16 NDF 24 66.25 42.17–83.02
ADF 22 47.04 12.79–83.83 ADF 24 49.49 25.81–70.01
HC 22 13.48 3.65–31.08 HC 24 16.76 9.99–24.46
Ls 22 23.88 3.00–49.18 Ls 24 25.44 6.75–49.84
CP 22 8.53 3.05–15.62 CP 24 17.54 8.11–32.90
SS 11 15.09 1.43–48.92
Ca 15 0.18 0.01–0.38 Leaves N Mean Range
K 15 1.28 0.04–2.97
Mg 15 0.14 0.07–0.24 HT 15 2.93 0.30–6.79
Na 15 0.01 0.00–0.02 CT 15 4.52 0.13–30.71
P 15 0.12 0.07–0.18 RD 16 3.28 0.00–34.11
Cu 15 12.32 5.39–19.23 Ash 16 7.70 2.66–14.47
Cr 5 0.93 0.25–1.48 NDF 16 63.93 42.17–80.63
Fe 15 76.00 29.33–167.84 ADF 16 47.54 25.81–69.53
Mn 15 143.06 19.54–332.07 HC 16 16.40 10.17–24.46
Zn 15 17.94 8.44–30.41 Ls 16 25.19 6.75–49.84
CP 16 18.86 12.30–32.90
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820 Remis, Dierenfeld, Mowry, and Carroll
Table II. (Continued )
Stems & vine N Mean Range Seeds N Mean Range
HT 8 1.09 0.66–2.43
CT 8 0.53 0.11–1.77 HT 6 2.32 0.05–7.62
RD 8 0.00 0–0 CT 6 2.06 0.12–11.05
Ash 8 7.10 5.42–8.83 RD 6 0.00 0.00–0.00
NDF 8 70.90 53.61–83.02
ADF 8 53.40 37.57–70.01
HC 8 17.49 9.99–22.61
Ls 8 25.95 13.93–43.98
CP 8 14.91 8.11–19.70
Stems (herb) N Mean Range
HT 6 0.93 0.66–1.32
CT 6 0.59 0.11–1.77
RD 6 0.00 0.00–0.00
Ash 6 7.21 5.68–8.83
NDF 6 67.41 53.61–77.05
ADF 6 49.22 37.57–67.06
HC 6 18.19 9.99–22.61
Ls 6 25.98 13.93–43.98
CP 6 16.87 11.20–19.70
Abbreviations and units as in Appendix A.
protein (p = 0.9412) content of herbaceous stems and woody leaves. Con-
versely, fruits contained more soluble sugars (p < 0.0001) and condensed
tannins (p = 0.0039), and a higher protein precipitating ability (as measured
by the RD assay, p = 0.0046) than in all foliage samples. Fruits and foliage
had similar amounts of NDF (p = 0.104) and ADF (p = 0.3638). As ex-
pected, most tannin in the foliage of Bai Hokou gorilla diet comes from tree
leaves instead of herbaceous stems; leaves are higher than stem/vines in HT
(p = 0.0131) and CT (p = 0.0489). In fact, there was no measurable amount
of tannin in any of the stem/vine samples based on the RD assay. Moreover,
none of the foliage samples tested positive for alkaloids (Appendix A).
Although fruits are commonly referred to as high quality foods, Bai
Hokou fruits are much more variable interspecifically in putative antifee-
dants—alkaloids, fiber and tannins—than either herbs or leaves are
(Appendix A). There is no evidence that tannins deter gorillas from eat-
ing fruit. Further, the fact that gorillas pass undigested seeds through their
guts, apparently is not a result of the presence of antifeedants. The few seeds
samples are lower in condensed tannins (p = 0.0219) and protein precipi-
tating ability (p = 0.039) than fruit pulp, (Tables II and III). In tests for the
presence of alkaloids, only two fruits, Tabernatum sp. and Ficus sp., gave
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Nutritional Aspects of Western Lowland Gorilla Diet 821
Table III. Differences in the composition of plant parts consumed by gorillas at Bai Hokou
Ripe vs. Fruit(98) vs. Imp. fruit vs. Fleshy vs.
unripe fruit fruit(89) other fruit dry fruit
Assay z-stat p-value z-stat p-value z-stat p-value z-stat p-value
HT 0.18902 0.8501 0.70014 0.4838 0.04129 0.4835 1.3525 0.1762
CT 0.6607 0.2909 0.06 0.7555
RD 0.77249 0.4398 2.4511 0.0142 1.5797 0.0571 0.28611 0.7748
Ash 1.7591 0.0786 0.67417 0.5002 0.7144 1.2752 0.2023
NDF 0.7304 <0.0001 0.3998 0.0426
ADF 0.7304 <0.0001 0.3056 0.0108
HC 0.6293 0.4698 0.9326 0.4809
Ls 0.9086 0.0076 0.2003 0.2261
CP 0.96151 0.3363 2.5667 0.0103 1.9799 0.0239 2.0106 0.0444
SS 0.2463 1.8998 0.0207 0.0139 ND ND
Foliage vs. Fruit vs. Leaves vs.
fruit seeds stems
Assay z-stat p-value z-stat p-value z-stat p-value
HT 1.0147 0.3103 0.94781 0.3432 0.0042
CT 2.8777 0.0039 0.0219 1.5186 0.0644
RD 2.8357 0.0046 2.0641 0.039 0.88641 0.1877
Ash 5.1269 <0.0001 ND ND 0.8579
NDF 0.104 ND ND 0.5414
ADF 0.3638 ND ND 0.9714
HC 0.0128 ND ND 0.2952
Ls 0.7758 ND ND 0.8579
CP 5.5011 <0.0001 ND ND 0.07374 0.9412
SS ND ND ND ND ND ND
Abbreviations as in Appendix A and B.
If no ties are present in Mann-Whitney U test, a z-statistic is not calculated.
strong positive results. Most of the tannins in the leaf portion of Bai Hokou
gorilla diet are present in only two tree species, Diospyros sp. and Egumdu.
Further, although mature leaves are commonly characterized as high in both
tannin and fiber, compared to fruit, fiber values are not significantly different
between the foliage and fruit samples.
Ripe vs. Unripe Fruit
Crude protein and fiber fraction (NDF, ADF) and lignin (Ls) values are
not significantly different between ripe and unripe fruit (Table III). Unripe
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822 Remis, Dierenfeld, Mowry, and Carroll
fruits have slightly higher mean condensed tannin concentrations and pro-
tein precipitating ability (RD) than those of ripe fruits, while ripe fruits are
generally higher in soluble sugars and ash. However, none of these trends is
significant (Tables II and III).
Fleshy vs. Dry Fruit
Dry fruits had dehiscent pods or fibrous, rather than fleshy, pulp. They
are higher in fiber (mean Dry NDF = 70.5%, mean Fleshy NDF = 58.22%,
p = 0.0426 and mean Dry ADF = 55.85%, mean Fleshy ADF = 49.32%,
p = 0.0108) and lower in crude protein content than fleshy fruits (mean Dry
CP = 6.50%, mean Fleshy CP = 9.39%, p = 0.0444). Tannin values are not
significantly different between dry and fleshy fruits. There were not enough
samples analyzed for SS content to allow comparison between dry and fleshy
fruits (Table III).
1989 vs. 1998 Fruits
Fruits eaten by gorillas at Bai Hokou in 1998 (n = 29), are higher in
crude protein (1998 mean CP = 9.49%, 1989 CP = 5.81%, p = 0.0103) and
have less fiber (1998 mean ADF = 38.98%, 1989 ADF = 70.59%, and 1998
mean NDF = 53.07%, 1989 NDF = 84.05%, p < 0.0001) and lignin (1998
mean Ls = 21.22%, 1989 Ls = 36.02%, p < 0.01) than the small sample of
fruits (n = 6) collected at the same site in 1989. Nevertheless, despite their
apparently higher nutritional content, 1998 fruits also appear to be higher in
tannins (1998 mean RD = 5.05%QTE, 1989 RD = 4.0%QTE, p = 0.0142)
(Table III).
Important Gorilla Fruits
We analyzed 24 species of fruits from Bai Hokou and divided them
into two separate categories: those most important (prevalent) in the gorilla
diet during 5 years of field observations, versus all others. Important fruits
(IMP) are higher in soluble sugars (mean IMP SS = 23.08%, OTHER SS =
8.03%, p = 0.0139) and lower in crude protein content (mean IMP
CP = 7.24%, OTHER CP = 9.58%, p = 0.0239) than all other fruits. Mean
lignin values are also lower for favored fruits, although not significantly
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Nutritional Aspects of Western Lowland Gorilla Diet 823
(Table III). Fiber values do not appear to vary between these two classes of
fruits. Contrary to expectations, important fruits appear to have higher tan-
nin content than other fruits, although differences do not reach significance
(mean IMP RD = 5.37%QTE, OTHER RD = 4.54%QTE) (Table III).
DISCUSSION
Gorillas are seasonal frugivores or mixed frugivores/folivores; their diet
shifts along a seasonal and interannual gradient at all low altitude sites, with
high variability in dietary proportions of fleshy fruit. Overall, Bai Hokou
gorillas consume 230 plant parts from 129 plant species, including 89 species
of fruits (Remis, 1997a). Because we collected plant samples during poor
fruit seasons and years, our nutritional study does not assess the full varia-
tion present in Bai Hokou gorilla diet. In particular, the fruit samples rep-
resent only 46% of the important fruit species used by Bai Hokou gorillas,
whereas 67% of the important non-fruit foods are represented in the anal-
ysis. Our data represent the nutritional profile of gorillas during seasons of
fruit scarcity at Bai Hokou.
Our results suggest that, during periods of fruit scarcity, the Bai Hokou
gorillas continue to find and to consume fruits in greater numbers than would
be expected from community-wide phenological data, though many of them
are quite fibrous and tannin-rich. The Bai Hokou gorillas appear to choose
fruit for sugar (energy), with fiber and tannin as secondary concerns, which
is similar to chimpanzees in Uganda (Conklin et al., 1998; Reynolds et al.,
1998). Conversely, most of the leaves of woody species consumed by Bai
Hokou gorillas are low in fiber and tannin.
The soluble sugar carbohydrate (SS) fraction is significantly negatively
correlated with all fiber fractions (NDF, ADF, and Ls) in the fruit samples,
whereas crude protein and fiber fractions (NDF, ADF) are significantly and
negatively correlated in foliage samples. Both sugars and soluble proteins
are associated with cell contents, rather than cell wall constituents, so these
inverse relationships are to be expected.
Our data lend further evidence that gorillas and other primates selec-
tively consume differing nutrients in different types of foodstuffs. Never-
theless, our limited results suggest that the gorilla preference for fleshy
fruits may represent an attempt to limit fiber intake. If selection against
high fiber is a consistent finding, concurrent intake of a higher proportion of
SS may then result with frugivory, as opposed to higher protein consumption
with folivory. Accordingly, fruit and foliage are complementary food sources
for gorillas; each provides different sources of energy and other nutrients,
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824 Remis, Dierenfeld, Mowry, and Carroll
including minerals, though they have been studied in less detail. It seems
likely that mixed frugivory/folivory provides the most suitable nutrient bal-
ance for gorillas and many other herbivores.
Comparison to Other Gorilla Study Sites
Our study confirms earlier findings that western lowland gorillas in pri-
mary and old secondary forests have a more diverse diet than those of go-
rillas in montane or more disturbed sites (Waterman et al., 1983; Calvert,
1985; Rogers et al., 1990). In fact, diets at Karisoke are distinctive; lower
altitude mountain gorillas, western and eastern lowland gorilla diets are all
more diverse, and include a variety of fruits and leaves from woody species
(Goldsmith, 1999). Among the subspecies, western lowland gorillas appear
to consume the most fruit and more secondary compounds (Rogers et al.,
1990; Popovich et al., 1997), even when ripe fruit is seasonally scarce. Never-
theless, there is considerable variation in the chemical composition of foods
consumed, even in the relatively intact lowland rain forest habitats at Lop´e
and Bai Hokou.
Throughout Africa, gorillas consume substantial amounts of relatively
high protein herbaceous stems as staples or fallback foods. At Karisoke, how-
ever, the vegetation is montane-adapted, fruit is scarce, and gorilla diet is
particularly herbaceous (Watts, 1996). Herbaceous stems eaten across moun-
tain and lowland sites are similar in nutrient content and low in condensed
tannins, but those eaten at Bai Hokou appear to have more crude protein
than those consumed elsewhere. In general, foods consumed by gorillas dur-
ing seasons of fleshy fruit scarcity at Bai Hokou have more crude protein
and fiber than values reported in analyses of average foods consumed by
gorillas at other sites (Table IV). High fiber content (ADF) distinguishes
the leaf and fruit portion of the diet from the seasonal Bai Hokou data and
foods collected at Campo, Cameroon from averages reported from Lop´e,
Gabon. Although foods consumed by gorillas at all lowland sites have more
condensed tannins than foods eaten at Karisoke, leaves consumed at Bai
Hokou appear lower in tannins than those analyzed from Lop´e or Campo
(Calvert, 1985; Rogers et al., 1990).
Plant Chemistry and Food Choice
It is always difficult to interpret the significance of nutritional analyses
of plant foods as plants sampled may not adequately capture temporal or
spatial variation in plant chemistry (Rogers et al., 1990). Further, we were not
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Nutritional Aspects of Western Lowland Gorilla Diet 825
Table IV. Site differences in foods consumed by Western lowland and Mountain Gorillas
Site Subspecies CT NDF ADF CP Ls n(#)
LEAVES
Bai Hokou, CAR G.g.gorilla 4.52 63.93 47.54 18.86 25.19 16
Lope, Gabon
2
G.g.gorilla 14.58 na 28.92 18.37 na 16
Campo, Cameroon
3
G.g.gorilla 7.3 46.1 42.6 16.6 19.4 8
Karisoke, Rwanda
4
G.g.beringei 1.1 na 35.5 15.5 na 21
FRUIT
Bai Hokou, CAR G.g.gorilla 12.33 58.4 44.42 8.91 23.92 29
Lope, Gabon G.g.gorilla 8.83 na 23.7 5.22 na 46
Campo, Cameroon G.g.gorilla 2 64.6 44.8 6.3 26.9 8
Karisoke, Rwanda G.g.beringei na na na na na na
STEMS (PITH ONLY)
Bai Hokou, CAR G.g.gorilla 0.59 67.41 49.22 16.87 25.98 7
Lope, Gabon G.g.gorilla 1.74 na 48.59 5.08 na 6
Campo, Cameroon G.g.gorilla 0.5 55.9 44.4 6.7 11.3 11
Karisoke, Rwanda G.g.beringei 0.72 na 49.3 6.2 na 12
2
Rogers et al., 1990.
3
Calvert, 1985.
4
Waterman et al., 1983.
Abbreviations and units as in Appendix A and B.
able to determine the relative importance of each food in the diet, though we
sampled most foods consumed in the short sample periods. The Bai Hokou
data provide a snapshot in time of gorilla diet during scarcity of preferred
foods. It is likely that the nutrient and other phytochemical profiles of gorilla
diet at Bai Hokou and other sites fluctuate considerably between seasons
and over the time interval of any sample.
Our results emphasize the importance of distinguishing between ripe
and unripe as well as fleshy and fibrous fruits when considering fruits as
high quality. The ripe and unripe Bai Hokou fruits have fiber and secondary
compound profiles similar to many leafy and woody foods (Nijboer et al.,
1997), though the fruits generally have more soluble sugars and less protein
than the foliage.
In general, gorillas across sites, including Bai Hokou, appear to avoid
nitrogen-based alkaloids, even when preferred foods are scarce. However,
carbon-based tannins are present in some gorilla foods . Although gorillas
seem to tolerate moderate levels of tannins, then may well be more prevalent
in the fruit than leaf portion of their diets. In fact, the majority of tannins
in the Bai Hokou foliage samples came from only two species, including
Diospyros species (ebony). Fruits of Diospyros are important in the gorilla
diet, the tannin-rich leaves are not among their preferred foods. Diospy-
ros has high levels of phenolics (Waterman, 1986), which gives the wood a
characteristic dark color.
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826 Remis, Dierenfeld, Mowry, and Carroll
Plant secondary compounds have long been thought to have a dele-
terious effect on herbivors and therefore should be avoided in the diet.
Nevertheless, increasing evidence suggests that some plant metabolites, e.g.,
phenolics, may be a necessary component of a mammalian herbivore diet
(Martin et al., 1987; Mole and Waterman, 1987). Spelman et al. (1989) found
that captive lemurs were extremely susceptible to excess iron deposition—
hemosiderosis—in the duodenum, liver, and spleen, which they attributed
a diet high in iron and ascorbic acid and low in tannins. We found that the
majority of fruits consumed by gorillas at Bai Hokou with Fe content >100
mg/kg, also had high CT levels (Appendix B). Tannins could help to con-
trol iron metabolism by binding to excess dietary iron (Roy and Mukherjee,
1979). Berry (1998) found hydrolyzable or condensed tannins or both in
many foods consumed by mountain gorillas in Bwindi Impenetrable Na-
tional Park, Uganda, and suggested that they may help to maintain a healthy
population of gut microbes. Specifically, Berry (1998) suggested that pheno-
lics help to control pathogenic microbes, while nonpathogenic, i.e., useful,
symbionts can be resistant to the effects of tannins. Finally, many mammals,
including primates, secrete proteins in saliva that have a high affinity for
tannins (Mehansho et al., 1987; Milton, 1998). These proteins might negate
detrimental effects that tannins could have during digestion (Fickel and
Joest, 1997).
The Effects of Gorilla Body Size on Dietary Adaptation
The large body size and digestive anatomy of gorillas lends them flex-
ibility to cope with scarcity of preferred foods by selecting and processing
large amounts of fiber. Consumption of large quantities of fibrous foods
may further facilitate consumption of tannins, as the efficiency of a tannin in
reducing protein digestibility is concentration-dependent (Cork and Foley,
1992; Simmen et al., 1999). Captive gorillas retained foods longer in their
gastrointestinal tracts (Remis, 2000) than chimpanzees given a similar diet
did (Lambert, 1997). Large surface areas of the colon and cecum along with
longer gut retention times should allow animals to maximize absorption of
nutrients (Chivers and Langer, 1994). Nevertheless, when ripe fruit is abun-
dant, the foliage component declines in western gorilla diet. We need further
studies to determine whether a commitment to a high-fiber strategy (Rogers
et al., 1992) could affect gorillas’ selection of fibrous foods even when fleshy
ripe fruit is plentiful.
Although gorillas consume more fruit, even during scarcity, than might
be predicted from their body size alone, they also consume more fiber than
the more persistently frugivorous chimpanzees do (Remis et al., unpublished;
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Nutritional Aspects of Western Lowland Gorilla Diet 827
Kuroda et al., 1996; Conklin-Brittain et al., 1998). The spectrum of variabil-
ity among the diets of the great apes at various sites complicates efforts to
argue for the existence of an ape-level adaptation to frugivory (Andrews,
1981; Temerin and Cant, 1983; Wrangham et al., 1998). Across sites, goril-
las appear to be more opportunistic in their frugivory than smaller-bodied
hominoids are. The Bai Hokou gorilla fruit sample contains higher amounts
of fiber and tannins than those consumed by monkeys and chimpanzees
at Kibale, Uganda (Lambert, 1997; Wrangham et al., 1998). Nevertheless,
they also contain more fiber and tannins than average gorilla foods from
Karisoke, Rwanda or Lop´e, Gabon (Waterman et al., 1983; Rogers et al.,
1990; Plumptre, 1995). Moreover, in preliminary ongoing species compar-
isons at Bai Hokou, there is no significant difference in fiber or tannin con-
tent of fruits consumed by sympatric gorillas (n = 31 fruits), chimpanzees
(n = 7 fruits) and grey-cheeked mangabeys (n = 25 fruits) during the brief
1998 data set on fruit scarcity (Remis et al., unpublished data). Site-specific
differences in plant biochemistry occur among sites across the tropics, which
likely relate to soil quality, altitude, sunlight and other abiotic factors (Van
Soest, 1994). Habitat differences shape the diets of the primates in different
locations and reduce our ability to identify specific or demic adaptations. We
plan to explore the nutritional aspects of diet in western lowland gorillas dur-
ing fleshy fruit abundance and to explore seasonal variation in niche overlap
and separation with respect to sympatric chimpanzees and cercopithecines.
Finally, little research has specifically focussed on nutrients other than
energy or protein in primates. Some interesting speculations on mineral nu-
trition of free-ranging primates are possible from our data. They provide
some rough guidelines for development of suitable substitute diets for cap-
tive populations, though captive and wild animals probably differ in gut
flora and fauna. For example, plants eaten by Bai Hokou gorillas were rel-
atively low in iron. Excess dietary iron has been related to Cu deficiency
(Morris, 1987), recently implicated in heart disease of captive gorilla pop-
ulations (Meehan pers. comm.). Clearly, the contribution of minerals from
dietary foliage needs to be better examined, as green leaves are an important
source of Ca and other essential minerals for herbivores.
Gastrointestinal diseases, obesity, and cardiovascular disease reported
in captive gorillas (Meehan, 1997) are likely influenced by diet and activity
levels. Hence defining nutrient concentrations in native diets can enhance
understanding of nutrient interactions in applied feeding programs. High
fiber diets, containing relatively moderate protein and soluble sugar concen-
trations, balanced mineral content, with possibly beneficial effects of dietary
tannins, should play important roles in nutritional health of captive and wild
gorilla populations.
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828 Remis, Dierenfeld, Mowry, and Carroll
Appendix A. Nutrient and secondary compound content of foods consumed by gorillas at Bai Hokou, Central African Republic.
Family Plant Latin BaAka name Part HT CT Alkaloid RD Ash NDF ADF Ls CP HC
Acanthaceae Whitfieldra elongata Indolu l ND ND ND 0.00 14.47 53.84 29.38 9.09 31.50 24.46
Annonaceae Annonidium mannii Mobai Fr 1.17 0.52 ND ND 5.06 19.84 12.79 3.00 8.53 7.05
Annonaceae Hexalobus sp. Pota Fu ND ND ND ND 3.39 64.02 46.97 23.17 10.74 17.05
Annonaceae Polyalthia suaveoleons Motunga Fr 24.12 17.63 0 5.08 3.00 62.68 48.45 8.08 12.21 14.23
Annonaceae Polyalthia suaveoleons Motunga se 1.03 0.55 0 0.00 ND ND ND ND ND
Apocynaceae Landolphia sp. Bossindja Fr 2.11 19.18 1 8.33 2.55 56.69 34.76 4.43 4.87 21.93
Apocynaceae Landolphia sp. Bossindja se 0.05 0.26 0 0.00 ND ND ND ND ND
Apocynaceae Orthopichonia barterii Mongenje l(v) 0.73 0.28 0 0.00 8.09 83.02 70.01 36.28 10.00 13.01
Apocynaceae Tabernaemontana sp. Etokoloko (big) Fr 1.81 9.99 2 4.83 5.13 43.72 33.94 18.96 14.17 9.78
Apocynaceae Tabernaemontana sp. Etokoloko Fr 0.99 0.17 0 ND 7.58 42.27 32.56 11.86 15.62 9.71
Apocynaceae Tabernaemontana sp. Etokoloko (big) se 0.75 11.05 0 0.00 ND ND ND ND ND ND
Burseraceae Santira trimera Baba Fr 6.06 0.43 0 0.00 ND ND ND ND ND ND
Burseraceae Santira trimera Baba se 7.62 0.24 0 ND ND ND ND ND ND ND
Caesalpinace Dialum zenkerei Mokombe Fr ND ND ND ND 2.44 52.60 21.52 10.78 8.95 31.08
Ebenaceae Diospyros bipindensis Embandja l 6.79 30.71 0 34.05 3.47 67.24 57.07 42.35 12.30 10.17
Gurke
Ebenaceae Diospyros iturensis Babango Fr ND ND ND ND 2.86 76.15 50.75 31.14 5.93 25.40
or bipendensis
Euphorbiacea Drypetes gilgiana or Timbu Fr 3.51 32.43 0 5.13 3.83 61.14 49.83 33.93 10.98 11.31
diopa
Euphorbiacea Drypetes gilgiana or Timbu Fr 1.07 0.24 0 0.00 3.33 48.89 33.39 15.17 8.92 15.50
diopa
Euphorbiacea Drypetes gilgiana or Timbu Fr ND ND ND ND ND ND ND ND ND ND
diopa
Euphorbiacea Drypetes gilgiana or Timbu Fr 0.99 1.15 0 0.00 3.59 44.78 32.57 16.07 10.37 12.21
diopa
Euphorbiacea Drypetes gilgiana or Timbu se 1.14 0.12 0 0.00 ND ND ND ND ND ND
diopa
Euphorbiacea Drypetes sp. Mosarakosarak l 2.42 0.50 0 0.00 7.33 77.06 55.59 23.31 17.00 21.47
Euphorbiacea Dryptes gossweileri? Ngama Fu 1.83 0.72 0 0.00 3.07 47.05 10.94 3.64 9.57 36.11
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Nutritional Aspects of Western Lowland Gorilla Diet 829
Gnetaceae Gnetum africanum Koko l(v) 5.53 1.89 0 0.00 8.71 52.07 36.75 12.43 19.80 15.32
Gnetaceae Gnetum africanum Koko l(v) 2.43 0.46 0 0.00 5.42 79.69 61.91 15.42 8.11 17.78
Irvingiaceae Irvingia excelsa Payo Fr 6.77 3.28 0 0.00 3.65 82.59 68.26 29.20 5.47 14.33
Irvingiaceae Irvingia excelsa Payo Fr 22.01 5.24ND ND ND ND ND ND ND ND
Irvingiaceae Irvingia excelsia Payo Fr 5.88 2.16ND ND ND ND ND ND ND ND
Irvingiaceae Klainedoxa gabonensis Bokoko Fr 2.35 0.72 0 0.00 2.71 84.41 70.76 31.85 3.05 13.65
Lauraceae Beilschmiedia obscura Ngala l 0.96 0.92 0 0.00 6.10 80.63 69.53 49.84 14.80 11.10
Meliaceae Entrandrophragma Boyo l 5.69 0.53 0 0.00 9.58 49.61 30.21 11.60 27.60 19.40
cylindricum
Meliaceae Lovoa sp. Gima l 1.25 0.58 1 0.00 7.66 76.51 61.01 38.41 22.70 15.50
Mimosaceae Maranthes glabra Mokandja Fr 0.44 0.46 0 0.00 2.59 94.16 83.83 48.92 4.54 10.33
chrysobalanceae
Mimosaceae Tetraplaura teraptera Ekombolo Fr 2.41 0.88 0 0.00 5.38 82.40 61.93 26.31 7.97 20.47
Moraceae Chlorophora excelsa Mobangui Fu 0.24 0.01 0 0.00 ND ND ND ND ND ND
Moraceae Ficus sp. Gumu Fr 1.14 0.71 2 0.00 5.47 76.55 67.81 42.01 7.77 8.74
Moraceae Ficus sp. Gumu Fr 2.06 13.02ND 2.83 3.70 49.95 40.25 22.70 5.19 9.70
Moraceae Treculia africana Efusa or Pusa Fr 1.81 0.47 0 0.00 6.10 73.27 69.22 49.18 12.29 4.05
Moraceae Treculia africana Efusa or Pusa se 3.34 0.12 0 0.00 2.34 21.66 13.47 6.00 14.61 8.19
Olacaceae Strombosia pustulata Embongo Fr 1.09 3.14 0 0.00 3.27 71.04 55.40 31.38 14.63 15.64
tetranda
Olacaceae Strombosia pustulata Embongo Fu 2.65 22.42 0 5.98 3.07 54.68 42.66 31.08 7.11 12.02
tetranda
Papilionaceae Lonchocarpus sp. Molindu l 0.85 0.67 0 0.00 4.41 70.38 57.59 38.03 14.50 12.79
(Molin)
Papilionaceae Pterocarpus soyauxii Embema Fu 2.15 0.51 0 2.26 3.27 68.39 50.58 16.39 10.04 17.81
Sapindaceae Zhana cf. Gulungensis Iwungu Fr 2.80 32.01 0 10.62 4.67 46.90 39.54 25.65 7.61 7.36
Sapotaceae Manilkara letouzeyi Monginza Fu 5.79 35.44 0 6.33 2.26 53.47 39.76 28.63 11.15 13.71
Sapotaceae Manilkara letouzeyi Monginza Fu 5.32 95.74 0 9.48 1.82 53.62 41.99 31.05 9.48 11.63
Sapotaceae Manilkara letouzeyi Monginza Fu ND ND 0.00 2.54 46.15 34.90 24.14 7.97 11.25
Sapotaceae Manilkara letouzeyi Monginza se ND ND 0 0.00 ND ND ND ND ND ND
(Continued )
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830 Remis, Dierenfeld, Mowry, and Carroll
Appendix A. (Continued )
Family Plant Latin BaAka name Part HT CT Alkaloid RD Ash NDF ADF Ls CP HC
Tiliaceae Desplatsia deweverei Liamba Fu 1.44 3.52 0 3.68 6.80 59.16 56.32 41.92 7.91 2.84
Tiliaceae Duboscia cf. Viridiflora Guruma (big) Fr 9.01 49.26 1 12.17 3.67 32.36 28.71 16.85 4.97 3.65
Tiliaceae Duboscia macrocarpa Guruma F 0.50 8.94 0 0.00 4.47 84.17 70.96 37.84 6.08 13.21
Tiliaceae Duboscia macrocarpa Guruma Fr 1.05 6.80 0 3.39 ND ND ND ND ND ND
Ulmaceae Celtis mildbraedii Gombe l 1.44 0.82 0 0.00 10.05 69.75 56.23 28.72 15.60 13.52
Ulmaceae Celtis sp. Gombe l 1.96 0.53 0 0.00 2.66 57.55 45.29 27.01 12.80 12.26
Ulmaceae Celtis sp. Gombe l 1.35 0.41 0 0.00 10.70 74.99 59.41 26.92 15.90 15.58
Zingiberacea Aframomum sp. Njokoko Fu ND ND ND ND ND ND ND ND ND ND
Zingiberacea Aframomum sp. Njokoko st 0.78 0.74 0 0.00 6.89 77.05 67.06 43.98 18.40 9.99
Zingiberacea Aframomum sp. Njokoko st 1.14 1.77 0 0.00 5.68 53.61 41.58 21.91 15.70 12.03
Zingiberacea Aframomum sp. Njokoko st 0.93 0.21 0 0.00 7.85 75.89 55.18 25.00 16.60 20.71
Zingiberacea Aframomum sulcatum Njombo st 1.32 0.41 0 0.00 7.06 61.21 39.91 20.03 19.60 21.30
Zingiberacea Aframomum sulcatum Njombo st 0.75 0.28 0 0.00 6.94 60.08 37.57 13.93 11.20 22.51
Zingiberacea Renalria sp. Dembelembe st 0.66 0.11 0 0.00 8.83 76.61 54.00 31.02 19.70 22.61
unknown unknown Essekelende Fr 0.69 3.42 ND ND 3.23 44.91 27.68 10.10 7.45 17.23
unknown unknown Batorro l 3.68 0.64 0 0.00 10.68 51.93 28.96 6.75 32.90 22.97
unknown unknown Mopusupusu l 3.55 0.87 0 0.00 6.39 77.42 58.50 31.26 18.80 18.92
unknown unknown Egumdu l 6.40 28.17 0 6.20 7.70 71.44 58.13 30.68 14.30 13.31
unknown unknown Mogombagumbu l 0.30 0.13 0 0.00 6.36 42.17 25.81 12.96 17.00 16.36
unknown unknown Demelle l 1.72 0.40 0 0.00 6.88 50.34 31.12 13.69 14.20 19.22
l = leaves; l(v) = vines; st = stems; Fr = ripe fruit; Fu = unripe fruit; se = seeds. Alkaloids scored on a scale of 0 (no reaction) to 3 (strong
reaction); hydrolyzable tannins (HT) expressed as mg/g hexahydroxydiphenylglucose; condensed and radial diffusion tannins (CT and RD)
as % dry weight quebracho tannin equivalents ash, neutral detergent fiber (NDF), acid detergent fiber (ADF), lignin (Ls), crude protein
(CP), and hemicellulose (HC) as % of dry matter.
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Nutritional Aspects of Western Lowland Gorilla Diet 831
Appendix B. Mineral Content of Fruits collected at Bai Hokou 1998
Family Latin Plant BaAka name Part Ca K Mg Na P Cu Cr Fe Mn Zn
Annonaceae Annonidium mannii Mobai Fr 0.013 0.0441 0.103 0.004 0.123 17.36 <.20 40.83 41.97 11.46
Annonaceae Hexalobus sp. Pota Fu 0.147 1.288 0.12 0.001 0.154 26.07 <.20 117.87 79.27 13.09
Annonaceae Polyalthia suaveoleons Motunga Fr 0.378 0.968 0.097 0 0.078 18.63 <.20 76.85 167.06 8.89
Apocynacea Landolphia sp. Bossindja Fr 0.051 0.757 0.09 0.003 0.1 10.16 <.20 167.84 52.42 18.65
Apocynacea Tabernaemontana sp. Etokoloko Fr 0.129 2.032 0.213 0.012 0.18 11.49 0.43 73.17 75.19 16.19
Apocynacea Tabernaemontana sp. Etokoloko Fr 0.235 2.968 0.243 0.009 0.125 19.23 <.20 43.9 168.34 23.24
Burseracea Santira trimera Baba Fr ND ND ND ND ND ND ND ND ND ND
Ebenaceae Diospyros iturensis Babango Fr 0.132 0.687 0.066 0.003 0.071 5.39 <.20 29.33 219.24 8.44
Euphorbiace Drypetes gilgiana or diopa Timbu Fr 0.263 1.225 0.119 0.01 0.12 14.92 <.20 31.17 225.74 20.77
Euphorbiace Drypetes gilgiana or diopa Timbu Fr 0.103 1.152 0.084 0.002 0.199 8.76 1.43 77.88 225.71 20.17
Euphorbiace Drypetes gilgiana or diopa Timbu Fr ND ND ND ND ND ND ND ND ND ND
Euphorbiace Drypetes gilgiana or diopa Timbu Fr 0.152 1.256 0.091 0.003 0.115 8.09 <.20 76.06 179.86 22.82
Euphorbiace Dryptes gossweileri? Ngama Fu 0.136 1.142 0.175 0.008 0.148 13.27 <0.20 96.11 142.81 32.92
Irvingiaceae Irvingia excelsa Payo Fr ND ND ND ND ND ND ND ND ND ND
Moraceae Chlorophora excelsa Mobangui Fu ND ND ND ND ND ND ND ND ND ND
Moraceae Ficus sp. Gumu Fr 0.375 0.979 0.182 0.001 0.126 10.53 0.25 79.37 53.54 17.78
Moraceae Treculia africana Efusa Fr 0.119 2.389 0.184 0.021 0.149 16.93 <.20 106.86 96.31 15.85
Olacaceae Strombosia pustulata tetranda Embongo Fr 0.143 0.041 0.113 0.009 0.163 8.81 1.48 63.06 216.04 30.41
Olacaceae Strombosia pustulata tetranda Embongo Fu 0.191 1.11 0.128 0.018 0.089 2.4 <.20 129.8 120.77 9.24
Papilionacea Pterocarpus soyauxii Embema Fu ND ND ND ND ND ND ND ND ND ND
Sapindaceae Zhana cf. gulungensis Iwungu Fr 0.086 1.689 0.096 0.017 0.086 11.12 <.20 150.34 72.91 25.93
Sapotaceae Manilkara letouzeyi Monginza Fu 0.192 1.053 0.11 0.015 0.154 4.1 1.13 70.96 89.99 21.4
Sapotaceae Manilkara letouzeyi Monginza Fu 0.095 0.557 0.015 0.002 0.073 1.74 0.52 55.07 60.15 9.79
Sapotaceae Manilkara letouzeyi Monginza Fu 0.229 0.435 0.134 0.006 0.087 2.82 <.20 30.98 127 8.65
Tiliaceae Desplatsia deweverei Liamba Fu 0.28 1.886 0.245 0.005 0.129 11.35 0.88 129.65 207.91 40.7
Tiliaceae Duboscia cf. viridiflora Guruma (big) Fr 0.166 1.336 0.179 0.005 0.092 9.16 1.06 58.44 19.54 14
Tiliaceae Duboscia macrocarpa Guruma Fr ND ND ND ND ND ND ND ND ND ND
Zingiberacea Aframomum sp. Jokoko Fu ND ND ND ND ND ND ND ND ND ND
unknown unknown Essekelende Fr 0.343 1.707 0.215 0.004 0.165 14.16 <.20 64.84 332.07 14.5
Plant parts as in Appendix A. Ca, K, Mg, Na, and P expressed as % of dry matter. Cu, Cr, Fe, Mn, and Zn expressed as mg/kg of dry matter.
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International Journal of Primatology [ijop] PP104-298611 October 1, 2001 10:23 Style file version Nov. 19th, 1999
832 Remis, Dierenfeld, Mowry, and Carroll
ACKNOWLEDGMENTS
We thank the Central African Government for permission to conduct
research in the Dzanga-Ndoki National Park. Our field research on gorilla
feeding ecology has benefited over the years from logistical and financial
support of many agencies, particularly World Wildlife Fund-US and the
Wildlife Conservation Society. Nutritional analysis was supported in part
by the The National Science Foundation (SBR-981584), Wildlife Conserva-
tion Society, Purdue University and The Berry College student work op-
portunity program. Plant samples were collected with the assistance of JB
Kpanou, C. Chiopoletta and other members of the Bai Hokou Research
Team. We appreciate the laboratory analytical assistance of M. Fitzpatrick,
M. Glick-Bauer, J. McCool, T. Ross, L. Silber, J. Deutsch and A. Wallace.
We appreciate the assistance of D. Shure, D. Jenkins, M.Cipollini, S. Green
and R. Fest. We thank D. Shure and four reviewers for comments on the
manuscript.
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... In this framework, western gorillas are particularly interesting as they show a flexible diet and respond to seasonal variation in fruit availability by dramatically modifying their diet from mainly folivorous (> 70% of leaves) to mainly frugivorous (> 70% of fruit; e.g. [22][23][24][25]66,76,78,79 ). On the other extreme, the Virunga population of mountain gorillas (G. ...
... Other species switch dietary choices to different food categories or to resources that are available year-round (e.g. birds: 18 ; antelopes: 19 ; monkeys: 2,20 ; elephants: 21 ; western gorillas: [22][23][24][25]. They may vary the daily range and travelling time to optimize foraging effort (e.g. ...
... Western gorillas are seasonal frugivores (e.g. [22][23][24][25]79 ) and our study showed that group size may influence their diet composition, feeding and social time, in response to seasonal variation in fruit availability. ...
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... Within Catarrhini the relationship between TRSA and diet has been assessed in a few extant taxa (Homo sapiens, Gorilla gorilla, Pongo pygmaeus, Pan troglodytes, and Papio anubis; Kupczik, 2003). Among the hominoids within this sample, the highly frugivorous P. troglodytes was found to have the smallest TRSA among the hominoids within the sample, while G. gorilla, which consumes a particularly folivorous diet among apes (Remis et al., 2001), was found to have the largest relative total postcanine TRSA (Kupczik, 2003). M 2 root surface area has also been examined relative to crown surface area in a larger catarrhine sample (n = 58; Kupczik et al., 2009). ...
... Contrary to previous examinations of hominoid TRSA relative to a facial size proxy (Kupczik, 2003;Kupczik & Dean, 2008), M 2 and total postcanine TRSA of G. gorilla relative to both GM and BM was smaller than or overlapping with that of the substantially frugivorous apes, not matched by similarly larger dentition, as has been observed for their molar crowns (Gingerich et al., 1982). While G. gorilla is often classified as a folivore, observations by Remis et al. (2001) suggest that although the diet of G. gorilla consists primarily of tough plant material during dry seasons, fruit makes up a similar proportion of total nutrition on an annual basis. At least one population of G. gorilla has also been observed feeding on hard-objects seasonally (van Casteren et al., 2019). ...
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... This relationship could indicate a potential trade-off in that foods they consume with the highest protein and caloric value also contain high amounts of tannins and other difficult to digest plant secondary metabolites. While some mammals might avoid food items high in particular plant secondary metabolites, evidence suggests that other mammals readily consume foods high in secondary metabolites if those foods are also high in energy, protein, or water (Felton et al., 2009;Lambert & Rothman, 2015;Remis et al., 2001;Simpson & Raubenheimer, 2001;Villalba & Provenza, 2005). The presence of tannin-or toxin-degrading bacteria in the gut microbiome could facilitate this behavior by allowing animals to tolerate higher concentrations of plant secondary metabolites in their diet. ...
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... However, the impact of mountain uplift on TRF clade diversification has never been tested. White et al., 1993;Remis et al., 2001;Rogers et al., 2004). Cauliflorous fruits in particular limit plant dispersal because they target understory dispersers, which are relatively sedentary and habitat specific (Onstein et al., 2018). ...
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... Gorillas also regularly eat fruits, many of which are high in soluble sugars (Remis et al., 2001). The cercopithecid species with ICATs (C. ...
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... Bachelor groups have a much shorter captive management history than mixed-sex groups. As such, mixed-sex groups have been extensively studied from veterinary, behavioral, and endocrine, perspectives both in wild and captive settings which has allowed for a comprehensive and detailed set of management protocols (Beck, 1984;Beck & Power, 1988;Gold & Maple, 1994;Remis et al., 2001;Rosenbaum et al., 2018). Bachelor groups, on the other hand, have only emerged as a captive social management strategy, and thus an area of particular focus, over the past 15 years (Levréro et al., 2006;Pullen, 2005;Stoinski et al., 2013Stoinski et al., , 2004bStoinski et al., , 2004aStoinski et al., , 2001. ...
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Intraspecies violence, including lethal interactions, is a relatively common phenomenon in mammals. Contrarily, interspecies violence has mainly been investigated in the context of predation and received most research attention in carnivores. Here, we provide the first information of two lethal coalitionary attacks of chimpanzees ( Pan troglodytes troglodytes ) on another hominid species, western lowland gorillas ( Gorilla gorilla gorilla ), that occur sympatrically in the Loango National Park in Gabon. In both events, the chimpanzees significantly outnumbered the gorillas and victims were infant gorillas. We discuss these observations in light of the two most widely accepted theoretical explanations for interspecific lethal violence, predation and competition, and combinations of the two-intraguild predation and interspecific killing. Given these events meet conditions proposed to trigger coalitional killing of neighbours in chimpanzees, we also discuss them in light of chimpanzees’ intraspecific interactions and territorial nature. Our findings may spur further research into the complexity of interspecies interactions. In addition, they may aid in combining field data from extant models with the Pliocene hominid fossil record to better understand behavioural adaptations and interspecific killing in the hominin lineage.
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Food calls are widespread across many mammal and bird species, but the reasons of this adaptive success are not yet fully understood. Using data from four habituated groups of western gorillas (Gorilla gorilla) in Central Africa, we investigated the possible influence of ecological (food type and quantity) and social factors (presence, arrival and departure of specific group members) on food call production. Western gorillas emitted food calls mainly in foraging contexts. The emission probability increased with food types of high quality (in relation to the season) and, particularly, with abundant food patches. Food calls elicited the arrival of group members at the feeding tree. Adult females, the most frequent signallers, mostly emitted food calls when the silverback and all offspring were absent at the feeding tree, when compared to the absence of other group members. From the receiver perspective, the probability that the silverback and all offspring arrived at the feeding tree increased when adult females emitted food calls. When calling, adult females likely benefit by increasing both nutritional intake and protection of their own offspring (by increasing spatial proximity with the silverback). Moreover, food calls emitted in the second part of the duration of tree visits had the strongest effect on the prolongation of the feeding session. Our results suggest that the adaptive reasons of food calls in one-male harem species may be increasing group cohesion/coordination and facilitating offspring survival. Significance statement The adaptive reasons for the widespread presence of food calls in many animal species aremultiple and not mutually exclusive. We showed that western gorilla emitted food calls mainly in foraging contexts; they attract other group members and deliver information on the presence of abundant resources of high quality. Food calls emitted in the second part of the tree visit seem to prolong the feeding session. Adult females, the most frequent signallers, emitted food calls more often when their offspring and the silverback are absent. These results suggest that, in species with rather stable society (such as one-male harem), this behaviour may serve to increase cohesion/coordination and to facilitate offspring survival. This study is a first step for increasing our understanding on the presence of functionally referential calls in wild western gorillas.