Effects of group dynamics and diet on the ranging patterns of a western gorilla group (Gorilla gorilla gorilla) at Bai Hokou, Central African Republic

This study describes how group dynamics and diet have influenced the ranging patterns of a western gorilla group at Bai Hokou, Central African Republic. The results are compared with those from an earlier study [Cipolletta, International Journal of Primatology, 2003], when the same group was larger and undergoing the process of habituation to humans. Data were obtained from maps of the gorillas' travel routes, direct observations, and analysis of fecal samples. Through the years, the group has experienced a decrease in size, from eight to three individuals, with periods of membership fluctuation. The male's search for new mates resulted in a larger home range than was recorded when the group consisted of more individuals. Moreover, despite an average group size of three throughout this study, the monthly range and mean daily path length (DPL) were also larger when the group was acquiring/losing members in new areas, than when no new members joined or left the group. Fruit was consumed year-round, although more heavily so during wet months. The influence of fruit consumption on the ranging patterns was concealed initially by the effect of habituation [Cipolletta, International Journal of Primatology, 2003], and later (at least partially) by the male's search for new mates. In the last 14 months of the study, when the group numbered only three individuals and was ranging in a restricted area, the average DPL, but not the monthly range, increased when the gorillas were consuming more fruit.     

Simulation of the population dynamics and social structure of the Virunga mountain gorillas

An agent-based model was developed to simulate the growth rate, age structure, and social system of the endangered mountain gorillas (Gorilla beringei beringei) in the Virunga Volcanoes region. The model was used to compare two types of data: 1) estimates of the overall population size, age structure, and social structure, as measured by six censuses of the entire region that were conducted in 1971-2000; and 2) information about birth rates, mortality rates, dispersal patterns, and other life history events, as measured from three to five habituated research groups since 1967. On the basis of the research-group data, the "base simulation" predicted a higher growth rate than that observed from the census data (3% vs. 1%). This was as expected, because the research groups have indeed grown faster than the overall population. Additional simulations suggested that the research groups primarily have a lower mortality rate, rather than higher birth rates, compared to the overall population. Predictions from the base simulation generally fell within the range of census values for the average group size, the percentage of multimale groups, and the distribution of females among groups. However, other discrepancies predicted from the research-group data were a higher percentage of adult males than observed, an overestimation of the number of multimale groups with more than two silverbacks, and an overestimated number of groups with only two or three members. Possible causes for such discrepancies include inaccuracies in the census techniques used, and/or limitations with the long-term demographic data set obtained from only a few research groups of a long-lived species. In particular, estimates of mortality and male dispersal obtained from the research groups may not be representative of the entire population. Our final simulation addressed these discrepancies, and provided a better basis for further studies on the complex relationships among individual life history events, group composition, population age structure, and growth rate patterns.     

Yaws disease in a wild gorilla population and its impact on the reproductive status of males

We evaluated the prevalence of skin lesions in a gorilla population in the Republic of Congo. The observed lesions were typical of yaws, a treponematosis described in gorillas and humans living in tropical regions. Among the 377 gorillas identified, 17% presented skin lesions, mainly on their faces. The worst cases presented physical handicaps because of the deep lesions. As in humans, lesions break out when individuals are young. Lesions were more prevalent among males than females above 8 years old. This sex-bias prevalence could result from the behavioral characteristics of males through a greater exposure to wounds. Lesions were also more prevalent in unmated adult males (either solitaries or those living in nonbreeding groups) than in males leading breeding groups. In the case of the latter, nonaffected and affected leading males had a similar number of infants and juveniles. Still, none of the leading males ever presented serious handicaps because of the skin lesions. This suggests that adult females could favor males without lesions. Finally, lesions were more prevalent among immature animals in nonbreeding groups than in breeding groups, suggesting that either young animals with lesions disperse earlier from their natal groups, or that the disease spreads faster in nonbreeding groups. Our results provide some insights into the spread of a disease in a wild population. Further studies are required to determine if the vigor of males affects the development of the disease and if affected individuals experience social discrimination inducing a negative impact on population dynamics.     

Functional analysis of social staring behavior in an all-male group of mountain gorillas

A total 202 social staring episodes (prolonged gazing by one individual toward another within a short distance) were observed in various social contexts among six unrelated, adult, and subadult male mountain gorillas. Staring was not accompanied by distinct facial expressions by actors or recipients, irrespective of their age or dominance rank. Younger, subordinate animals tended to stare at elder, dominant animals more frequently than vice versa. Staring may have multiple functions depending on the social context. In the initiation of non-agonistic interactions, staring rarely occurred, but was very successful for younger males in eliciting play or homosexual interactions from older animals. Staring was also directed by younger males to older males for greeting or appeasement. It may possibly play a role in reducing the increased social tension that occurs during or after conflict and in averting the potential conflict among older males. Younger males occasionally supplanted older males by staring at feeding spots. This slow supplantation is similar to the phenomenon of food sharing achieved through begging behavior in chimpanzees and bonobos. A prolonged gaze including staring by subordinates towards dominants may characterize the frequent and prolonged face-to-face interactions in the African great apes, and contrasts with a frequent gaze aversion by subordinates towards dominants in macaques or baboons. A difference between gorillas and other apes is that, chimpanzees and bonobos make positive contact with each other through eye contact, while gorillas simply stare at another without physical contact in greeting or appeasement process. Staring may serve an effective strategy for younger male gorillas to intervene safely in olders' conflict and sometimes to suppress or inhibit olders' performance, but their non-agonistic contacts or supporting attacks may not contribute to the establishment or support of social bonds between them. It is possible that staring may be common among the African great apes and man, and that it has evolved as a tactics to mask the dominant/subordinate relationships between individuals with multiple functions.     

Sex differences in play among western lowland gorilla (Gorilla gorilla gorilla) infants: implications for adult behavior and social structure

Mammalian play is believed to improve motor skills as well as facilitate the development of social relationships. Given the marked sexual dimorphism in gorilla body size and the role assumed by the male in protecting the group from conspecifics and predators, the motor-training hypothesis of play predicts that male infants should exhibit higher frequencies of social play than female infants, and that males should prefer to play with other males. Given that adult female gorillas are strongly attracted to adult breeding males and form only weak social bonds with unrelated adult females, the social-relationship hypothesis of play predicts that female infants should prefer to play with males. These hypotheses were tested in a 22-month study of 12 gorilla infants, aged between 0-5 years, living in three zoological parks in Chicago and Atlanta. Consistent with the hypotheses, male infants played more than female infants did, and both male and female infants preferred to play with males rather than with females. These findings suggest that sex differences in play in the great apes and other primates can be predicted by the characteristics of adult behavior and social structure above and beyond the patterns of sex-biased dispersal or coalition formation with same-sex kin.     

Seasonality in the Ecology and Life Histories of Mountain Gorillas (Gorilla gorilla beringei)

The abundance of food, especially that of fruit and often that of young leaves, varies considerably over time for most primates. This variation can depend on or be independent of seasonality in rainfall. Mountain gorillas (Gorilla gorilla beringei) in the Virungas are exceptional: their habitat contains almost no edible fruit, and they mostly eat perennially available herbs and vines that are densely and evenly distributed in much of their habitat. Earlier studies documented little consistent temporal variation in mountain gorilla diets and habitat use, except for seasonal use of bamboo by some groups, and documented no birth seasonality. Long-term data ( 7 years) on 6 mountain gorilla groups confirm these results for habitat use, except that they show some seasonality in use of the upper altitudinal extremes of the gorillas' home ranges for unclear reasons. Relatively low and inconsistent variation in habitat quality over time should lower the costs of grouping for gorillas compared to other apes. Long-term data also confirm the absence of seasonality in births and conceptions. However, they show that mortality rates and risk of respiratory infections vary directly with rainfall. These relationships are probably causal and may be mediated through thermoregulatory stress.     

Feeling a bit peckish: Seasonal and opportunistic insectivory for wild gorillas

Objectives

Insectivory likely contributed to survival of early humans in diverse conditions and influenced human cognitive evolution through the need to develop harvesting tools. In living primates, insectivory is a widespread behavior and frequently seasonal, although previous studies do not always agree on reasons behind this. Since western gorillas (Gorilla gorilla) diet is largely affected by seasonal variation in fruit availability, we aimed to test three non-mutually exclusive hypotheses (habitat use, frugivory and rainfall) to explain seasonality in termite feeding across age/sex classes in three habituated groups (N_individuals = 27) in Central Africa.

Materials and Methods

We used 4 years of ranging, scan and continuous focal sampling records of gorillas (N_{ranging days} = 883, N_scans = 12,384; N_hours = 891) in addition to 116 transects recording vegetation and termite mound distribution.

Results

Depending on the age/sex classes, we found support for all three hypotheses. Time spent in termite-rich vegetation positively impacted termite consumption in all age/sex classes, but subadults. Lengthier travels increased termite feeding in females but decreased it in subadults. Frugivory decreased termite consumption in adults. Daily rainfall had a positive effect on termite feeding and foraging in silverbacks and juveniles, but a negative effect in subadults. For females, rainfall had a positive effect on termite feeding, but a negative effect for termite foraging.

Discussion

In great apes, seasonal insectivory seems to be multifactorial and primarily opportunistic with important differences among age/sex classes. While insectivory has potentials to be traditional, it likely played a crucial role during primate evolution (including ours), allowing diet flexibility in changing environments.     

The Choice to Access Outdoor Areas Affects the Behavior of Great Apes

Outdoor access is often cited as a critical component of appropriate housing for great apes in captivity, and although studies have shown that offering primates choices can improve welfare, choice to access specific areas has been empirically neglected. Behavioral data were collected on chimpanzees and gorillas housed in naturalistic enclosures while (a) restricted to an indoor enclosure and (b) permitted free access to an adjacent outdoor area. To isolate the factor of choice, only the sessions in which apes remained indoors were compared. With choice, chimpanzees showed more frequent social, F(1, 5) = 20.526, p = .006, and self-directed behaviors, F(1, 5) = 13.507, p = .014, and lower inactivity levels, F(1, 5) = 9.239, p = .029. Gorillas were more frequently inactive, F(1, 8) = 22.259, p = .002, and produced lower levels of object manipulation, F(1, 8) = 8.243, p = .021, and feeding, F(1, 8) = 5.407, p = .049. Results are consistent with an association between choice and the expression of species-typical and arousal behaviors in chimpanzees. The effects are less evident in gorillas, but this outcome may be buffered by the species' lower motivation to utilize the outdoor spaces. Findings highlight species-specific reactions to access to choice that may offer insight for enclosure design, management, and nonhuman animal welfare.     

Diet and Mandibular Morphology in African Apes

Investigations seeking to understand the relationship between mandibular form, function, and dietary behavior have focused on the mandibular corpus and symphysis. African apes vary along a gradient of folivory/frugivory, yet few studies have evaluated the morphology of the mandibular corpus and symphysis in these taxa, and the investigations have yielded mixed results. Specifically, studies using external metrics have identified differences in mandibular proportions that analysis of cortical bone distribution has not substantiated. I contribute to the ongoing debate on the relationship between jaw form and dietary behavior by comparing mandibular corporal and symphyseal shapes in African apes. Importantly, and in contrast to previous studies of African ape internal geometry, I include the Virunga mountain gorillas (Gorilla beringei beringei), the ape most specialized toward a folivorous diet. I test the hypotheses that 1) Gorilla beringei beringei always has significantly more robust mandibular corpora and symphyses, relative to mandibular length, than all other African apes and 2) all gorillas have significantly more robust mandibular corpora and symphyses, relative to mandibular length, than Pan. Results demonstrate that the folivorous mountain gorillas consistently exhibit a relatively wider mandibular symphysis and corpus than all other African apes. Furthermore, all gorillas consistently exhibit relatively more robust mandibular corporal and symphyseal dimensions than Pan. The results indicate that among African apes, mountain gorillas are better able to counter lateral transverse bending (wishboning) loads at the symphysis and torsional loads at the corpus. All gorillas are likewise better able to resist wishboning and vertical bending at the symphysis, and sagittal bending and torsion at the corpus, than Pan, findings that are consistent with masticating relatively tougher foods, repetitive loading of the jaws, or both. I offer possible explanations for the lack of concordance in results between studies that have analyzed the biomechanical properties of African ape mandibles and others that have relied on external metrics. More comprehensive study of the internal geometry of the mandible is needed to resolve whether African apes differ morphologically in ways predicted by dietary variation.