Ecology of gorillas and its relation to female transfer in mountain gorillas

Understanding the principles that underly primate social evolution depends on integrated analysis of data on behavioral ecology, demography, life history tactics, and social organization. In this paper, data on the behavioral ecology of gorillas are reviewed and comparisons made among the three subspecies. Gorillas are selective feeders; and, their patterns of food choice are consistent with models of feeding by large generalist herbivores. They rely heavily on terrestrial herbaceous vegetation, which provides an abundant supply of densely distributed food. Availability of this food varies little in space and time; and, gorilla foraging activity can maintain its productivity. The level of frugivory and the extent of seasonal variation in diet and habitat use vary among and within populations. Low variability in food distribution patterns makes cooperative defense of foraging areas not worthwhile; but, it also means that ecological costs associated with gregariousness are low. However, demographic and life history data on mountain gorillas show that these costs may be sufficient to reduce female reproductive success as group size increases. Advantages to being with high quality males apparently can outweigh these costs. The implications of these data for the evolution of the mountain gorilla social system, and the possible roles of male protection, predation, and female/female competition in this regard, are discussed.     

Sharing natural resources: mountain gorillas and people in the Parc National des Volcans,Rwanda

The compatibility of natural resource use by people and mountain gorillas (Gorilla beringei beringei) within the Parc National des Volcans was studied. The distribution of gorillas was modelled using a Maximum Entropy algorithm. Biophysical predictor variables were trained with daily GPS locations of gorillas during 2006. Elevation, as a climate surrogate, was the best predictor (58%) of the occurrence of gorillas. The mid‐altitudes (2500–3500 m a.s.l.) contained the bulk of the gorilla groups. Incoming solar radiation, as proxy for comfortable nesting sites, was the second best predictor (17%). Vegetation types, as foliage provider, (13%) and slope steepness for providing security (12%) were contributing predictors. The modelled and actual gorilla distributions were together overlaid with people’s resource use in the park. Both people and gorillas were congregated in the areas identified as most suitable for gorillas. However, within these areas spatial segregation was found between human natural resource‐users and gorillas. Therefore, the number of gorillas is likely to be limited by the human natural resource use within the park. A perimeter fence, the introduction of community‐based natural resource management, and a buffer zone are discussed as short‐, medium‐ and long‐term mitigation measures.     

Intestinal parasites and bacteria of mountain gorillas (Gorilla beringei beringei) in Bwindi Impenetrable National Park, Uganda

A survey in 1994 examined intestinal helminths and bacterial flora of mountain gorillas (Gorilla beringei beringei) in Bwindi Impenetrable National Park, Uganda. Parasites and bacteria were identified to genus in the feces of two groups of tourist-habituated and one group of non-tourist-habituated mountain gorillas. Eggs were identified as those of an anoplocephalid cestode, and nematode eggs representative of the genera: Trichuris, Ascaris, Oesophagostomum, Strongyloides, and Trichostrongylus. This is the first report of Ascaris lumbricoides-like eggs in mountain gorillas. Fecal samples (n=76) from all groups contained helminth eggs, with strongyle eggs and anoplocephalid eggs being the most common. Salmonella and Campylobacter were found in both gorilla groups. Regular long-term non-invasive fecal monitoring of the populations of mountain gorillas is essential for the prevention and identification of potential health threats by intestinal parasites and bacteria in this highly endangered subspecies.This revised version was published online in April 2005 with corrections to the cover date of the issue.     

An experimental study of nettle feeding in captive gorillas

Mountain gorillas (Gorilla beringei beringei) in Karisoke, Rwanda, feed on the stinging nettle Laportea alatipes by means of elaborate processing skills. Byrne [e.g. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences 358:529–536, 2003] has claimed that individuals acquire these skills by means of the so‐called program‐level imitation, in which the overall sequence of problem‐solving steps (not the precise actions) is reproduced. In this study we present western lowland gorillas (Gorilla gorilla gorilla) with highly similar nettles. Twelve gorillas in three different groups (including also one nettle‐naïve gorilla) used the same program‐level technique as wild mountain gorillas (with differences mainly on the action level). Chimpanzees, orangutans, and bonobos did not show these program‐level patterns, nor did the gorillas when presented with a plant similar in structural design but lacking stinging defenses. We conclude that although certain aspects (i.e. single actions) of this complex skill may be owing to social learning, at the program level gorilla nettle feeding derives mostly from genetic predispositions and individual learning of plant affordances. Am. J. Primatol. 70:584–593, 2008. © 2008 Wiley‐Liss, Inc.     

Clinical and pathologic manifestation of oesophagostomosis in African great apes: does self‐medication in wild apes influence disease progression?

Nodular worms (Oesophagostomum spp.) are common intestinal parasites found in cattle, pig, and primates including humans. In human, they are responsible for serious clinical disease called oesophagostomosis resulting from the formation of granulomas, caseous lesions or abscesses in intestinal walls. In wild great apes, the fecal prevalence of this parasite is high, but little information is available concerning the clinical signs and lesions associated. In the present study, we describe six cases of multinodular oesophagostomosis in free-ranging and ex-captive chimpanzees and captive gorillas caused by Oesophagostomum stephanostomum. While severe clinical signs associated with this infection were observed in great apes raised in sanctuaries, nodules found in wild chimpanzees do not seem to affect their health status. One hypothesis to explain this difference would be that in wild chimpanzees, access to natural environment and behavior such as rough leaves swallowing combined with ingestion of plants having pharmacological properties would prevent severe infection and decrease potential symptoms.     

Ranging Patterns of a Western Gorilla Group During Habituation to Humans in the Dzanga-Ndoki National Park,Central African Republic

The ranging patterns of western gorillas have been described in relation to their more frugivorous diet as compared to mountain gorillas. However, the lack of data from habituated groups prevents a full understanding of their ranging behavior and its implications for western gorilla social behavior. Unhabituated gorillas often flee from approaching researchers, possibly resulting in longer daily ranges and home range expansions and/or shifts. I report on the influence of habituation on the ranging patterns of a western gorilla group. I analyzed progress in habituation through changes in gorilla reactions, displayed upon detecting the observers. I measured daily path length and home range while following the group's trails and monitored diet via fecal analyses and direct observations. During the study period, habituation progressed and the gorillas increasingly ignored the arrival of observers. As a result, daily path lengths were longer during the early stages of habituation, when the gorillas were avoiding observers. There was no range expansion or shift, though the measure is more difficult to investigate, being subject to error due to variation in sample size. We recorded seasonal changes in the consumption of fruit by gorillas, but no direct effect on ranging patterns is detectable with the data available, which is likely a consequence of the combined effect of habituation and seasonality on group movements. The findings are not only important for understanding the extent of human influence while conducting research on wild gorillas but also suggest the need for caution when interpreting results from non-habituated gorillas.     

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.     

Effects of hand‐rearing on the reproductive success of western lowland gorillas in North America

This study sought to assess the potential effects of hand‐rearing by evaluating the relationships among rearing type and reproductive success in the American Zoo and Aquarium Association's Species Survival Plan^® for western lowland gorillas. Our study included data on 697 gorillas: 257 wild‐born (WB) and 440 born at zoos or related facilities in North America. We found no significant differences in the number of reproductive zoo‐born (ZB) and WB females, but more WB males sired infants than their ZB counterparts. This was influenced by a skew in the number of reproductive years for WB males in the studbook. ZB males showed no difference in infants produced per reproductive year, as compared to WB males, while ZB females produced more infants per reproductive year than did WB females. Mother‐reared (MR), ZB females produced more offspring and used more reproductive opportunity than hand‐reared (HR) females, whereas rearing had no effect on the reproductive success of ZB males. Moreover, MR and partially hand‐reared (PHR) females were more likely to become nurturing mothers themselves. Zoo Biol 21:389–401, 2002. © 2002 Wiley‐Liss, Inc.     

Regurgitation and reingestion in captive gorillas: Description and intervention

Direct observation of regurgitation and reingestion (r/r) of captive gorillas was used to describe and catalog this behavior and thus lay the groundwork for possible intervention in the r/r cycle. Analysis of questionnaires regarding 117 gorillas at 17 zoos suggests that social deficits during early development contribute to the occurrence of r/r later in life. Wild-caught and captive-born, hand-reared gorillas show higher incidence of r/r than that of mother-reared infants. R/r increases the daily ingestion time. Feeding browse decreases r/r and increases time spent feeding from about 11% of the day to 27% of the day.