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.     

Sexual behavior and hormonal estrus cycles in captive aged lowland gorillas (Gorilla gorilla)

To evaluate whether observed cycles in proceptive behavior in aging lowland gorilla females (age 40+) at Brookfield Zoo were driven by ovarian activity, we compared monthly behavioral data to estradiol and progestogen cycles based on fecal hormone assessments. Progestogen peaks showed regularity and close coincidence with monthly sexual behaviors. Estradiol was more variable. Progestogen peaks varied between 22+/-5 days for the control female (29 years old), to 24+/-2.5 and 29+/-8 for the two aged subjects. In the first aged female, which was housed with other females and a silverback, the high degree of cyclicity in sexual behavior, regularity of progestogen cycles, and close concordance between hormonal cycling and sexual behavior strongly compared to patterns found (in this and other studies) in gorilla females <35 years old. Cyclical progestogen peaks were longer and more variable in the second aged female-perhaps because she lacked the social mediation of other females or a male. For husbandry reasons she is not housed with the gorilla group, behavioral data were not collected from her. The value of our longitudinal study is in obtaining reproductive profiles of primate females that are approaching maximum lifespan. This pilot study is part of a larger research project on reproductive senescence that will include other captive females >35 years old, a population that is rapidly increasing in North American zoos as gorillas continue to age.     

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.     

Changes in ovarian ultrastructure and ecdysteroid titer during the aging process of female Xyleborus ferrugineus (Coleoptera: Scolytidae)

Striking ultrastructural and hormonal parameters of premature menopause and aging are reported in female Xyleborus ferrugineus fed cholesterol, rather than 7-dehydrocholesterol, as a sole dietary sterol. The titer of free ecdysteroids in such 63-day-old females remained abnormally elevated through the period of the ovarian cycle. A similar plateauing of such elevated titer also occurred in 147-day-old, irregularly cycling females fed only cholesterol as the dietary sterol. These hormonal changes in menopausing X. ferrugineus females seem especially analogous to the maintenance of an elevated concentration of 17-beta-estradiol through the estrous, as well as the proestrous, ovary of aged irregularly cycling rats. The highly abnormal ultrastructure of ovaries of X. ferrugineus females aged 216 days on a diet containing cholesterol as the sole sterol seems quite analogous to that of the nonovulatory follicles in older, irregularly cycling rats. Our new findings involving aging X. ferrugineus females indicate further the usefulness of an insect model to study aging processes.     

Western lowland gorillas (Gorilla gorilla gorilla) change their activity patterns in response to frugivory

The most important environmental factor explaining interspecies variation in ecology and sociality of the great apes is likely to be variation in resource availability. Relatively little is known about the activity patterns of western lowland gorillas (Gorilla gorilla gorilla), which inhabit a dramatically different environment from the well‐studied mountain gorillas (G. beringei beringei). This study aims to provide a detailed quantification of western lowland gorillas' activity budgets using direct observations on one habituated group in Bai Hokou, Central African Republic. We examined how activity patterns of both sexes are shaped by seasonal frugivory. Activity was recorded with 5‐min instantaneous sampling between December 2004 and December 2005. During the high‐frugivory period the gorillas spent less time feeding and more time traveling than during the low‐frugivory period. The silverback spent less time feeding but more time resting than both females and immatures, which likely results from a combination of social and physiological factors. When compared with mountain gorillas, western lowland gorillas spend more time feeding (67 vs. 55%) and traveling (12 vs. 6.5%), but less time resting (21 vs. 34%) and engaging in social/other activities (0.5 vs. 3.6%). This disparity in activity budgets of western lowland gorillas and mountain gorillas may be explained by the more frugivorous diet and the greater dispersion of food resources experienced by western lowland gorillas. Like other apes, western lowland gorillas change their activity patterns in response to changes in the diet. Am. J. Primatol. 71:91–100, 2009. © 2008 Wiley‐Liss, Inc.     

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.     

Physical maturation,life‐history classes and age estimates of free‐ranging western gorillas—insights from Mbeli Bai,Republic of Congo

Physical maturation and life‐history parameters are seen as evolutionary adaptations to different ecological and social conditions. Comparison of life‐history patterns of closely related species living in diverse environments helps to evaluate the validity of these assumptions but empirical data are lacking. The two gorilla species exhibit substantial differences in their environment, which allows investigation into the role of increased frugivory in shaping western gorilla life histories. We present behavioral and morphological data on western gorilla physical maturation and life‐history parameters from a 12.5‐year study at Mbeli Bai, a forest clearing in the Nouabalé‐Ndoki National Park in northern Congo. We assign photographs of known individuals to different life‐history classes and propose new age boundaries for life‐history classes in western gorillas, which can be used and tested at other western gorilla research sites. Our results show that western gorillas are weaned at a later age compared with mountain gorillas and indicate slower physical maturation of immatures. These findings support the risk‐aversion hypothesis for more frugivorous species. However, our methods need to be applied and tested with other gorilla populations. The slow life histories of western gorillas could have major consequences for social structure, mortality patterns and population growth rates that will affect recovery from population crashes of this critically endangered species. We emphasize that long‐term studies can provide crucial demographic and life‐history data that improve our understanding of life‐history evolution and adaptation and help to refine conservation strategies. Am. J. Primatol. 71:106–119, 2009. © 2008 Wiley‐Liss, Inc.     

Socioecological factors influencing population structure of gorillas and chimpanzees

Differences in distribution and density between gorillas and chimpanzees are reconsidered with special reference to population structure. Both ecological and social factors influencing population structure are compared between species and between habitats within species. Gorillas and chimpanzees respond differently to a decline in food quality, such as fruit scarcity: gorillas change diet and decrease range, while chimpanzees do not change diet but may expand range. These responses result in different effects on their grouping patterns. For gorillas the dispersed distribution and reduction of range size decreases the rate of inter-unit encounters and female transfer. The concentration of social units increases the rate of aggressive contact between units and stimulates female transfer. Social units of gorillas may crowd or disperse in order to attain the optimal density. This tendency may result in similar densities of gorillas across habitats. By contrast, the distribution patterns or range size may not affect inter-unit relationships of chimpanzees. Within a single unit-group, various reproductive strategies are adopted by both sexes. Independent travel of females and flexible grouping patterns enable them to survive at very low density in extraordinary large ranges. Density and inter-unit relationships are good criteria for a healthy population of gorillas, while the size of unit-group and inter-individual relationships are good criteria for chimpanzees. Conservation planners should consider these differences for sympatric and allopatric survival in these species.     

Age-related variation in immunity in a wild mammal population

Age‐related changes in immunity are well documented in humans and laboratory mammals. Using blood samples collected from wild Soay sheep, we show that pronounced differences in T‐cell subsets and inflammatory markers amongst age classes are also evident under natural conditions. These shifts parallel those observed in mammals experiencing protected environments. We found progressive declines in the proportion of naïve CD4 T cells with age, a precipitous drop in γδ T cells after the second year of life and an increase in acute phase protein levels amongst geriatric sheep. Our findings suggest immune aging patterns observed in laboratory and domestic mammals may generalize to more complex, challenging environments and could have fitness costs under natural conditions.     

Age-related patterns of reproductive success among female mountain gorillas

A key goal of life history theory is to explain the effects of age and parity on the reproductive success of iteroparous organisms. Age-related patterns may be influenced by changes in maternal experience or physical condition, and they may reflect maternal investment trade-offs between current versus future reproduction. This article examines the influences of age and parity upon the interbirth intervals (IBI), offspring survival, and birth rates of 66 female mountain gorillas in the Virunga Volcano region from 1967-2004. Fertility was relatively low for females below age 12; improved as they matured; and then declined as they aged further. Primiparous mothers had 50% higher offspring mortality and 20% longer IBI than second-time mothers, though only the difference with IBI was statistically significant. The length of subsequent IBI was positively correlated with birth order but not with the mother's age. Mountain gorillas showed no evidence of an extended postreproductive lifespan. Age-related patterns seem most likely to reflect changes in the physical condition of the mother, but more detailed studies are needed to quantify those physical differences, and to obtain behavioral evidence that would provide more direct measures of maternal investment and experience.     

Relative growth,ontogeny, and sexual dimorphism in gorilla(Gorilla gorilla gorilla and G. g. beringei): Evolutionary and ecological considerations

Gorillas are the largest and among the most sexually dimorphic of all extant primates. While gorillas have been incorporated in broad-level comparisons among large-bodied hominoids or in studies of the African apes, comparisons between gorilla subspecies have been rare. During the past decade, however, behavioral, morphological, and molecular data from a number of studies have indicated that the western lowland (Gorilla gorilla gorilla) and eastern mountain (Gorilla gorilla beringei) subspecies differ to a greater extent than has been previously believed. In this study I compare patterns of relative growth of the postcranial skeleton to evaluate whether differences between subspecies result from the differential extension of common patterns of relative growth. In addition, patterns of ontogeny and sexual dimorphism are also examined. Linear skeletal dimensions and skeletal weight were obtained for ontogenetic series of male and female G.g. gorilla (n = 315) and G.g. beringei (n = 38). Bivariate and multivariate methods of analysis were used to test for differences in patterns of relative growth, ontogeny, and sexual dimorphism between sexes of each subspecies and in same-sex comparisons between subspecies. Results indicate males and females of both subspecies are ontogenetically scaled for postcranial proportions and that females undergo an earlier skeletal growth spurt compared to males. However, results also indicate that the onset of the female growth spurt occurs at different dental stages in lowland and mountain gorillas and that mountain gorillas may be characterized by higher rates of growth. Finally, data demonstrate lowland and mountain gorilla females do not differ significantly in adult body size, but mountain gorilla males are significantly larger than lowland gorilla males, suggesting mountain gorillas are characterized by a higher degree of sexual dimorphism in body size. Thus, although lowland and mountain gorillas do not appear to have evolved novel adaptations of the postcranium which correlate with differences in locomotor behavior, the present investigation establishes subspecies differences in ontogeny and sexual dimorphism which may be linked with ecological variation. Specifically, these findings are evaluated in the context of risk aversion models which predict higher growth rates and increased levels of sexual dimorphism in extreme folivores. Am. J. Primatol. 43:1–31, 1997. © 1997 Wiley-Liss, Inc.     

Behavioral indices of estrus in a group of captive African elephants (Loxodonta africana)

This study investigated behavioral signals of estrus by systematically monitoring the interactions of one male with four female African elephants housed in a naturalistic outdoor enclosure at Disney's Animal Kingdom over a period of 11 months. We measured changes in five spatial behaviors and 22 tactile‐contact behaviors, as well as changes in serum progestagen and LH concentrations, across three ovarian cycles for each female. Two females did not cycle during the study. Three different phases of the ovarian cycle were identified: mid luteal, anovulatory follicular, ovulatory follicular. The male followed more and carried out more genital inspections, flehmen, and trunk‐to‐mouth behaviors toward cycling females during their ovulatory phase. Genital inspections by the male peaked above baseline levels on the day of an LH surge, and up to 9 days before, in both cycling females and, thus, might be a useful behavioral index of estrus. The male also carried out more genital inspections, flehmen, and trunk touches to the back leg toward ovulatory cycling than noncycling females. Overall, our results indicated that: 1) a single subadult African elephant male could discriminate two females in the ovulatory phase of their cycle (i.e., during the 3 weeks preceding ovulation) from the mid luteal phase; 2) the male also discriminated two cycling females in the ovulatory and anovulatory follicular phases from two noncycling females; 3) two females in the ovulatory phase of the cycle displayed a greater variety of tactile‐contact behavior toward the male compared to the other cycle phases. Zoo Biol 0:1–19, 2005. © 2005 Wiley‐Liss, Inc.     

Hormonal changes during the mating and conception seasons of wild northern muriquis (Brachyteles arachnoides hypoxanthus)

We investigated hormonal and behavioral changes in wild male and female northern muriquis (Brachyteles arachnoides hypoxanthus) at the Estação Biológica de Caratinga, Minas Gerais, Brazil, during a 6‐mo period that encompassed the onset of the 1998–1999 mating and conception seasons. Individual females resumed mating with the resumption of ovarian cycling, which was not synchronized among them or related to their cortisol levels. Females experienced two to seven cycles prior to conceiving, and the first conception occurred 2 mo after the onset of the group's mating season. There were no differences in female cortisol levels across their premating, mating, and conception conditions. Cortisol levels were significantly higher in females than in males prior to the conception season, consistent with the prediction that energy reserves may be associated with breeding readiness in females, but not males, in this species. The sustained elevation in male cortisol occurred after the peak in their sexual activity, which resulted in the first conception of the year. Male cortisol levels were positively correlated between years that were similar in rainfall, but differed in the timing of sexual and reproductive events. The timing of cortisol elevations in males appears to be generally regulated by environmental cues, but is responsive to fine‐tuning by social and behavioral cues related to the unpredictable timing of reproductive opportunities within their extended mating season. Am. J. Primatol. 61:85–99, 2003. © 2003 Wiley‐Liss, Inc.     

Infanticide and social flexibility in the genus <Emphasis Type="Italic">Gorilla</Emphasis>

Based on the cases of infanticide by male mountain gorillas reported from the Virunga volcanic region, the socioecological and life history features of gorillas satisfy the conditions for which infanticide may be expected. However, there are considerable variations in the occurrence of infanticide between habitats. We analyze the recent reports of infanticides that were directly observed or are suspected based on field evidence in two populations of eastern and western lowland gorillas (Kahuzi and Mbeli Bai, respectively) along with previous reports on mountain gorillas, and consider which social features are linked with and which factors influence the occurrence of infanticide in the gorilla populations. All victims were suckling infants and most of them were killed by males who seemed unrelated to them. Dependent infants are most vulnerable to infanticide when the protector male (its putative father in most cases) is absent, and so male protection ability seems to be important in determining female transfer decisions. Two cases observed in Kahuzi suggest that the infanticidal male may discriminate between infants to accept and those to kill according to his previous interactions with their mothers. Mating for a prolonged period prior to parturition is necessary for immigrant females to avoid infanticide by the new male of the group that they join. Infanticide was usually associated with female transfer, and the patterns of female association at transfer may shape variations in social structure between populations. Female mountain gorillas prefer large groups with multiple males and tend to transfer alone in order to seek more protection against infanticide in Virunga. By contrast, female eastern and western lowland gorillas tend to transfer with other females to small groups or solitary males, and maturing silverbacks take females to establish new groups through group fission in Kahuzi and Mbeli Bai. These differences may result in more multi-male and larger groups in the Virungas than in Kahuzi and Mbeli Bai. Rapid changes in density of gorilla social units and their relations following drastic environmental changes caused by recent human disturbances may also increase the probability of infanticide.     

Behavioral and Hormonal Analysis of Social Relationships Between Oldest Females in a Wild Monogamous Group of Common Marmosets (Callithrix jacchus)

Social organization in common marmosets (Callithrix jacchus) appears to be based on a hierarchy of dominance, in which dominant females limit the reproductive functioning of subordinates. Reduced fertility was documented for captive but not for wild females. We investigated social dynamics and hormonal profiles of females living in a wild group. We collected behavioral and hormonal data during 7 mo from the reproductive and two nonreproductive adult females (probably daughters), and their interactions with the reproductive male. The subjects lived in a monogamous group at Nísia Floresta field station in northeastern Brazil. We collected behavioral data as all day follows, once a week and fecal material twice a week. We analyzed fecal samples for cortisol and progesterone using enzyme immuno assay (EIA) techniques. During the first three months we monitored the reproductive female (GRE) and the oldest nonreproductive female, (GIO). After the disappearence of GIO, we monitored the next oldest subordinate adult female, (GRA). GRE had just given birth at the beginning of the study, and she gave birth 6 mo later. During the last 2 weeks of GRE's gestation, GRA showed elevated progesterone levels, indicating ovarian cycling. Cortisol levels from GRE and GRA reached very high levels then. During GRA's cycling period, her interactions with the reproductive male (GT) were lower than when she was not cycling. We saw sexual interactions only between GT and GRE. The results provide evidence that ovarian function in mature subordinate females might be affected by the reproductive condition of the dominant female.     

Mountain gorilla life histories,reproductive competition,and sociosexual behavior and some implications for captive husbandry

New data on modal patterns of, and variability in, mountain gorilla life history tactics and sociosexual behavior, collected during long-term fieldwork at the Karisoke Research Centre, are presented here. These data show that immature males and females develop sociosexual relationships with both peers and adults, and that these relationships—which contribute to more complex social relationships—often lead to mating between natal females and males who are “familiar” partners. They also show that within-group reproductive competition between males can reach considerable levels, but that immature males are sometimes able to copulate with both nulliparous and parous females; that unrelated silverbacked males can reside in the same all-male group but become intolerant of each other if females join the group; and that, whereas females sometimes compete with each other for opportunities of copulate, their probability of conceiving is probably not decreased by any inability to control the mating situation. This information may contribute to continued improvement in the husbandry of captive gorillas.     

Social and ecological factors alter stress physiology of Virunga mountain gorillas (Gorilla beringei beringei)

Living in a rapidly changing environment can alter stress physiology at the population level, with negative impacts on health, reproductive rates, and mortality that may ultimately result in species decline. Small, isolated animal populations where genetic diversity is low are at particular risks, such as endangered Virunga mountain gorillas (Gorilla beringei beringei). Along with climate change‐associated environmental shifts that are affecting the entire population, subpopulations of the Virunga gorillas have recently experienced extreme changes in their social environment. As the growing population moves closer to the forest's carrying capacity, the gorillas are coping with rising population density, increased frequencies of interactions between social units, and changing habitat use (e.g., more overlapping home ranges and routine ranging at higher elevations). Using noninvasive monitoring of fecal glucocorticoid metabolites (FGM) on 115 habituated Virunga gorillas, we investigated how social and ecological variation are related to baseline FGM levels, to better understand the adaptive capacity of mountain gorillas and monitor potential physiological indicators of population decline risks. Generalized linear mixed models revealed elevated mean monthly baseline FGM levels in months with higher rainfall and higher mean maximum and minimum temperature, suggesting that Virunga gorillas might be sensitive to predicted warming and rainfall trends involving longer, warmer dry seasons and more concentrated and extreme rainfall occurrences. Exclusive use of smaller home range areas was linked to elevated baseline FGM levels, which may reflect reduced feeding efficiency and increased travel efforts to actively avoid neighboring groups. The potential for additive effects of stress‐inducing factors could have short‐ and long‐term impacts on the reproduction, health, and ultimately survival of the Virunga gorilla population. The ongoing effects of environmental changes and population dynamics must be closely monitored and used to develop effective long‐term conservation strategies that can help address these risk factors.     

Characterization of dermatologic changes in geriatric rhesus macaques

Abstract: Nonhuman primates are frequently used for aging studies. We observed a high prevalence of skin disease among a group of geriatric rhesus monkeys (mean age ? 25 years; n ? 9) used in aging behavioral studies. Gross and histopathologic changes in the skin of these geriatric rhesus were compared with skin from control adult monkeys (mean age ? 10; n ? 4) and sun-exposed monkeys (mean age ? 11; n = 4) to characterize age-related skin changes. Biopsy specimens were taken from four specified skin locations (lateral to bridge of nose, ventral midline, dorsal midline, perineal area) and from additional areas where skin lesions were present. Samples were routinely processed and evaluated by light microscopy. Blood samples were collected and tested for estrogen, thyroid-stimulating hormone, triiodothyronine, thyroxine, and Cortisol levels. The axilla was swabbed and samples were obtained for bacterial culturing. All nine of the geriatric monkeys had notable dermal lesions, while one of the control monkeys and one of the sun-exposed monkeys had abnormal findings. Major gross findings included increased areas of erythematous skin, wrinkling, focal skin scaling, thinning of hair, foot calluses, and exudative lesions. Histologic skin changes included subacute dermatitis, acanthotic dermatitis, and a lesion resembling an early solar lentigo in the sun-exposed animal. These changes were not associated with hormonal abnormalities or bacterial pathogens. Histologic changes are compatible with nonspecific skin changes observed in elderly humans. This study establishes a baseline of dermatologic changes of the aging rhesus macaque.     

Testosterone and Visceral Fat in Midlife Women: The Study of Women's Health Across the Nation (SWAN) Fat Patterning Study

Visceral fat (VF) increases with the menopause and is an independent predictor of the metabolic syndrome, diabetes, and cardiovascular disease (CVD) in women. Little is known about how hormonal changes during the menopausal transition are related to the increase in VF. We aimed to determine the relationship between bioavailable testosterone and VF in middle‐aged women at various stages of the menopausal transition and whether this relationship is independent of age and other CVD risk factors. The Study of Women's Health Across the Nation (SWAN) is a longitudinal, community‐based study. This report uses baseline data from a population‐based longitudinal ancillary study at the Chicago site to examine the cross‐sectional relationship between testosterone and computed tomography (CT)–assessed VF in women at different stages of the menopausal transition. Included are 359 women (47.2% black), aged 42–60 years, who were randomly selected from a complete community census in which a 72% participation rate was achieved. In multivariate models, bioavailable testosterone was associated with VF independent of age, race, percent total body fat, and other cardiovascular risk factors. Bioavailable testosterone was a stronger predictor than estradiol and was interchangeable in its strength of association with sex hormone–binding globulin (SHBG). As bioavailable testosterone was associated with VF even after adjusting for insulin resistance, this suggests that it plays an important role in regional fat distribution. Our findings may have direct implications in explaining the effect of menopause‐related testosterone predominance on VF accumulation and subsequent cardiovascular risk.     

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.