Nest Building in Captive Gorilla gorilla gorilla

Although various aspects of gorilla nest building have been described in wild populations, nest-building behavior of captive gorillas has not been subject to much scientific review. We observed nest building in 17 gorillas during three periods: summer baseline, winter baseline, and winter treatment, in which the amount of available nesting material was doubled. We conducted observations exclusively in the indoor holding area in the hour following evening departure of animal care staff. During baseline, gorillas engaged in nest-building on 3.1% of scans and were on a constructed nest on 27.9% of scans. Overall, gorillas spent significantly more time on elevated nests than on floor nests. There were no statistically significant sex, age class, or rearing history differences in nest building, time spent on a nest, or nest location preference. Nest building consisted of both gathering and manipulating materials. The gorillas spent significantly more time building nests in winter than in summer. Additionally, they were more often on elevated nests during winter than summer. Doubling the amount of nesting material did not increase nest building, but it increased the number of floor nests. Our results suggest that providing adequate materials to captive gorillas for nest building may facilitate performance of species-typical nest-building, minimize competition among individuals for nesting sites and materials, and permit individual thermoregulation.     

Socialization processes in a female lowland gorilla

A low-interactive, captive, female lowland gorilla, Molly, was studied following the introduction into her enclosure of three gorillas, two males and a female, raised from birth in captive gorilla groups. Observations were made 6 mo after the new gorillas were introduced. Throughout the period of observation, Molly interacted in an affiliative manner with one of the males, playing or sitting quietly with him in a tree (where Molly spent most of her time) and occasionally on the ground. Agonistic displays between Molly and the new female decreased after they were released in the enclosure without the males for a series of days. Molly, however, continued to react to the other male, the most dominant, in an agonistic manner, and usually retreated from his reach, climbed the tree, and/or grimaced and piloerected whenever he approached. Although Molly's continued avoidance of the dominant male impeded her complete socialization, we propose that the interventions employed in this study—introduction of new younger gorillas into and an enclosure, and a series of dyadic separations between the noninteractive gorilla and each of the new group members—are possible strategies that can be used to facilitate socialization of captive, noninteractive gorillas.     

Genetic identification of elusive animals: re-evaluating tracking and nesting data for wild western gorillas

Western gorillas Gorilla gorilla have been exceedingly difficult to habituate to the presence of human observers. Nevertheless, researchers have amassed a wealth of information on population densities and group structure for this ape species by locating and counting the sleeping nests of wild individuals. Such nest-count studies have suggested that western gorilla groups often have multiple silverbacks and these multimale groups occasionally divide into smaller subgroups. However, observational data from forest clearing sites and from a few recently habituated western gorilla groups show no evidence of multimale family groups or of subgrouping. This discrepancy underscores a long-standing question in ape research: How accurately do nesting sites reflect true group compositions? We evaluated these indirect measures of group composition by using DNA from faeces and hair to genetically identify individual gorillas at nesting sites. Samples were collected from unhabituated wild western gorillas ranging near Mondika Research Center in the Central African Republic and Republic of Congo. DNA extracted from these samples was genotyped at up to 10 microsatellite loci and one X–Y homologous locus for sex identification. Individuals were then identified at nesting sites by their unique multilocus genotypes, thus providing a 'molecular census' of individual gorillas. Results confirm that western gorillas often build more than one nest at a nesting site and, thus, nest counts can be highly inaccurate indicators of group size and composition. Indeed, we found that nest counts can overestimate group size by as much as 40%, indicating that true gorilla population numbers are probably lower than those reported from census surveys. This study demonstrates how genetic analysis can be a valuable tool for studying and conserving elusive, endangered animals.     

Development of Independence from the Mother in Gorilla gorilla gorilla

In gorillas, the mother takes full responsibility for carrying and feeding offspring, and lactational amenorrhea prevents her from investing in another offspring while suckling. Therefore, mothers should encourage immature gorillas to become gradually more independent until they are physically able to acquire sufficient resources, travel independently, and manage social relationships unaided. We investigated the development of independence in a population of wild western gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Nouabalé-Ndoki National Park, Republic of Congo. We observed the spatial proximity, the development of travel, patterns of suckling and weaning, and the nature of mother-offspring interactions for 42 immature gorillas to investigate the nature of the relationship between mother and offspring. Though mother-offspring distance and independent travel increased during immaturity, with few active interactions between the dyads, offspring still continued to suckle until a median age of 4.6 yr, longer than in mountain gorillas. The tolerance of the mother toward suckling by older offspring may reflect the importance of continued nutritional investment: the temporal nature of the diet of western gorillas means that succulent fruits—likely to be preferred weaning foods—are not always available. We propose that to buffer against potential fruit shortage, mothers continue to nurse their offspring to decrease the likelihood of death from care-dependent factors and to improve an offspring’s chances of survival.     

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.     

A preliminary behavioral comparison of two captive all-male gorilla groups

Gorillas live in polygamous harem groups, generally composed of one male, several adult females, and their offspring. With an equal numbers of male and female gorillas born in captivity, however, housing gorillas in social breeding units inevitably means that some males will not have access to female social partners. Thus, the future of the captive gorilla population depends on the collective ability of zoos to house equal numbers of males and females. This study examined the behavioral profiles of two all-male groups of captive lowland gorillas (Gorilla gorilla gorilla) to provide information on this type of housing situation. One group consisted of three subadult individuals, while the other consisted of two subadults and a silverback. Data were collected during two 6-month intervals, for a total of 284 hr. The behavioral profiles of the animals were stable over the course of the study but proximity patterns changed. Differences in feeding, solitary play, and object-directed behavior were found between groups, while no significant differences were observed in affiliative or agonistic social behavior. At both institutions, group cohesion appeared to be high, particularly between subadults; these individuals spent approximately 10% of their time engaging in social behavior and 25-50% of their time in close proximity (within 5 m). However, the Zoo Atlanta males spent significantly more time within 1 m and 5 m of each other than the Santa Barbara males, which may reflect a higher level of cohesiveness among members of the Zoo Atlanta group. The behavioral profiles of the animals in this study were similar to those found in bachelor groups of wild mountain gorillas. One notable exception was the absence of homosexual behavior between the silverback and subadults in Santa Barbara and the low frequency of this behavior between subadults in both groups. Although more longitudinal data are needed, these data suggest that all-male groups can be a feasible housing strategy for males at certain periods of their life span. Zoo Biol 20:27-40, 2001. Copyright 2001 Wiley-Liss, Inc.     

Non‐invasive fecal hormone analysis and behavioral observations for monitoring stress responses in captive western lowland gorillas (Gorilla gorilla gorilla)

Non‐invasive techniques for monitoring the stress response in captive western lowland gorillas (Gorilla gorilla gorilla) were investigated. Fecal samples for cortisol measurement and concurrent behavioral data were collected from six individuals in a socially housed gorilla group (one adult male, three adult females and their three offspring) over a 7‐month period. Despite inter‐individual variation in the dynamics of fecal cortisol concentrations over time, several major secretory peaks coincided across individuals. High cortisol concentrations in feces were correlated with induced stressors or behavioral observations indicating high social tension, with a 1–2 day lag period. Entry progression order of the gorillas into a den complex and a supplant‐based dominance index were suitable indicators of overall dominance hierarchies, and fluctuations over time reflected periods of instability. Diurnal variation in fecal cortisol was not apparent when comparing afternoon and morning samples, however the sample collection interval was relatively short (3–5 hr). These results demonstrate the feasibility of monitoring stress responses based on the dynamics of both fecal cortisol excretion and behavior. This non‐invasive approach may be used for gauging responses to changes in husbandry, environment and group structure of captive gorillas. Zoo Biol 0:1–15, 2005. © 2005 Wiley‐Liss, Inc.     

Insectivory by Gorilla gorilla gorilla in Southeast Cameroon

Our study extends quantitative analyses of insect-eating by gorillas (Gorilla gorilla gorilla) to Cameroon. During a 2-mo period (May–June 2001), we recorded and analyzed feeding traces on plants and insects and in gorilla feces. We found 180 feeding traces, 17% of which involved insectivory. Seventy-eight percent of the fecal samples (n = 36) contained insects. Ants were found in 61% of the samples, termites in 39%, while 56% of the samples contained remains of other insects. We added 14 new species to the known insect diet of western lowland gorillas. Overall, social insects are predominant. The choice of prey by the Ntonga gorillas gives new clues for the existence of cross-cultural differences among gorilla populations. A comparison of the overall frequency of insectivory with those at other sites in Central Africa indicate a possible effect of forest disturbance on the insectivorous behavior of gorillas. The study suggests the existence of temporal variation in ant- and termite-eating by gorillas.     

Long‐term proximity relationships in a captive social group of western lowland gorillas (Gorilla gorilla gorilla)

To visualize long‐term social relationships among 12 gorillas in a captive breeding group at the San Diego Wild Animal Park, San Diego, CA, the multidimensional scaling (MDS) procedure was applied to proximity (within 5 m) values collected in five different periods over 2.5 years. The resulting two‐dimensional representations clearly show that in samples taken during this 2.5‐year period, the silverback male was surrounded by five adult females, while three infant or young juvenile females, gathered in a body, remained between the silverback male and adult females. Some adult females maintained proximity to the silverback male more frequently than other adult females throughout the five periods. Unlike mountain gorillas in the wild, females with dependent offspring did not tend to stay near the silverback male more frequently than other females, and related females did not tend to spend more time near each other than non‐related females. Three older juvenile or young adult males were plotted the furthest from the silverback male, with gradual changes with increasing age. The usefulness of the MDS procedure, which can be used to visualize easily and clearly the social relationships among individuals, is discussed from the viewpoint of the management of breeding groups of captive gorillas. Zoo Biol 20:197–209, 2001. © 2001 Wiley‐Liss, Inc.     

Preference for structural environmental features in captive lowland gorillas (Gorilla gorilla gorilla)

Increased interest in the behavioral effects of captive environments and advances in zoo design have resulted in a proliferation of “naturalistic” animal habitats. Although scientists have demonstrated that such exhibits have some positive effects on behavior, very little is known about the complex relationships between these specialized environments and behavior. This study sought to determine preferences for structural environmental features, such as slope and trees, exhibited by captive lowland gorillas (Gorilla gorilla gorilla). An average of 496 location scans were collected on each of 23 gorillas, housed in 6 groups. The results indicate that enclosure usage was significantly associated with the features of slope, nearness to holding facility, and presence of environmental components additional to substrate (e.g., tree or rock). In particular, areas with flat ground that were near the holding building and that included at least one component additional to substrate were preferred by all animals. © 1993 Wiley-Liss, Inc.     

Testicular atrophy in captive gorillas (Gorilla g. gorilla)

Testes were obtained at post mortem from three Western lowland gorillas, two Bornean orang-utans and one Chimpanzee. All specimens were adults which had died in captivity. Histological examination revealed that two gorillas exhibited complete degeneration of the seminiferous tubules and that the testes contained large amounts of interstitial tissue. Some, less obvious, signs of degeneration were noted in the third specimen but poor preservation of the material hampered interpretation of these features. The testes of the Orang-utans and Chimpanzee were structurally normal. Testicular atrophy in captive gorillas may be more common than is generally realized and there is a need for research on fertility and reproductive endocrinology of male specimens. At the moment it is not possible to identify the causes of testicular dysfunction in captive gorillas.     

Sexual and Hormonal Cycles in Geriatric Gorilla gorilla gorilla

As our closest living relatives, great apes likely experience behavioral and physiological patterns associated with reproductive aging and menopause that are similar to human patterns. We present results from a nationwide zoo-based study on behavioral and hormonal changes in female western gorillas. We evaluated progestogen concentrations via daily fecal sampling in 30 gorillas, 22 of which were geriatric (≥30). We collected concurrent behavioral data 1–3 times weekly on 16 of the females. While control females cycled regularly, ca. 23% of geriatric females are acyclic (menopausal), and another 32% show variable hormonal patterns suggesting perimenopause. Patterns included increased variability in cycle length and peak progestogen values, and frequent insufficient increases in progestogen levels during the luteal phase. Acyclic females have significantly lower overall progestogen concentrations than the self cycling females, though differences are not significant when cycle phase is incorporated. We detected behavioral estrus in 9 of 10 cycling females for which data were available. In all but 1 case, proceptive behavior occurred during the follicular phase, preceding ovulation on average by 6.6 d. Females spent more time in proximity to the silverback male while in behavioral estrus than during other periods. To date, maximum longevity in captive female gorillas is 52 yr, with poor reproductive prognosis beginning from the age of 37. We demonstrate that both perimenopause and menopause characterize aged female gorillas, which may experience a postreproductive lifespan of >25% of their lives. Continued study of aging apes is warranted, and apes may serve as models for age-related reproductive changes in humans.     

Spatial memory and foraging competition in captive western lowland gorillas (Gorilla gorilla gorilla)

Spatial memory and foraging competition were investigated in three mother/offspring pairs of western lowland gorillas,Gorilla gorilla gorilla, using a naturalistic foraging task at the Toronto Zoo. Sixteen permanent food sites were placed throughout the animals’ enclosures. All of the sites were baited and a pair of animals was free to visit the sites and collect the food. Five of the subjects collected the food with accuracy better than chance. Most of the subjects visited the sites using a pattern, and for half the subjects this was one of adjacency. The high accuracy of five of the subjects and the lack of a consistent adjacency pattern suggest that the animals did in fact use spatial memory. Furthermore, the gorillas tended to avoid visiting food sites that had been previously depleted by their partner. They also appeared to split their search of the enclosures, each visiting only a proportion of the food sites. This indicated that the animals were competing and altering their foraging behaviour based on the behaviour of their partner. Therefore, accuracy was recalculated to take this into account. When the sites depleted by either animal in a pair during a given trial were worked into the accuracy calculations for individual animals, three of the animals still maintained accuracy above chance. This suggests that the animals were not only able to remember which sites they had depleted, but those sites depleted by their foraging partner as well.     

Social structure and life-history patterns in western gorillas (Gorilla gorilla gorilla)

Life-history traits and ecological conditions have an important influence on primate social systems. Most of what we know about the life-history patterns and social structure of gorillas comes from studies of eastern gorillas (Gorilla beringei sp.), which live under dramatically different ecological conditions compared to western gorillas (Gorilla gorilla sp.). In this paper we present new data on western gorilla social structure and life histories from four study sites, and make comparisons with eastern gorilla populations. Data were obtained from two study sites with gorilla groups undergoing the habituation process (Lossi, Democratic Republic of Congo and Bai Hokou, Central African Republic) and two "bai" studies (Maya Nord and Mbeli Bai, Republic of Congo). The size and structure of these groups were similar to those seen in eastern gorillas. However, differences in the occurrence of various group transitions (group formations, changes between one-male and multimale composition, and group disintegrations) exist, and western gorillas notably exhibit much higher rates of male emigration and correspondingly fewer multimale groups compared to mountain gorillas. Certain phenomena have been observed only rarely, including predation by leopards. The preliminary data show no significant differences in birth rates between western gorillas and mountain gorillas. The ecological variability across gorilla habitats likely explains the flexibility in the social system of gorillas, but we need more information on the social relationships and ecology of western gorillas to elucidate the causes for the similarities and differences between western and eastern gorillas on the levels of individuals, social groups, and population dynamics.     

Proximity patterns of female western lowland gorillas (Gorilla gorilla gorilla) during the six months after parturition

Studies of wild mountain gorillas have demonstrated strong preferences among new mothers for the silverback. Protection against infanticide has been theorized to be the primary cause for this attraction. We examined social partner preferences in captive western lowland mothers during the 6 mo after parturition and found that juveniles and females were the primary members of the new mothers' social network. Mothers spent significantly more time in close proximity with both of these subgroups than with the silverback. Proximity patterns changed over time: new mothers spent more time near females in the month after parturition than in the month prior to parturition, and spatial proximity increased again in months 2-6 postparturition. These findings differ from those reported for wild mountain gorillas, which may reflect the lack of infanticide risk in captivity. Given current hypotheses that infanticide may be a limited in western lowland gorillas, the social partner preferences observed here may be indicative of patterns in wild populations.     

The genetic and demographic status of the Western lowland gorilla (Gorilla g. gorilla) in captivity

There are over 600 gorillas in captivity of which 43% are now captive born. Because the wild population is threatened and trade represents a major threat to its long-term survival, the captive population needs to be managed to be demographicalry stable over the long term, while retaining a substantial amount of genetic variation. Projected trends in population size and structure, based on recent fecundity and survival rates, suggest that the captive population will decline slowly unless infant mortality rates can be reduced. From a genetic standpoint, however, the large number of wild-born animals and large population size suggest that there should be no difficulty in achieving genetic aims to preserve 90% heterozygosity over the next 200 years.     

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.     

Use of space in a non-naturalistic environment by chimpanzees (Pan troglodytes) and lowland gorillas (Gorilla gorilla gorilla)

Chimpanzees and gorillas are the two most common species of great ape in captive facilities in North America. This study examined patterns of space use by 14 gorillas and six chimpanzees housed in similar non-naturalistic environments at Lincoln Park Zoo in Chicago, IL. The location of each individual was recorded in relation to elements of the environment over a two-year period. These data were compared to volumetric measurements of the enclosures to determine “preferences” for particular environmental elements. Chimpanzees preferred the highest tier of the enclosure and the gorillas preferred the floor level. Both species showed preferences for doorways, corners and the mesh barriers adjacent to keeper areas. These data supplement data from wild populations of apes and provide information useful for those seeking to design new ape enclosures or renovate existing facilities.     

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.     

Genetic diversity of North American captive‐born gorillas (Gorilla gorilla gorilla)

Western lowland gorillas (Gorilla gorilla gorilla) are designated as critically endangered and wild populations are dramatically declining as a result of habitat destruction, fragmentation, diseases (e.g., Ebola) and the illegal bushmeat trade. As wild populations continue to decline, the genetic management of the North American captive western lowland gorilla population will be an important component of the long‐term conservation of the species. We genotyped 26 individuals from the North American captive gorilla collection at 11 autosomal microsatellite loci in order to compare levels of genetic diversity to wild populations, investigate genetic signatures of a population bottleneck and identify the genetic structure of the captive‐born population. Captive gorillas had significantly higher levels of allelic diversity (t₇ = 4.49, P = 0.002) and heterozygosity (t₇ = 4.15, P = 0.004) than comparative wild populations, yet the population has lost significant allelic diversity while in captivity when compared to founders (t₇ = 2.44, P = 0.04). Analyses suggested no genetic evidence for a population bottleneck of the captive population. Genetic structure results supported the management of North American captive gorillas as a single population. Our results highlight the utility of genetic management approaches for endangered nonhuman primate species.