Diet and seasonal changes in sympatric gorillas and chimpanzees at Kahuzi–Biega National Park

Based on 8 years of observations of a group of western lowland gorillas (Gorilla beringei graueri) and a unit-group of chimpanzees (Pan troglodytes schweinfurthii) living sympatrically in the montane forest at Kahuzi–Biega National Park, we compared their diet and analyzed dietary overlap between them in relation to fruit phenology. Data on fruit consumption were collected mainly from fecal samples, and phenology of preferred ape fruits was estimated by monitoring. Totals of 231 plant foods (116 species) and 137 plant foods (104 species) were recorded for gorillas and chimpanzees, respectively. Among these, 38% of gorilla foods and 64% of chimpanzee foods were eaten by both apes. Fruits accounted for the largest overlap between them (77% for gorillas and 59% for chimpanzees). Gorillas consumed more species of vegetative foods (especially bark) exclusively whereas chimpanzees consumed more species of fruits and animal foods exclusively. Although the number of fruit species available in the montane forest of Kahuzi is much lower than that in lowland forest, the number of fruit species per chimpanzee fecal sample (average 2.7 species) was similar to that for chimpanzees in the lowland habitats. By contrast, the number of fruit species per gorilla fecal sample (average 0.8 species) was much lower than that for gorillas in the lowland habitats. Fruit consumption by both apes tended to increase during the dry season when ripe fruits were more abundant in their habitat. However, the number of fruit species consumed by chimpanzees did not change according to ripe fruit abundance. The species differences in fruit consumption may be attributed to the wide ranging of gorillas and repeated usage of a small range by chimpanzees and/or to avoidance of inter-specific contact by chimpanzees. The different staple foods (leaves and bark for gorillas and fig fruits for chimpanzees) characterize the dietary divergence between them in the montane forest of Kahuzi, where fruit is usually scarce. Gorillas rarely fed on insects, but chimpanzees occasionally fed on bees with honey, which possibly compensate for fruit scarcity. A comparison of dietary overlap between gorillas and chimpanzees across habitats suggests that sympatry may not influence dietary overlap in fruit consumed but may stimulate behavioral divergence to reduce feeding competition between them.     

The role of taste in food selection by African apes: implications for niche separation and overlap in tropical forests

Ripe fruit eating shapes the behavior of most of the apes. Gorillas (Gorilla gorilla) and chimpanzees (Pan troglodytes) are very different sizes and, consequently, have been traditionally viewed as ecologically distinct, but few studies have explored the behavioral and physiological foundations of their diets. Debate continues on the extent that large-bodied gorillas may be less selective and more opportunistic fruit eaters than chimpanzees. Taste responses have been predicted to relate to body size and digestive strategies. This study employs laboratory research on taste perception and discrimination among captive zoo-housed chimpanzees and relates it to previous work on gorillas to better characterize diets and niche separation among these apes. During the captive trials, differences were recorded in consumption patterns of water and varying concentrations of dilute aqueous fructose (sweet) and tannic acid solutions (astringent), compounds commonly found in wild foods. The chimpanzees exhibited similar preference thresholds for fructose (50 mM) to other primates studied. They exhibited slightly lower inhibition thresholds for tannic acid solutions than gorillas, but higher than smaller primates studied to date. These preliminary findings suggest that tannin tolerance may well be mediated by body size, though possible species differences in salivary proteins or other sensory differences remain to be explored. This research furthers our efforts to understand the roles of body size and physiological adaptations in shaping diet and niche separation of chimpanzees and gorillas.     

Diet of Grauer's Gorillas in the Montane Forest of Kahuzi,Democratic Republic of Congo

We describe the diet of a semihabituated group of Grauer's gorillas (Gorilla beringei graueri) inhabiting the montane forest of Kahuzi-Biega National Park, Democratic Republic of Congo, based on direct observations, feeding remains in their fresh trails, and fecal samples collected over 9 yr. We examined fruit availability in their habitat; consumption of fruit, vegetative, and animal food; and daily intake of vegetative plant food using a transect, fruit monitoring trails, fecal analysis, and tracing of the animal's daily trails between consecutive nest sites. The fruit food repertoire of Kahuzi gorillas resembles that of western and eastern lowland gorillas inhabiting lowland tropical forests, while their vegetative food repertoire resembles that of mountain gorillas inhabiting montane forests. Among 236 plant foods (116 species), leaves, pith, and barks constitute the major parts (70.2%), with fruit making up the minor part (19.7%). About half (53.2%) of the total fecal samples included fruit remains. The gorillas used leaves, stems and other vegetative plant parts as staples. Their fruit intake was similar to that reported for mountain gorillas in Bwindi. They ate animal foods, including earthworms, on rare occasions. Variation in fruit consumption was positively associated with variation in fruit production. The gorillas ate fig fruits frequently; fig intake is positively correlated with that of other fruits, and figs were not fallback foods. They relied heavily on bamboo shoots on a seasonal basis; however, no bamboo shoots were available for several years after a major flowering event. Our results support the argument that variation in gorilla diets mostly reflects variation in vegetational composition of their habitats.     

Behavioral Ecology of Sympatric Chimpanzees and Gorillas in Bwindi Impenetrable National Park,Uganda: Diet

Via a field study of chimpanzees (Pan troglodytes schweinfurthii) and gorillas (Gorilla gorilla beringei) in Bwindi Impenetrable National Park, Uganda, we found that their diets are seasonally similar, but diverge during lean seasons. Bwindi chimpanzees fed heavily on fruits of Ficus sp., which were largely ignored by the gorillas. Bwindi gorilla diet was overall more folivorous than chimpanzee diet, but was markedly more frugivorous than that of gorillas in the nearby Virunga Volcanoes. During 4 mo of the year Bwindi gorilla diet included more food species than that of the chimpanzees. Three factors in particular—seasonal consumption of fibrous foods by gorillas, interspecific differences in preferred fruit species, and meat consumption by chimpanzees—contributed to dietary divergence between the two species. When feeding on fruits, gorillas ate Myrianthus holstii more frequently than chimpanzees did, while chimpanzees included more figs in their annual diet. Chimpanzee diet included meat of duikers and monkeys; gorilla frequently consumed decaying wood.     

Insect-eating by sympatric Lowland gorillas (Gorilla g. gorilla) and chimpanzees (Pan t. troglodytes) in the Lopé Reserve,Gabon

Sympatric populations of lowland gorillas (Gorilla gorilla gorilla) and chimpanzees (Pan troglodytes troglodytes) in the Lopé Reserve in central Gabon consumed insects at similar average frequencies over a 7-year period (30% versus 31% feces contained insect remains). Data came mostly from fecal analysis supplemented by observation and trail evidence. The weaver ant (Oecophylla longinoda) was the species eaten most frequently by both gorillas and chimpanzees. Other species of insects wore eaten but there was virtually no overlap: Chimpanzees used tools to eat Apis bees (and their honey) and two large species of ants; gorillas ate three species of small ants. Thus, despite their shared habitat, the esources utilized were not identical as gorillas do not show the tool-use “technology” of chimpanzees. The frequency of insect-eating by both species of ape varied seasonally and between years but in different ways. This variation did not seem to be related to the ratio of fruit to foliage in their diets. Gorillas of all age-classes ate insects at similar rates. Comparisons with insectivory by other populations of gorillas indicate differences exist. Mountain gorillas (Gorilla g. beringei) in the Virunga Volcanoes, Rwanda, consume thousands of invertebrates daily, eating them inadvertently with handfuls of herbaceous foods but they deliberately ingest insect-foods only rarely. Lowland gorillas at Lopé habitually ate social insects, and their selective processing of herbaceous foods probably minimizes inadvertent consumption of other invertebrates. Gorillas at Belinga in northeastern Gabon, 250 km from Lop6, ate social insects at similar rates but ignored weaver ants in favor of Cubitermes sulcifrons, a small species of termite that occurs at Lopé but was not eaten by gorillas. This indicates that local traditions similar to those reported for chimpanzees also exist amongst populations of gorillas. © 1992 Wiley-Liss, Inc.     

Sympatric Chimpanzees (<Emphasis Type="Italic">Pan troglodytes troglodytes</Emphasis>) and Gorillas (<Emphasis Type="Italic">Gorilla gorilla gorilla</Emphasis>) in Loango National Park,Gabon: Dietary Composition,Seasonality, and Intersite Comparisons

Dietary overlap of sympatric apes is complex and understudied, but its examination is essential to further our understanding of species distribution, abundance, and community ecology. Over 3 yr we studied food availability and dietary composition of central chimpanzees (Pan troglodytes troglodytes) and western gorillas (Gorilla gorilla gorilla) in Loango National Park, Gabon. We predicted that living in a habitat dominated by mature forest with sparse terrestrial herbaceous vegetation would lead to an increase in frugivory by gorillas, resulting in increased dietary overlap between the 2 ape species vs. other sites, but that chimpanzees would remain more frugivorous than gorillas. Through fecal analysis we measured overlap in fruit consumption between the 2 species on a bimonthly basis using the Renkonens method. Mean overlap was 27.5% but varied greatly seasonally, ranging between 0.3% and 69%, indicating that when examined on a finer scale, the degree of overlap appears much lower than at other study sites. In contrast to studies elsewhere, there was not a positive correlation between rainfall and fruit availability in Loango, and the long dry season was a period of high fruit production. As observed elsewhere, we found a positive correlation between fruit consumption and fruit availability for both chimpanzees and gorillas and we found that chimpanzees were more frugivorous than gorillas. A very low availability of herbs did not lead to increased frugivory by gorillas nor increased overlap between the 2 ape species vs. other field sites. We conclude that forest composition, fruit availability, and dietary variability of sympatric species can vary greatly between locations, and that chimpanzees and gorillas can adapt to primary forest with little undergrowth, where they concentrate their diet on fruit and leaves.     

Food Preferences Among Captive Western Gorillas (Gorilla gorilla gorilla) and Chimpanzees (Pan troglodytes)

I used a zoological park setting to address food preferences among gorillas (Gorilla gorilla gorill) and chimpanzees (Pan troglodytes). Gorillas and chimpanzees are different sizes, and consequently, have been traditionally viewed as ecologically distinct. Sympatric western gorillas and chimpanzees have proved difficult to study in the wild. Limited field data have provided conflicting information about whether gorillas are fundamentally different from chimpanzees in diet and behavior. Fruit eating shapes the behavior of most apes, but it is unclear whether the large-bodied gorillas are an exception to this rule, specifically whether they are less selective and more opportunistic fruit eaters than chimpanzees are. My research provides experimental observational data to complement field data and to better characterize the diets and food preferences of the African apes. During laboratory research at the San Francisco Zoological Gardens, I examined individual and specific differences in food preferences of captive gorillas and chimpanzees via experimental paired-choice food trials with foods that varied in nutritional content. During the study, I offered 2500 paired-food choices to 6 individual gorillas and 2000 additional pairs to them as a group. I also proffered 600 food pairs to 4 individual chimpanzees. Despite expectations of the implications of body size differences for diet, gorillas and chimpanzees exhibited similar food preferences. Both species preferred foods high in non-starch sugars and sugar-to-fiber ratios, and low in total dietary fiber. Neither species avoided foods containing tannins. These data support other suggestions of African apes sharing a frugivorous adaptation.     

Fallback foods and dietary partitioning among Pan and gorilla

Recent findings on the strong preference of gorillas for fruits and the large dietary overlap between sympatric gorillas and chimpanzees has led to a debate over the folivorous/frugivorous dichotomy and resource partitioning. To add insight to these arguments, we analyze the diets of sympatric gorillas and chimpanzees inhabiting the montane forest of Kahuzi-Biega National Park (DRC) using a new definition of fallback foods (Marshall and Wrangham: Int J Primatol 28 [2007] 1219–1235). We determined the preferred fruits of Kahuzi chimpanzees and gorillas from direct feeding observations and fecal analyses conducted over an 8-year period. Although there was extensive overlap in the preferred fruits of these two species, gorillas tended to consume fewer fruits with prolonged availability while chimpanzees consumed fruits with large seasonal fluctuations. Fig fruit was defined as a preferred food of chimpanzees, although it may also play a role as the staple fallback food. Animal foods, such as honey bees and ants, appear to constitute filler fallback foods of chimpanzees. Tool use allows chimpanzees to obtain such high-quality fallback foods during periods of fruit scarcity. Among filler fallback foods, terrestrial herbs may enable chimpanzees to live in small home ranges in the montane forest, whereas the availability of animal foods may permit them to expand their home range in arid areas. Staple fallback foods including barks enable gorillas to form cohesive groups with similar home range across habitats irrespective of fruit abundance. These differences in fallback strategies seem to have shaped different social features between sympatric gorillas and chimpanzees. Am J Phys Anthropol 140:739–750, 2009. © 2009 Wiley-Liss, Inc.     

Insectivory by Gorilla gorilla gorilla in Southeast Cameroon

Our study extends quantitative analyses of insect-eating by gorillas (Gorilla gorilla gorilla) to Cameroon. During a 2-mo period (May–June 2001), we recorded and analyzed feeding traces on plants and insects and in gorilla feces. We found 180 feeding traces, 17% of which involved insectivory. Seventy-eight percent of the fecal samples (n = 36) contained insects. Ants were found in 61% of the samples, termites in 39%, while 56% of the samples contained remains of other insects. We added 14 new species to the known insect diet of western lowland gorillas. Overall, social insects are predominant. The choice of prey by the Ntonga gorillas gives new clues for the existence of cross-cultural differences among gorilla populations. A comparison of the overall frequency of insectivory with those at other sites in Central Africa indicate a possible effect of forest disturbance on the insectivorous behavior of gorillas. The study suggests the existence of temporal variation in ant- and termite-eating by gorillas.     

Male and female western gorilla diet: Preferred foods,use of fallback resources,and implications for ape versus old world monkey foraging strategies

Most of what is currently known about western gorilla (Gorilla gorilla) diet is based on indirect studies using fecal samples and trail signs rather than measures based on direct observations. Here we report results on adult male and female western gorilla foraging behavior, based on systematic focal observations and nutritional analyses of foods. We found that western gorillas, like other apes, are highly selective ripe fruit specialists, seeking fruit high in energy, low in antifeedants, and rare in the environment. During seasonal fruiting peaks, fruit accounted for up to 70% of feeding time. When ripe fruit was scarce, gorillas increased time spent feeding on leaves and nonpreferred fruits and herbs. Leaves were the major fallback food, accounting for up to 70% of feeding time in males and 50% in females during periods of fruit scarcity. In spite of large differences in body size, the sexes were remarkably similar in their overall diet, not differing in time spent feeding on fruit or preferred herbs. However, the male consistently fed more often and on a greater variety of leaves than did females, whereas females fed more often on fallback herbs and termites. Our findings, when considered in light of previous findings on sympatric mangabeys, indicate that the foraging strategy of western gorillas is broadly similar to that of chimpanzees and orangutans, and distinct from that of old world monkeys. Am J Phys Anthropol 140:727–738, 2009. © 2009 Wiley-Liss, Inc.     

Ecological Constraints on the Foraging Effort of Western Gorillas (Gorilla gorilla gorilla) at Bai Hoköu, Central African Republic

Recent studies demonstrate that western lowland gorillas incorporate much more fruit into their diet than Virunga mountain gorillas do. Very little is known, however, about how the frugivorous behavior of western gorillas influences their daily ranging behavior, which may ultimately affect social factors such as group size and structure. I examined the influence of diet and the spatiotemporal availability of plant foods on the foraging effort of nonhabituated western lowland gorilla groups during 17 months at Bai Hoköu in the Dzanga-Ndoki National Park, Central African Republic. I determined diet from indirect methods and gorilla plant food availability and spatial distribution from phenology and line transects. Daily path length gives an estimate of foraging effort and was the distance paced, following fresh gorilla trails, from morning to evening nest sites. The availability and distribution of fruit and its consumption by gorillas varied seasonally. When concentrating on fruits, gorillas traveled significantly farther (mean = 3.1 km/day) than when their diet consisted mostly of nonfruit vegetation, such as leaves and woody pith, stems, and bark (mean = 2.1 km/day). The amount of herbaceous vegetation in the diet did not vary seasonally and did not influence daily path length. The best environmental predictor of foraging effort was fruit density, or a measure combining both density and spatial pattern: coefficient of dispersion. In addition, when fruit patches were small, path length tended to increase but not significantly. Compared with results of other studies, gorillas at Bai Hoköu travel farther (mean = 2.6 km/day) than gorillas in Gabon (mean = 1.7 km/day) and five times farther than mountain gorillas in the Virungas (mean = 0.5 km/day). Increased foraging effort of gorillas in this region, especially during the fruiting season, may have profound effects on group size and structure.     

Investigation of the RH locus in gorillas and chimpanzees

The human Rh blood-group system is encoded by two homologous genes,RhD andRhCE. TheRH genes in gorillas and chimpanzees were investigated to delineate the phylogeny of the humanRH genes. Southern blot analysis with an exon 7-specific probe suggested that gorillas have more than twoRH genes, as has recently been reported for chimpanzees. Exon 7 was well conserved between humans, gorillas, and chimpanzees, although the exon 7 nucleotide sequences from gorillas were more similar to the humanD gene, whereas the nucleotide sequences of this exon in chimpanzees were more similar to the humanCE gene. The intron between exon 4 and exon 5 is polymorphic and can be used to distinguish the humanD gene from theCE gene. Nucleotide sequencing revealed that the basis for the intron polymorphism is anAlu element inCE which is not present in theD gene. Examination of gorilla and chimpanzee genomic DNA for this intron polymorphism demonstrated that theD intron was present in all the chimpanzees and in all but one gorilla. TheCE intron was found in three of six gorillas, but in none of the seven chimpanzees. Sequence data suggested that theAlu element might have previously been present in the chimpanzeeRH genes but was eliminated by excision or recombination. Conservation of theRhD gene was also apparent from the complete identity between the 3′-noncoding region of the human D cDNA and a gorilla genomic clone, including anAlu element which is present in both species. The data suggest that at least twoRH genes were present in a common ancestor of humans, chimpanzees, and gorillas, and that additionalRH gene duplication has taken place in gorillas and chimpanzees. TheRhCE gene appears to have diverged more thanRhD among primates. In addition, theRhD gene deletion associated with the Rh-negative phenotype in humans seems to have occurred after speciation. Correspondence to: C.M. Westhoff     

Home Range and Frugivory Patterns of Mountain Gorillas in Bwindi Impenetrable National Park,Uganda

Mountain, western, and Grauer's gorillas exhibit broad differences in ecological patterns with western gorillas eating more fruit and having larger home ranges than their largely folivorous counterparts in the Virunga Volcanoes. We studied the home range and frugivory patterns of one group of Gorilla beringei beringei in the little-studied population of Bwindi Impenetrable National Park, Uganda, to compare with other populations and to investigate whether there was any relationship between patterns of frugivory and home range size. During the 3-year study, the gorillas ate 16 species of fruit on 27% of observation days. There was high variability in frugivory among the 3 years and no consistent seasonal pattern. Annual home range size was ca. 21 km² for Years 1 and 2, and it increased dramatically to 40 km² in Year 3. Home range size varied considerable between months and seasons, but there is no clear relationship between occurrence of fruit-eating and home range size. The group exhibited more fruit-eating and a larger home range size those ofthe gorillas in the Virunga Volcanoes. Their home range size is comparable to that of western gorillas, though Bwindi gorillas consumed less fruit. Home range size and utilization by all gorillas probably depends on a complex relationship between the distribution and abundance of both fruit and herbaceous vegetation and social factors such as male mating tactics.     

Population Density of Chimpanzees and Gorillas in the Petit Loango Reserve,Gabon: Employing a New Method to Distinguish Between Nests of the Two Species

We studied the vegetation and population density of chimpanzees (Pan troglodytes troglodytes) and gorillas (Gorilla gorilla gorilla) in the Petit Loango Reserve, Gabon. The study site consists mainly of primary and secondary forest, and it also includes patches of savanna, inundated forest, and coastal scrub. The density of herbaceous plants of Marantaceae and Zingiberaceae, which serve as important foods for great apes at other study sites, is extremely low. It was very difficult to distinguish between the nests of chimpanzees and gorillas because most gorilla nests were made in trees. We developed a new method to identify the builder of nests via a combination of field observation of nests and fecal samples, and examination of hair samples under a scanning electron microscope. Nests of chimpanzees were mainly in primary forest, and the estimated density (0.78 individual per km ² ) is higher than the average density in Gabon and comparable to the nest density in the Lopé Reserve, Gabon. Nests of gorillas were mainly in secondary forest, and their estimated density (0.21) is similar to the average next density in Gabon but lower than the gorilla nest density in the Lopé Reserve. The low density of herbaceous foods seemed to influence the density of gorillas but not the density of chimpanzees.     

Seasonal change in the composition of the diet of eastern lowland gorillas

Details are presented of the composition of the diet of eastern lowland gorillas, derived mainly from a study of their fresh trails and fecal analysis, during the course of an entire year in the tropical lowland forests of the Itebero region, Zaire. Gorillas ate 194 plant foods from 121 species and 45 families. They consumed 48 species of fruits; and 89% of fecal samples contained fruit seeds, but fruits were a relatively small part (25%) of the total number of food items. The composition of their diet changed seasonally. When consumption of fruit decreased in the long rainy and the long dry seasons, the gorillas ate, in addition to Zingiberaceae and Marantaceae, many kinds of leaf and bark, which may be an important buffer against the shortage of fruits. Gorillas also fed regularly on ants (Ponerinae), and the frequency of consumption showed small seasonal variations. From a comparison of diet composition, eastern lowland gorillas appeared to be intermediate between the other two subspecies. The choice of food showed differences in preference of fruits and insects between subspecies and may reflect high similarities within subspecies in lowland and montane forests.     

Diet composition of chimpanzees inhabiting the montane forest of Kahuzi,Democratic Republic of Congo

The diet of chimpanzees was investigated by direct observations, feeding remains, and fecal analysis from January 1994 to December 2000 in the montane forest of Kahuzi-Biega National Park. A total of 171 food items were identified, among which 156 items were plant materials belonging to 114 species from 57 taxonomic families. Chimpanzees consumed 66 species of fruits (62 species of pulps and four species of seeds). Results of fecal analysis showed that fig fruits were the most frequently eaten. Their seeds occurred in 92% of a total of 7212 chimpanzee fecal samples. The chimpanzees changed their diet according to seasonal and annual variations in both abundance and diversity of fruit species. However, they are very selective frugivores. Only a few pulp-fruit species are regularly identified in their fecal samples. During the rainy season, when ripe fruit was scarce, chimpanzees relied heavily on piths and leaves. They swallowed leaves of two species of Commelinaceae without chewing, probably for medical purposes. Animal foods were eaten infrequently. The montane forest of Kahuzi, where chimpanzees range up to 2600 m above sea level, may be the highest altitudinal limit ever recorded for their distribution. Compared to other chimpanzee habitats, Kahuzi has a low diversity of fruit species and the availability of a few pulp-fruit species may be critical to the survival of Kahuzi chimpanzees.     

Western lowland gorilla diet and resource availability: new evidence,cross-site comparisons,and reflections on indirect sampling methods

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 high-quality 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.     

Monitoring the reproductive status of female gorillas (Gorilla gorilla gorilla) by measuring the steroid hormones in fecal samples

The reproductive status of female gorillas (Gorilla gorilla gorilla) was estimated by measuring the sex steroid hormones in fecal samples instead of in blood samples. Fecal samples from female gorillas were used to examine the reliability of this non-invasive assay system, which included the extraction method for estradiol-17β (E₂) and progesterone (P) from fecal samples. Fecal samples from three female gorillas were collected daily for about 55 days, and fecal E₂ and P were assayed to clarify the fluctuation patterns of these steroids in the feces. Fecal sampling from one female was repeated for another 50-day period (starting 75 days after the end of the first observation period) and assayed to confirm if the menstrual cycle of this subject was ovulatory. Although fecal E₂ concentration measurements were quantitative by using this assay system, fecal P concentration measurements were semi-quantitative. Relative amounts of fecal P in fecal samples were estimated by using the values of B/B₀ (bound/total binding in the radioimmunoassay system). Two of the four fluctuation patterns of fecal hormones observed throughout the menstrual cycle for the three female gorillas were typical for normal ovulatory cycles. In the subject observed for two periods, one pattern was typical and the other atypical. The results show that this non-invasive method is simple and practical for monitoring the reproductive status of great apes as well as Old World monkeys.     

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

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.     

Predation on mammals by chimpanzees in the montane forest of Kahuzi, Zaire

The rate of predation on mammals by chimpanzees was determined from carcasses and from fecal specimens found on fresh trails during a 16-month period in the montane forest of Kahuzi-Biega National Park, Zaire. A unit-group of semi-habituated chimpanzees, composed of 22 – 23 individuals including 8 adult or adolescent males, appeared to kill about 18 – 30 mammalian prey (16 – 28Cercopithecus monkeys) per year, if the multiple kills by chimpanzees were not considered. A juvenile l'Hoest's monkey was recorded for the first time as the prey of chimpanzees in this study. Predation occurred in the late dry and the early rainy seasons, when the diversity of ripe fruits was the highest during the year. The Kahuzi chimpanzees tended to kill mammals less frequently but to killCercopithecus monkeys more frequently than chimpanzees in other habitats. The absence of red colobus monkeys, which are the most frequent prey in Gombe, Mahale, and Tai, might be responsible for the low predation rate. However, the estimated rate of predation onCercopithecus monkeys is the highest record among various chimpanzee habitats. At least 11 – 18% of theCercopithecus population seemed to be lost annually as a result of being killed by chimpanzees. Chimpanzees may be the most important predators on these monkeys in the absence of leopards at Kahuzi. The examination of fecal samples and carcasses suggested that adult (probably male) or adolescent chimpanzees tended to eat juvenile or subadult monkeys most frequently, as is also seen for chimpanzees in Gombe, Mahale, and Tai.