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.     

Genetic analysis reveals population structure and recent migration within the highly fragmented range of the Cross River gorilla (Gorilla gorilla diehli)

Recently developed methods of individual-based analysis of genetic data allow an unprecedented opportunity to understand the relationships among fragmented populations. By defining population structure and identifying migrant individuals, such analyses can provide a framework to aid in evaluating the threats posed by inbreeding and reduced genetic variability as a consequence of limited gene flow among fragments. Here we investigate population structure in the critically endangered Cross River gorilla (Gorilla gorilla diehli) by applying a suite of individual-based analyses to data obtained from between one-quarter and one-third of the estimated total population through the use of noninvasively collected DNA samples. The population structure inferred using data from 11 autosomal microsatellite loci was broadly consistent with geography and habitat fragmentation, but showed no simple isolation-by-distance effects. In contrast to previous field surveys, which suggested that all gorilla localities were isolated from one another, we infer low levels of gene flow and identify migrants between habitat fragments as well as individuals of admixed ancestry, suggesting persistent recent reproductive contact between many of the localities. These results are encouraging for the conservation of the Cross River gorilla population. Conservation efforts should strive to maintain connectivity between subpopulations that are still in migratory contact and attempt to restore connectivity where it has been lost.     

Behavioral Responses of Silverback Gorillas (Gorilla gorilla gorilla) to Videos

This study examined the impact of video presentations on the behavior of 4 silverback, western lowland gorillas (Gorilla gorilla gorilla). On each of 5 occasions, gorillas viewed 6 types of videos (blue screen, humans, an all-male or mixed-sex group engaged in low activity, and an all-male or mixed-sex group engaged in agonistic behavior). The study recorded behavioral responses and watching rates. All gorillas preferred dynamic over static videos; 3 watched videos depicting gorillas significantly more than those depicting humans. Among the gorilla videos, the gorillas clearly preferred watching the mixed-sex group engaged in agonistic behavior; yet, this did not lead to an increase in aggression or behavior indicating agitation. Further, habituation to videos depicting gorillas did not occur. This supports the effectiveness of this form of enrichment, particularly for a nonhuman animal needing to be separated temporarily due to illness, shipment quarantine, social restructuring, or exhibit modification.     

Third-Party Intervention in Wild Western Lowland Gorillas (Gorilla gorilla gorilla)

International Journal of Primatology -     

Taste Responses to Fructose and Tannic Acid Among Gorillas (Gorilla gorilla gorilla)

We investigated the taste responses to fructose and tannic acid compounds among 6 western gorillas (Gorilla gorilla gorilla) at the San Francisco Zoo. We presented the subjects with 12 reference concentrations of 3–300 mM fructose, paired with tap water in paired-choice experimental trials. We subsequently presented them with tap water paired with 8 dilute concentrations of tannic acid (0.25–6 mM) dissolved in 100 mM fructose solutions and 8 concentrations (0.25–6 mM) dissolved in 300 mM fructose solutions. The gorillas exhibited a broadly similar preference threshold for fructose (50 mM) to those of other nonhuman primate and human samples. The gorillas tolerated moderate levels of tanninc acid, especially when presented in a very sweet package. The depressing effect of tannic acid on the ingestion of fructose solutions appeared to increase progressively with tannin concentration and was lower as fructose concentration increased. The inhibition threshold for tannic acid solutions was reached at 4 mM tannic acid in 100 mM fructose solution, and at 6 mM tannic acid in 300 mM fructose solutions. These results suggest that gorillas use sweetness as a criterion for food selection and regard those with moderate levels of tannins as palatable. Our findings corroberate food preference studies and nutritional analyses of wild gorilla foods indicating that they prefer sugary foods and readily consume ones containing moderate levels of tannins. Taste responses may facilitate a flexible frugivorous/folivorous diet among gorillas.     

Socioecological Factors Influencing Intergroup Encounters in Western Lowland Gorillas (Gorilla gorilla gorilla)

International Journal of Primatology - Socioecological variables influence the rate and nature of encounters between conspecific primate groups. Both social (i.e., type of encounter) and ecological...     

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.     

Gorilla Mothers Also Matter! New Insights on Social Transmission in Gorillas (Gorilla gorilla gorilla) in Captivity

The present paper describes two distinct behaviors relating to food processing and communication that were observed in a community of five separately housed groups of lowland gorillas (Gorilla gorilla gorilla) in captivity during two study periods one decade apart: (1) a food processing technique to separate wheat from chaff, the so-called puff-blowing technique; and (2) a male display used to attract the attention of visitors, the so-called throw-kiss-display. We investigated (a) whether the behaviors were transmitted within the respective groups; and if yes, (b) their possible mode of transmission. Our results showed that only the food processing technique spread from three to twenty-one individuals during the ten-year period, whereas the communicative display died out completely. The main transmission mode of the puff-blowing technique was the mother-offspring dyad: offspring of puff-blowing mothers showed the behavior, while the offspring of non- puff-blowing mothers did not. These results strongly support the role mothers play in the acquisition of novel skills and vertical social transmission. Furthermore, they suggest that behaviors, which provide a direct benefit to individuals, have a high chance of social transmission while the loss of benefits can result in the extinction of behaviors.     

The Intestinal Entodiniomorph Ciliates of Wild Lowland Gorillas (Gorilla gorilla gorilla) in Gabon,West Africa

Examination of 109 fecal samples from wild lowland gorillas revealed the presence of five species of entodiniomorph ciliates: Troglodytella ahrassarti, Troglodytella gorillae, and three unclassified species. These latter three species were also found in the feces of a captive gorilla in Gabon and are considered to be intestinal parasites or commensals.     

Effects of Selected Behavioral Enrichment Devices on Behavior of Western Lowland Gorillas (Gorilla gorilla gorilla)

Environmental complexity plays an integral role in the activity and psychological well-being of primates. The experiment described in this article evaluates the effects of nonintrusive, inexpensive, and easily managed behavioral enrichment devices on the behavior of a group of captive Western lowland gorillas. Devices used included cardboard boxes containing food items, paper bags containing food items, burlap rags, and willow and maple browse. The enrichment devices increased foraging, social play, and solitary play behaviors. Sedentary behaviors decreased. Rags, bags, browse, and boxes did not statistically decrease the incidence of regurgitation/reingestation or coprophagy. Depending on the type of enrichment item used, the effects on agonism and manipulation of enrichment item were variable. To make informed management decisions about the psychological well-being of captive animals, it is important to objectively quantify and examine the influences on their behavior.     

Food-Associated Calling in Gorillas (Gorilla g. gorilla) in the Wild

Many nonhuman primates produce food-associated vocalizations upon encountering or ingesting particular food. Concerning the great apes, only food-associated vocalizations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) have been studied in detail, providing evidence that these vocalizations can be produced flexibly in relation to a variety of factors, such as the quantity and quality of food and/or the type of audience. Only anecdotal evidence exists of eastern (Gorilla beringei) and western gorillas (Gorilla gorilla) producing food-associated vocalizations, termed singing or humming. To enable a better understanding of the context in which these calls are produced, we investigated and compared the vocal behavior of two free-ranging groups of western lowland gorillas (Gorilla g. gorilla) at Mondika, Republic of Congo. Our results show that (a) food-associated call production occurs only during feeding and not in other contexts; (b) calling is not uniformly distributed across age and sex classes; (c) calls are only produced during feeding on specific foods; and (d) normally just one individual gives calls during group feeding sessions, however, certain food types elicit simultaneous calling of two or more individuals. Our findings provide new insight into the vocal abilities of gorillas but also carry larger implications for questions concerning vocal variability among the great apes. Food-associated calls of nonhuman primates have been shown to be flexible in terms of when they are used and who they are directed at, making them interesting vocalizations from the viewpoint of language evolution. Food-associated vocalizations in great apes can offer new opportunities to investigate the phylogenetic development of vocal communication within the primate lineage and can possibly contribute novel insights into the origins of human language.     

Effects of selected behavioral enrichment devices on behavior of Western Lowland Gorillas (Gorilla gorilla gorilla)

Environmental complexity plays an integral role in the activity and psychological well-being of primates. The experiment described in this article evaluates the effects of nonintrusive, inexpensive, and easily managed behavioral enrichment devices on the behavior of a group of captive Western lowland gorillas. Devices used included cardboard boxes containing food items, paper bags containing food items, burlap rags, and willow and maple browse. The enrichment devices increased foraging, social play, and solitary play behaviors. Sedentary behaviors decreased. Rags, bags, browse, and boxes did not statistically decrease the incidence of regurgitation/reingestation or coprophagy. Depending on the type of enrichment item used, the effects on agonism and manipulation of enrichment item were variable. To make informed management decisions about the psychological well-being of captive animals, it is important to objectively quantify and examine the influences on their behavior.     

Gorillas (Gorilla gorilla gorilla) in the Likouala swamp forests of north central Congo: Preliminary data on populations and ecology

Until recently it was generally believed that gorillas avoid water. In two recent, independent faunal surveys in the Likouala swamp of north central Congo, we have found that gorillas occur at high densities, extending the known range of Gorilla gorilla gorillavery close to the Oubangui River. The gorillas in this area fed on the abundant ground vegetation of herbaceous monocotyledons in the swamp forest. In adjacent islands of terra firmathere was little evidence of gorilla nesting or feeding, indicating that they spend at least part of the year almost exclusively in the swamp forests. Recent reports from several sites in West Africa indicate that swamp forests may generally provide abundant food resources that are used regularly by gorillas.     

The birth of a wild mountain gorilla (Gorilla gorilla beringei)

The live birth of a wild mountain gorilla is described. Details of the birth process are given along with the mother’s periparturitional behaviour and the responses of others to the birth. The mother, a multiparous female, gave no observable indication that birth was imminent, and delivered her infant quickly and easily. During parturition, a subadult male showed interest in the proceedings. Following delivery, several animals approached the mother, who then left the birth site. The mother cleaned her infant and by two hours postpartum had established ventro-ventral contact and, possibly, nursing. These data are related to relevant observations from the literature and unpublished data on other animals from this study.     

Habitat Use and Structural Preferences of Captive Western Lowland Gorillas (Gorilla gorilla gorilla(: Effects of Environmental and Social Variables

Exhibit naturalism has become the architectural standard for new zoo exhibits, yet our scientific understanding of how animals interact with naturalistic environments is very limited. Given the expense incurred in building new exhibits, it is essential that more information on animal-environment interactions be obtained and that it be incorporated at the outset into exhibit designs. We documented four years of habitat use and structural preferences of western lowland gorillas at Zoo Atlanta. We found that quality of space rather than quantity of space was important, as subjects spent 50% of their time in <15% of the exhibits. Subjects showed strong preferences for the areas near structures, particularly the holding building, and spent significantly less time away from structures than expected. Temperature interacted with structural preferences in that the subjects spent more time away from structures when temperatures were cold and more time near them when temperatures were hot. There was no difference in habitat use and structural preferences due to age, sex, and rearing history, but social factors appear to play a role. Our results are similar to those found a decade earlier in the same population and to other studies of space use in apes but are the first to include significant temperature effects. Additionally, they suggest that managers and designers need to take into account factors such as quality of space, attractiveness of the holding building, and the interaction of structural preferences with variables such as temperature and social factors when designing exhibits.     

A Case Study of Primiparous Maternal and Infant Gorilla (Gorilla gorilla gorilla) Behavior

A primiparous mother and her infant were subjects of a longitudinal behavioral study at Lincoln Park Zoo in Chicago, Illinois. From November 1998 to November 1999, we collected a total of 100 hr of focal nonhuman animal instantaneous point sampling and all occurrence data on the mother and her infant. After 8 months, we introduced the 4-member focal group to an 8-member gorilla group, thus providing an opportunity to study the effect of the introduction on mother-infant behavior. Overall, time the pair spent physically apart was notably high with contact and proximate scores steadily decreasing over time. The infant was largely responsible for maintaining social proximity, rarely leaving the mother and frequently approaching her. In contrast, the mother left the infant frequently and approached the infant minimally. Maternal affiliative and nonaffiliative behaviors fluctuated throughout the study. Over time, both decreased. Despite a precarious maternal relationship, infant developmental trends were typical for captive gorillas. The results of this study suggest that allowing flexibility in judging maternal conduct can be of benefit to successful gorilla husbandry.