The role of foraging behaviour in the sexual segregation of the African elephant

Elephants (Loxodonta africana) exhibit pronounced sexual dimorphism, and in this study we test the prediction that the differences in body size and sociality are significant enough to drive divergent foraging strategies and ultimately sexual segregation. Body size influences the foraging behaviour of herbivores through the differential scaling coefficients of metabolism and gut size, with larger bodied individuals being able to tolerate greater quantities of low-quality, fibrous vegetation, whilst having lower mass-specific energy requirements. We test two distinct theories: the scramble competition hypothesis (SCH) and the forage selection hypothesis (FSH). Comprehensive behavioural data were collected from the Pongola Game Reserve and the Phinda Private Game Reserve in South Africa over a 2.5-year period. The data were analysed using sex as the independent variable. Adult females targeted a wider range of species, adopted a more selective foraging approach and exhibited greater bite rates as predicted by the body size hypothesis and the increased demands of reproductive investment (lactation and pregnancy). Males had longer feeding bouts, displayed significantly more destructive behaviour (31% of observations, 11% for females) and ingested greater quantities of forage during each feeding bout. The independent ranging behaviour of adult males enables them to have longer foraging bouts as they experience fewer social constraints than females. The SCH was rejected as a cause of sexual segregation due to the relative abundance of low quality forage, and the fact that feeding heights were similar for both males and females. However, we conclude that the differences in the foraging strategies of the sexes are sufficient to cause spatial segregation as postulated by the FSH. Sexual dimorphism and the associated behavioural differences have important implications for the management and conservation of elephant and other dimorphic species, with the sexes effectively acting as distinct “ecological species”.     

Sex differences in giraffe foraging behavior at two spatial scales

We test predictions about differences in the foraging behaviors of male and female giraffes (Giraffa camelopardalis tippelskirchi Matchie) that derive from a hypothesis linking sexual size dimorphism to foraging behavior. This body-size hypothesis predicts that males will exhibit specific behaviors that increase their dry-matter intake rate relative to females. Foraging behavior was examined at two hierarchical levels corresponding to two spatial and temporal scales, within patches and within habitats. Patches are defined as individual trees or shrubs and habitats are defined as collections of patches within plant communities. Males were predicted to increase dry-matter intake rate within patches by taking larger bites, cropping bites more quickly, chewing less, and chewing faster. Within habitats, males were expected to increase intake rate by increasing the proportion of foraging time devoted to food ingestion as opposed to inter-patch travel time and vigilance. The predictions were tested in a free-ranging population of giraffes in Mikumi National Park, Tanzania. Males spent less total time foraging than females but allocated a greater proportion of their foraging time to forage ingestion as opposed to travel between patches. There was no sex difference in rumination time but males spent more time in activities other than foraging and rumination, such as walking. Within patches, males took larger bites than females, but females cropped bites more quickly and chewed faster. Males had longer per-bite handling times than females but had shorter handling times per gram of intake. Within habitats, males had longer average patch residence times but there was no significant sex difference in inter-patch travel times. There was no overall difference between sexes in vigilance while foraging, although there were significant sex by habitat and sex by season interactions. Although not all the predictions were confirmed, overall the results agree qualitatively with the body-size hypothesis. Sex-related differences in foraging behavior led to greater estimated intake rates for males at the within-patch and within-habitat scales. Received: 20 November 1995 / Accepted: 5 November 1996     

Movement patterns and herd dynamics among South African giraffes (Giraffa camelopardalis giraffa)

Giraffes reside in a fission–fusion social system, with sex, age proximity, kinship and home range overlap accounting for some of the variance in herd composition, while season, sex, age and time of day influence diet, home range size and distance travelled. To increase our knowledge of habitat use and fission–fusion herd dynamics, we placed GPS devices on eight adult female South African giraffes (Giraffa camelopardalis giraffa) living in the Khamab Kalahari Nature Reserve (South Africa). We tested four predictions about how season, kinship, home range and travel patterns influence habitat use and herd dynamics. Our two key findings were that females with a greater degree of home range overlap were more likely to form herds, but the degree of overlap was independent of the amount of time that they spent together in a herd, and that on the day prior to herd formation, females travelled about twice as far as their daily average and tended to move directly towards their future herd mate. We conclude that habitat use and movement patterns regulating fission–fusion dynamics reflect an interaction of ecological, social and reproductive factors operating in tandem, not independently.     

Sexual segregation by Masai giraffes at two spatial scales

In this paper alternative explanations for observed patterns of sexual segregation by giraffes are examined at two spatial scales: within-habitats and within-landscape. Habitats are defined as recognizable plant associations and the landscape as the collection of all available habitat types. The study was conducted in Mikumi National Park, Tanzania. At the within-landscape level, all sex and age classes of giraffes exhibited high degrees of preference for riverine habitats. Sex differences in habitat selection were mostly due to females with young, who tended to select open floodplain habitats in which their vigilance time was lowest. Males, and females without young, preferred more heavily-wooded habitat. Habitat preferences were not related to observed habitat-specific forage intake rates for either males or females. Within habitats, male and female giraffes selected different feeding heights, males feeding higher in the canopy than females. Females showed a strong tendency to generalize with respect to feeding height. It is suggested that a sexual dimorphism–body size hypothesis provides a parsimonious explanation for the observed feeding height selection patterns, whereas a reproductive strategy hypothesis can explain sex-differences in habitat selection patterns within the landscape.     

Effects of body size and social context on the arboreal activities of lowland gorillas in the Central African Republic

The objectives of this 27 month study were to document the positional behaviors used by lowland gorillas (Gorilla gorilla gorilla) in the Central African Republic and to compare the effects of body size dimorphism on the use of arboreal substrates. During this study, despite their great size, all gorillas used trees regularly. Predictions concerning the relationship of body size to arboreal behavior were generally upheld. Small branch and suspensory activities were rare for silverbacks. Females used smaller and multiple substrates and suspensory postures more frequently than males. Although females foraged in the periphery of trees, males stayed close to the cores and rarely used terminal branches. In addition to body size, this study found that party size, social rank, and tree structure all influence an animal's substrate choice and subsequent positional activities. Lone males typically remained in the cores of trees where substrates are large. Group males may have been forced to use all parts of trees because others were present. Lone males used small crown trees which provided easy access to terminal branch foods. Males and females foraging together used larger trees (containing more feeding sites) than single sex groups. Female positional behavior may have been affected by the presence of males. When apart from males, females used the cores of trees and larger substrates more than when foraging with males. As habitat and social context both influence substrate use, they should be considered essential components of body-sized based interpretations of the behavior of fossil or extant species. © 1995 Wiley-Liss, Inc.     

Seasonal and geographical influences on the feeding ecology of giraffes in the Luangwa Valley,Zambia: 1973–2014

Obtaining longitudinal data about the feeding ecology of long‐lived iteroparous mammals is rare, but enhances our understanding of how the environment influences niche breadth and dietary diversity within a species. We analysed forty years of feeding records obtained from a population of Thornicroft's giraffes (Giraffa camelopardalis thornicrofti) living in the Luangwa Valley, Zambia. Giraffes are browsers that have been reported to feed primarily upon Acacia leaves, but their feeding ecology in some locations conflict with this interpretation. Giraffes in the Luangwa Valley fed on 93 identified plant species, but only a few contributed to the bulk of the diet. Niche breadth was quite large (Shannon‐Weiner Diversity Index H′ = 3.699) and about 13% more diverse during the dry, than wet, season. Key species eaten during the dry season were very consistent across decades, with Kigelia africana and Capparis tomentosa prominent at this time. The evolutionary ecology of giraffes has probably benefitted from a foraging strategy that includes a variable and high‐quality diet during the hot, dry season, when feeding pressures are greatest. Giraffe feeding ecology has evolved in conjunction with their physiology, anatomy and morphology, resulting in an animal that is well adapted for survival in an arid environment.     

Intraspecific Strategic Responses of African Elephants to Temporal Variation in Forage Quality

ABSTRACT Mammalian herbivores adopt foraging strategies to optimize nutritional trade-offs against restrictions imposed by body size, nutritional requirements, digestive anatomy, physiology, and the forage resource they exploit. Selective or generalist feeding strategies scale with body size across species. However, within species, where constraints should be most similar, responses to limitation have rarely been examined. We used African elephants (Loxodonta Africana) to test for changes in seasonal diet quality of individuals of differing body size and sex through measurement of fecal nitrogen and phosphorus. We measured physiological stress response of these age and sex classes to seasonal change by fecal glucocorticoid metabolite levels (i.e., stress hormones). Large body size increased tolerance to lower-quality forage. Adult males and females exhibited divergent trends; females had higher diet quality than males, irrespective of body size. When limited by forage availability or quality during the dry season, diet quality declined across all body sizes, but weaned calves ingested a higher-quality diet than larger-bodied adults. On release from restriction during the wet season, weaned calf nitrogen concentrations were consistently high and stress hormone levels decreased, whereas adult female phosphorus levels were highest and less variable and stress hormone levels were unchanged. The ability to adjust forage quality is an important strategy used to ensure adequate nutritional intake according to body size limitations. Although body size is a key determining factor of dietary differences between adult elephants, foraging strategies are also driven by specific nutritional requirements, which may override the body size effects driving foraging decisions in some cases. The diversity of intraspecific response highlights ecologically segregated entities within a species and should be a concern for population management planning, particularly for threatened species. Fecal diet quality and stress hormone analysis could provide an early and sensitive indicator for monitoring age and sex class responses to resource restriction in high-density elephant populations.     

Sexual segregation of seasonal foraging habitats in a non-migratory marine mammal

Many animal species segregate by sex. Such segregation may be social in nature, or ecological, or both. Grey seals (Halichoerus grypus), like many large mammals, are sexually size dimorphic. In size dimorphic species, allometric differences in morphology, metabolic rate and reproductive costs are likely. Such differences may require the sexes to use different foraging strategies or different habitats. To investigate sexual segregation of habitat in grey seals, we used satellite tracks from 95 (male 46; female 49) adults breeding at Sable Island, Nova Scotia (44 degrees N, 60 degrees W) collected from 1995 to 2005. Location estimates were made from satellite fixes using a state-space movement model to estimate true locations and regularize them in time. Location estimates were used to calculate home range kernels of male and female habitat use each month. Month by sex kernel home ranges revealed striking differences and dynamics in habitat use between males and females on spatial scales broader than most terrestrial examples and at temporal and spatial resolutions rarely available for marine species. Differences were most pronounced just before (October-December) and immediately after breeding (February-March). During both periods, males primarily used areas along the continental shelf break, while females mainly used mid-shelf regions. Coupled with previously identified sex-specific seasonal patterns of energy storage, diving and diet, our findings suggest that males and females differ profoundly in their spatial foraging strategies. These differences may serve to maximize fitness by reducing intersexual competition during key foraging periods.     

Effects of size and sex on the courting success and foraging behaviour of Embiotoca jacksoni

To test life-history theory that body size and sex should influence how animals allocate time to foraging versus reproductive activities, we measured the effects of size and sex on courting success and foraging behaviour of black surfperch Embiotoca jacksoni off Santa Catalina Island, southern California. Observations of focal fish were made while snorkelling, during which the length of each fish (estimated to the nearest cm), total duration of courting encounters and foraging rates were recorded. We made observations during and outside the mating season. Courtship occurred only between pairs and its duration increased with the size of both the male and female. Although males would court females that were smaller or larger than themselves, pairs that were closely matched in size had long courting sessions, whereas those that differed considerably in size courted only briefly. Small fish foraged more than larger fish, both during and outside the mating season. Males and females foraged at similar rates outside of the mating season, but during the mating season males reduced their foraging rates to less than half that seen outside of the mating season, whereas females continued to forage at the same rate. This decrease in foraging rate of males during the mating season was seen in all sizes of males but was proportionally greatest in the largest males. These observations indicate that males trade off time spent on foraging for time spent courting during the mating season, whereas females do not.     

Sex and size related differences in the dry season feeding patterns of elephants in Chobe National Park, Botswana

Differences in feeding patterns of the African elephant were examined by sex and age during the dry season in a dystrophic savanna-woodland ecosystem in northern Botswana. Adult males had the least diverse diet in terms of woody plant species, but they consumed more plant parts than family units. The diameter of stems of food plants broken or bitten off was also greater for adult males than for females and subadult males. Adult males spent more time foraging on each woody plant than did females. The number of woody plant species and individuals present were higher at feeding sites of family units than at feeding sites of adult males, indicating that family units positioned themselves at feeding sites with higher species diversity than those of males. We argue that the most likely explanation for these differences is related to the pronounced sexual size dimorphism exhibited by elephants, resulting in sex differences in browsing patterns due to the allometric relationships that govern the tolerance of herbivores for variation in diet quality. From our results this Body Size Hypothesis is accepted rather than the alternative Scramble Competition Hypothesis, which predicts that adult male elephants consume lower quality browse because they are displaced from preferred browse as an outcome of scramble competition with adult females and their offspring. If the feeding patterns of adult male elephants were affected by intersexual scramble competition, we would expect adult males to browse at a higher level in the canopy than the smaller-bodied females and their offspring. No evidence was found for this, although adult females were found to browse at a higher level in the canopy when feeding in close proximity to subadults and juveniles than when feeding alone. Sex differences in elephant browsing patterns are, we propose, of relevance to understanding and managing elephant impacts on African woodlands.     

Relationship between reversed sexual dimorphism,breeding investment and foraging ecology in a pelagic seabird,the masked booby

Reversed sexual dimorphism (RSD) may be related to different roles in breeding investment and/or foraging, but little information is available on foraging ecology. We studied the foraging behaviour and parental investment by male and female masked boobies, a species with RSD, by combining studies of foraging ecology using miniaturised activity and GPS data loggers of nest attendance, with an experimental study where flight costs were increased. Males attended the chick more often than females, but females provided more food to the chick than males. Males and females foraged during similar periods of the day, had similar prey types and sizes, diving depths, durations of foraging trips, foraging zones and ranges. Females spent a smaller proportion of the foraging trip sitting on the water and had higher diving rate than males, suggesting higher foraging effort by females. In females, trip duration correlated with mass at departure, suggesting a flexible investment through control by body mass. The experimental study showed that handicapped females and female partners of handicapped males lost mass compared to control birds, whereas there was no difference for males. These results indicate that the larger female is the main provisioner of the chick in the pair, and regulates breeding effort in relation to its own body mass, whereas males have a fixed investment. The different breeding investment between the sexes is associated with contrasting foraging strategies, but no clear niche differentiation was observed. The larger size of the females may be advantageous for provisioning the chick with large quantities of energy and for flexible breeding effort, while the smaller male invests in territory defence and nest guarding, a crucial task when breeding at high densities. In masked boobies, division of labour appears to be maximal during chick rearing—the most energy-demanding period—and may be related to evolution of RSD.     

Ecological selection and sexual dimorphism in the sooty oystercatcher,Haematopus fuliginosus

Male and female sooty oystercatchers (subspecies Haematopus fuliginosus fuliginosus; Haematopodidae) have an average difference in bill length of 19%. We studied the relationship between this sexual dimorphism and foraging ecology at coastal sites in southern New South Wales, Australia. Intersexual foraging divergence was most striking in diet, with seven prey classes eaten exclusively by one sex (male: 4, female: 3), and all shared prey classes eaten in different proportions. Intersexual diet partitioning was also observed in energetic rewards gained from foraging, with females gaining highest energetic benefits from eating ascidians and males from eating limpets. Furthermore, within the most commonly consumed prey item, limpets, females gained higher energetic benefit from eating smaller sizes while males gained greater rewards from the largest limpet sizes. Intersexual divergence was also observed in several aspects of foraging behaviour. Finally, there was a significant effect of tidal cycles upon intersexual niche partitioning in this species; the degree of diet divergence varied between tide conditions and females had a consistently more efficient dietary intake on neap tides than males. Diet divergence in the sooty oystercatcher is greater than previously observed in any oystercatcher, and is correlated with the largest sexual bill dimorphism recorded in this family. It is argued that intersexual competition between territorial pairs is operating to diverge male and female bill morphology.     

Variation in selection pressure acting on body size by age and sex in a reverse sexual size dimorphic raptor

Body size strongly influences fitness, with larger individuals benefiting in terms of both greater productivity and survivorship; for reverse sexual size dimorphic (RSD) species, this relationship may be more complex. We examined the selection pressures acting on body size in male and female Merlins Falco columbarius to assess whether larger or smaller individuals of this RSD species were favoured in terms of survival and breeding performance. For males and females there were clear links between body size and survival but the exact relationship varied by sex. Among males, birds that survived each year class were larger than those that died and yearlings were on average smaller than older birds, but there were no measurable differences among adult males (age 2+). Among females, larger individuals aged 1 and 2 years were more likely to survive, but this size‐based pattern was not apparent in older age classes. Size early in life predicted the lifespan in male Merlins but not as strongly as for females and not for the largest individuals. Reproductive performance based on brood size was not associated with body size in either males or females, but there was a weak positive relationship between female body size and lifetime reproductive success. Selection appears to favour larger males and females but there is no indication that the population is evolving towards bigger individuals, perhaps in part due to selection against the largest birds. Increased survival may allow larger and higher quality individuals to occupy higher quality territories as they age and thereby to accrue greater lifetime reproductive success in the process.     

Sexual differences in the foraging ecology of 19th century beluga whales (Delphinapterus leucas) from the Canadian High Arctic

Marine mammals often exhibit significant sexual segregation in their diet and habitat use but these differences have not been studied systematically in historic or ancient populations due to the difficulties associated with determining the sex of skeletal elements based on gross morphology. Using a combined ancient DNA and stable isotope approach, we document a sexual difference in the foraging ecology of late 19th century beluga whales (Delphinapterus leucas) from the Canadian High Arctic. Using two PCR assays that coamplify fragments of the Y-linked SRY and X-linked ZFX genes, we assigned reproducible sex identities to 35 beluga specimens. This provided a basis for investigating sex-specific differences in foraging ecology using stable carbon and nitrogen isotope analyses of bone collagen. These isotopic data demonstrate that although both males and females primarily consumed Arctic cod, males utilized a wider range of prey than females, feeding on high trophic level benthic prey (sculpins) to a greater extent. Because bone collagen integrates prey isotopic compositions over the course of several years these sex-based differences in beluga bone collagen isotopic compositions reflect long-term and sustained sexual differences in foraging.     

The individual counts: within sex differences in foraging strategies are as important as sex‐specific differences in masked boobies Sula dactylatra

Understanding how animals allocate their foraging time is a central question in behavioural ecology. Intrinsic factors, such as body mass and size differences between sexes or species, influence animals’ foraging behaviour, but studies investigating the effects of individual differences in body mass and size within the same sex are scarce. We investigated this in chick‐rearing masked boobies Sula dactylatra, a species with reversed sexual dimorphism, through the simultaneous deployment of GPS and depth‐acceleration loggers to obtain information on foraging movements and activity patterns. Heavier females performed shorter trips closer to the colony than lighter females. During these shorter trips, heavier females spent higher proportions of their flight time flapping and less time resting on the water than lighter females did during longer trips. In contrast, body mass did not affect trip duration of males, however heavier males spent less time flapping and more time resting on the water than lighter males. This may occur as a result of higher flight costs associated with body mass and allow conservation of energy during locomotion. Body size (i.e. wing length) had no effect on any of the foraging parameters. Dive depths and dive rates (dives h^?1) were not affected by body mass, but females dived significantly deeper than males, suggesting that other factors are important. Other studies demonstrated that females are the parent in charge of provisioning the chick, and maintain a flexible investment under regulation of their own body mass. Variation in trip length therefore seems to be triggered by body condition in females, but not in males. Consequently, shorter trips are presumably used to provision the chick, while longer trips are for self‐maintenance. Our findings underline the importance of accounting for the effects of body mass differences within the same sex, if sex‐specific foraging parameters in dimorphic species are being investigated.     

Life expectancy,maximum longevity and lifetime reproductive success in female Thornicroft's giraffe in Zambia

Gestation and longevity scale with body mass across taxa, yet within size dimorphic taxa, males tend to have reduced lifespans compared with females. Testing life history models, and accounting for sex differences in longevity, requires obtaining accurate longitudinal data from wild populations. We provide the first report describing key life history parameters from a long‐term study of giraffes in Africa. We followed a population of Thornicroft's giraffe (Giraffa camelopardalis thornicrofti) in Zambia for over 40 years. Maximum longevity among females was approximately 28 years, with lifespan accounting for 81% of the variance in lifetime reproductive success. Average adult female life expectancy was no different than average adult male life expectancy. However, the breeding lifespan of males was about half that of females, while maximum lifespan of males was 75% that of females. Our findings support the suggestion that sex differences in maximum lifespan arise from stronger selection for lengthy lives in females than in males. Among females, longer lives are associated with greater reproductive output.     

Dietary choice for a balanced nutrient intake increases the mean and reduces the variance in the reproductive performance of male and female cockroaches

Sexual selection may cause dietary requirements for reproduction to diverge across the sexes and promote the evolution of different foraging strategies in males and females. However, our understanding of how the sexes regulate their nutrition and the effects that this has on sex‐specific fitness is limited. We quantified how protein (P) and carbohydrate (C) intakes affect reproductive traits in male (pheromone expression) and female (clutch size and gestation time) cockroaches (Nauphoeta cinerea). We then determined how the sexes regulate their intake of nutrients when restricted to a single diet and when given dietary choice and how this affected expression of these important reproductive traits. Pheromone levels that improve male attractiveness, female clutch size and gestation time all peaked at a high daily intake of P:C in a 1:8 ratio. This is surprising because female insects typically require more P than males to maximize reproduction. The relatively low P requirement of females may reflect the action of cockroach endosymbionts that help recycle stored nitrogen for protein synthesis. When constrained to a single diet, both sexes prioritized regulating their daily intake of P over C, although this prioritization was stronger in females than males. When given the choice between diets, both sexes actively regulated their intake of nutrients at a 1:4.8 P:C ratio. The P:C ratio did not overlap exactly with the intake of nutrients that optimized reproductive trait expression. Despite this, cockroaches of both sexes that were given dietary choice generally improved the mean and reduced the variance in all reproductive traits we measured relative to animals fed a single diet from the diet choice pair. This pattern was not as strong when compared to the single best diet in our geometric array, suggesting that the relationship between nutrient balancing and reproduction is complex in this species.     

Sex promotes spatial and dietary segregation in a migratory shorebird during the non-breeding season

Several expressions of sexual segregation have been described in animals, especially in those exhibiting conspicuous dimorphism. Outside the breeding season, segregation has been mostly attributed to size or age-mediated dominance or to trophic niche divergence. Regardless of the recognized implications for population dynamics, the ecological causes and consequences of sexual segregation are still poorly understood. We investigate the foraging habits of a shorebird showing reversed sexual dimorphism, the black-tailed godwit Limosa limosa, during the winter season, and found extensive segregation between sexes in spatial distribution, microhabitat use and dietary composition. Males and females exhibited high site-fidelity but differed in their distributions at estuary-scale. Male godwits (shorter-billed) foraged more frequently in exposed mudflats than in patches with higher water levels, and consumed more bivalves and gastropods and fewer polychaetes than females. Females tended to be more frequently involved and to win more aggressive interactions than males. However, the number of aggressions recorded was low, suggesting that sexual dominance plays a lesser role in segregation, although its importance cannot be ruled out. Dimorphism in the feeding apparatus has been used to explain sex differences in foraging ecology and behaviour of many avian species, but few studies confirmed that morphologic characteristics drive individual differences within each sex. We found a relationship between resource use and bill size when pooling data from males and females. However, this relationship did not hold for either sex separately, suggesting that differences in foraging habits of godwits are primarily a function of sex, rather than bill size. Hence, the exact mechanisms through which this segregation operates are still unknown. The recorded differences in spatial distribution and resource use might expose male and female to distinct threats, thus affecting population dynamics through differential mortality. Therefore, population models and effective conservation strategies should increasingly take sex-specific requirements into consideration.     

The Race for Females: The Mating System of the Red Mason Bee, Osmia rufa (L.) (Hymenoptera: Megachilidae)

The ecology of a species strongly influences the strategies with which males and females maximize their lifetime reproductive success. When males and females do not invest equally in offspring, the sex with the higher parental investment becomes a rare resource for the other. The spatial and temporal distribution of the limiting sex forms the basis of the mating system. In nest-constructing Aculeata such as the red mason bee, Osmia rufa, females perform intensive brood care, whereas males do not invest in their offspring but instead compete for access to mates. Receptive females of this species are widely distributed and do not assemble at certain places. Therefore, territorial behavior is not an advantageous mating tactic for males, which instead search for females within individual home ranges usually centered around food plants. The unmarked or defended home ranges of different males may completely overlap. Competitive searching leads to a random distribution of matings among males that is largely independent of body size. The mating system of O. rufa can be described as scramble competition polygyny.     

Tree structure and sex differences in arboreality among western lowland gorillas (Gorilla gorilla gorilla) at Bai Hokou, Central African Republic

The aim of six months of research in 1995 on the gorillas (Gorilla gorilla gorilla) at Bai Hokou, Central African Republic, was to study the effects of tree structure, relative numbers of arboreal feeding sites, and sex differences in body size on arboreal foraging. The analysis presented here also documents inter-annual variation in fruit availability and climbing by silverback gorillas by comparing the 1995 results to those from earlier research, 1990–1992. This analysis suggests that female gorillas maintain similar levels of arboreality in fruit-rich and fruit-poor seasons and years, but silverbacks may be more terrestrial when fruit is scarce or difficult to access. Trees of different shapes present different numbers of feeding sites to bigger males and smaller females. Male climbing is affected by the availability of fruit, and small trees with narrow crowns that lend easy access to fruit from the core. This study suggests that the energetics of vertical climbing and biomechanical constraints imposed by small branch feeding sites in the periphery of trees may constrain the arboreal behavior of male gorillas. Fine-tuned comparisons of food availability, tree structure, and variation in social context of behavior across habitats, will assist efforts to understand differences in ecology among populations and species of African apes.