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.     

Genital swelling in females of the monogamous gibbon, Hylobates (H.) lar.

Cyclical changes in the vulvae of five adult lar gibbons (Hylobates [H.] lar) were studied and compared with those of eight lowland gorillas. The results reveal that the gibbons have relatively conspicuous and specialized sexual swellings that alter shape and appearance during the ovarian cycle. At maximum extent, the genital swellings of gorillas are relatively and absolutely smaller than those of gibbons, and lack the distinctive coloration seen in the genital swellings of the smaller apes. We conclude that the female gibbon's sexual swelling is a far more conspicuous and effective signal of estrus status than that of the gorilla, and that this is not explicable in terms of allometry. Previous investigators have pointed to one-male mating systems, monogamous pair-bonding, or an arboreal habitat as reasons that some primates should have less conspicuous signals of estrus than others. Our findings for the gibbon are the reverse of these predictions, and indicate that sexual selection other than by intermale competition for estrous females is implicated in the ultimate causation of the gibbon's swelling. The adaptive value and significance of the female gibbon's sexual signals remain unclear, however.     

Comparative performance of orangutans (Pongo spp.), gorillas (Gorilla gorilla gorilla), and drills (Mandrillus leucophaeus), in an ephemeral foraging task

A goal of the comparative approach is to test a variety of species on the same task. Here, we examined whether the factors that helped capuchin monkeys improve their performance in a dichotomous choice task would generalize to three other primate species: orangutans, gorillas, and drill monkeys. In this task, subjects have access to two options, each resulting in an identical food, but one (the ephemeral option) is only available if it is chosen first, whereas the other one (the permanent option) is always available. Therefore, the food‐maximizing solution is to choose the ephemeral option first, followed by the permanent option for an additional reward. On the original version (plate task), the options were discriminated by the color and pattern of the plates holding the food, while on two subsequent versions we used altered cues that we predicted would improve performance: (1) the color of the foods themselves (color task), which we hypothesized was relevant to primates, who choose foods rather than substrates on which foods are found when foraging, and (2) patterned cups covering the foods (cup task), which we hypothesized would help primates avoid the prepotent response associated with visible food. Like capuchins, all three species initially failed to solve the plate task. However, while orangutans improved their performance from the plate to the color task, they did not for the cup task, and only a few gorillas and no drills succeeded in either task. Unfortunately, our ability to interpret these data was obscured by differences in the subjects' level of experience with cognitive testing and practical constraints that precluded the use of completely identical procedures across species. Nonetheless, we consider what these results can tell us, and discuss the value of conducting studies across multiple sites despite unavoidable differences.     

Comparing ape densities and habitats in northern Congo: surveys of sympatric gorillas and chimpanzees in the Odzala and Ndoki regions

The conservation status of western lowland gorillas and central chimpanzees in western equatorial Africa remains largely speculative because many remote areas have never been surveyed and the impact of emergent diseases in the region has not been well documented. In this study, we compared ape densities and habitats in the Lokoué study area in Odzala National Park and the Goualougo Triangle in Nouabalé-Ndoki National Park in northern Republic of Congo. Both of these sites have long been considered strongholds for the conservation of chimpanzees and gorillas, but supposedly differ in vegetative composition and relative ape abundance. We compared habitats between these sites using conventional ground surveys and classified Landsat-7 ETM+ satellite images. We present density estimates via both standing-crop and marked-nest methods for the first time for sympatric apes of the Congo Basin. The marked-nest method was effective in depicting chimpanzee densities, but underestimated gorilla densities at both sites. Marked-nest surveys also revealed a dramatic decline in the ape population of Lokoué which coincided with a local Ebola epidemic. Normal baseline fluctuations in ape nest encounter rates during the repeated passages of marked-nest surveys were clearly distinguishable from a 80% decline in ape nest encounter rates at Lokoué. Our results showed that ape densities, habitat composition, and population dynamics differed between these populations in northern Congo. We emphasize the importance of intensifying monitoring efforts and further refinement of ape survey methods, as our results indicated that even the largest remaining ape populations in intact and protected forests are susceptible to sudden and dramatic declines.     

Food transfers in immature wild western lowland gorillas (Gorilla gorilla gorilla)

The transfer of food items between individuals has been described in primates as serving an informative purpose in addition to supplementing the diet of immature individuals. This behaviour has yet to be described in western lowland gorillas (Gorilla gorilla gorilla), and results are presented here of observations of food transfers in immature gorillas at Mbeli Bai, Republic of Congo. The frequency of food transfers decreased with increasing immature age, while the frequency of independent feeding and processing of food increased. Transfers between mothers and infants were the most frequent, with infants attempting to take items from the mother. These attempts were not always successful and the item was relinquished on less than 50% of attempts. Mothers also took items from their offspring. The results point to the functional significance of food transfers in western lowland gorillas being informational. In a bai environment, where one species forms the majority of a visiting gorilla’s diet despite other species being available, the initiation of food transfers by immatures is proposed to serve the purpose of familiarising them with which species, and which parts of those species, may be eaten.     

Nuclear integrations of mitochondrial DNA in gorillas

Great ape systematics, particularly at the species level and below, is currently under debate, due in part to the recent influx of molecular data. The phylogenies of previously published mitochondrial control region (or D-loop) DNA sequences in gorillas show deep splits within West African gorillas (Gorilla gorilla gorilla), and very high levels of nucleotide diversity in this subspecies. Here we demonstrate that several previously reported D-loop haplotypes from West African gorillas are in all likelihood nuclear integrations of mitochondrial DNA. Revised estimates of the amount and pattern of mitochondrial DNA diversity in gorillas are provided, revealing two reciprocally monophyletic and highly divergent groups of gorillas, concurrent with their geographic distribution.     

Are the gorillas in Bwindi Impenetrable National Park “true” mountain gorillas?

The gorillas that inhabit Bwindi Impenetrable National Park in Uganda are the least known of the eastern gorillas. Because they are an allopatric population living a minimum of 25 km from the well‐studied population of mountain gorillas (Gorilla beringei beringei) in Rwanda and have certain morphological and ecological differences from these gorillas, their taxonomic status has been in question in recent years. This study presents new craniodental metrics from Bwindi individuals and compares them to Virunga individuals as well as to eastern lowland gorillas, G. gorilla graueri. Multivariate statistics, including MANCOVA, least‐squares, regression, and principal components analyses, were used to evaluate how closely the Bwindi crania resemble the Virunga crania and how both relate to G. g. graueri. Results indicate that the Bwindi gorillas have generally smaller crania than the Virunga gorillas, but when metrics are log‐transformed, the only variable that distinguishes the Bwindi individuals is a longer face. When both populations are compared to G. g. graueri, they cluster together separately from the eastern lowland gorillas, sharing such features as higher rami, wider bigonia, longer mandibles, and wider and shorter mandibular symphyses in relation to G. g. graueri. Functional morphological explanations for these differences are discussed, but lacking measurements of the physical properties of G. g. graueri, they cannot fully be explained. Results clearly indicate that at least pertaining to the cranium, upon which most gorilla taxonomy is based, the Bwindi gorillas are proper mountain gorillas (G. b. beringei). Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Potential for female kin associations in wild western gorillas despite female dispersal

Female philopatry and male dispersal are the norm for most mammals, and females that remain in their natal region often derive foraging or social benefits from proximity to female kin. However, other factors, such as constraints on group size or a shortage of potential mates, may promote female dispersal even when female kin associations would be beneficial. In these cases, female kin associations might develop, not through female philopatry, but through female emigration to the same group. To date, little attention has been focused on the potential for kin-biased behaviour between females in female-dispersing species. Here we investigate the genetic relationships among adults in eight wild groups of unhabituated western gorillas (Gorilla gorilla) at the Mondika Research Center using microsatellite genotyping of DNA collected from hair and faeces. We found that almost half (40%) of adult females had an adult female relative in the same group and average within-group relatedness among females was significantly higher than that expected under a model of random dispersal. This provides the first genetic evidence that females can maintain social associations with female relatives in spite of routine natal and secondary dispersal. In addition, we show that females appear to avoid related silverback males when making dispersal decisions, suggesting that a strategy of non-random female dispersal may also function to avoid inbreeding.     

Manual pressure distribution patterns of knuckle‐walking apes

Differences in how the hands of gorillas and chimpanzees contact the ground while knuckle walking have been noted but generally not quantified. It is widely believed that gorillas maintain a pronated arm and contact the ground with digits 2–5 consistently, while chimpanzees have variable arm position and digit contact. To further test these generalizations, distribution of pressure across the manus, peak digital pressures, and hand position were quantified using a pressure mat in eight captive chimpanzees (Pan troglodytes) and seven gorillas (Gorilla gorilla). Chimpanzees and gorillas make initial ground contact with the ulnar aspect of the hand and pressure moves radially. They differ in which digit usually makes final contact and receives maximum pressure, and hand position during contact. Gorillas regularly use a palm‐back hand position and touch‐off with digit 2. They show less variation in pressure application across the digits. Chimpanzees are more variable in hand position and pressure application. In both, hand position plays a key role in determining which digit acts as the final touch‐off element. Am J Phys Anthropol 152:44–50, 2013. © 2013 Wiley Periodicals, Inc.