Patterns of wounding in mixed-sex social groups of western lowland gorillas (Gorilla gorilla gorilla)

https://doi.org/10.1016/j.applanim.2021.105226Get rights and content

Highlights

  • Incidences of wounding were evaluated over a 24 month period in 131 gorillas living in mixed-sex groups.

  • Sex differences were observed with females receiving significantly more wounds than males.

  • Wounds were not received uniformly across the body. Wounds to the trunk of adults and face of immatures occurred most frequently.

  • Presence of a neighboring group did not affect wounding further suggesting that this management approach does not negatively affect welfare.

  • These data are informative to guiding husbandry practices and population management of gorillas living in zoos.

Abstract

Long-term, multi-institutional assessments have become a reliable tool for evaluating patterns of wounding in zoo-living primates, with results informing on best practices for species-specific care protocols and population management strategies. For western lowland gorillas (Gorilla gorilla gorilla) there has been no detailed reports focused on mixed-sex groups, the most common social group type for this species in zoos. In this study, wounding data were collected over 26 months for 131 gorillas living in 26 social groups at 18 zoos and analyzed in relation to age, sex, and group demographic variables. Adult females received wounds at significantly higher rates than adult males suggesting sexual dimorphism and the disparate social roles of male and female gorillas may differentially affect wounding rates. Group size was associated with increased wounding in adult gorillas, however, wounding rates overall were low in frequency and severity. Wound locations were not randomly located, with wounds to the trunk accounting for 24 % of all wounds. These data provide useful insights that may help inform and improve management of gorillas in mixed-sex groups. Specifically, understanding that vulnerability to wounding may be more prevalent in females compared to males can inform strategies aimed at improving husbandry, health, and welfare and have a positive impact on population management practices for the species.

Introduction

Social group composition, and the associated behavioural impacts of group dynamics, can have a significant influence on the welfare of animals in human care (Rault, 2012). Although aggressive interactions may be typical of social species and may be worrisome when observed by caregivers, they have been described as an enriching aspect of social life (de Waal, 1992). Aggression resulting in injury can, however, negatively affect animal welfare. Those caring for animals should be aware of factors contributing to wounding in order to minimize occurrences that may ultimately affect an animal’s welfare. North American zoos have recently taken a strong interest in this area of study regarding primates, coordinating long-term, multi-institution studies of wounding in chimpanzees (Pan troglodytes; Ross et al., 2009), western lowland gorillas (Gorilla gorilla gorilla; Leeds et al., 2015), hamadryas baboons (Papio hamadryas; Wiley et al., 2018), and Japanese macaques (Macaca fuscata; Cronin et al., 2020) which have added to a long history of similar research in laboratories (e.g. Baker et al., 2000; Judge et al., 1994; Ruehlmann et al., 1988). Findings from these studies have informed on how both natural history and husbandry contribute to patterns of wounding in each respective species, ultimately providing information to guide care and population management decisions.

Bachelor groups of gorillas are generally considered a temporary social grouping in nature (Gatti et al., 2004), however, they represent a long-term management strategy for zoo populations, providing socialization opportunities for males not immediately required or destined for breeding situations (Lukas et al., 2019). Those caring for and studying bachelor male gorillas in zoos have made large investments in evaluating their behaviour (e.g. Stoinski et al., 2004, 2013). Findings from this research focus have emphasized that age, and not group type, is one of the greatest determinants in male gorilla behavior (Leeds et al., 2015; Stoinski et al., 2013). With respect to agonistic behaviour, the presence of young silverbacks (males aged 14–20 years) corresponds with a doubling of wounding rates in bachelor groups (Leeds et al., 2015); however, less is known about characteristics of wounding in males and females in mixed-sex groups. Leeds et al. (2015) only described basic patterns of wounding in mixed-sex groups, reporting that gorillas in mixed-sex groups averaged approximately three wounds per year and almost 90 % of these wounds were considered a minor scrape and/or cut. No evaluations of wounding in relation to individual or group demographic variables were conducted.

On average, two-thirds of the approximately 350 western lowland gorillas in North American zoos are managed in mixed-sex groups (Lukas et al., 2019). In nature, mixed-sex groups are generally composed of one adult male, on average three to four adult females, and their respective immature offspring (Breuer et al., 2010; Gatti et al., 2004; Stokes, 2004). Therefore, recommendations for mixed-sex groups in zoos follow a similar composition (Lukas et al., 2019). Free-ranging gorilla groups are socially tolerant, highlighted in nature by predominately peaceful intergroup encounters (Cooksey et al., 2020; Magliocca and Gautier-Hion, 2004), and by evidence suggesting fluid group membership (Forcina et al., 2019) and localized dispersion of kin (Bradley et al., 2004; Morrison et al., 2019). Despite these patterns of behaviour, intragroup conflict still occurs (Stokes, 2004).

Field studies of western lowland gorillas suggest the majority of intragroup aggressive interactions occur between adult males and females (Stokes, 2004). Breuer, Robbins and Robbins (2018) evaluated this pattern of behaviour in more detail and found that (1) males directed aggression at females without dependent offspring at higher rates than females with dependent offspring and (2) aggression towards females occurred more frequently in the presence of a conspecific group, indicating male gorillas utilize sexual coercion to maintain females in their groups and preferentially target females more likely to emigrate. Similar patterns of aggression may also occur in zoos and predispose female gorillas to injurious aggression. Furthermore, it draws into question if housing multiple groups at one facility affects the frequency of injurious aggression. In bachelor groups, behavioural differences have been reported in the presence of mixed-sex groups (Grand et al., 2013) but differences in wounding have not been observed (Leeds et al., 2015). This observed difference in bachelor groups, as well as previous primatological studies (Baker et al., 2000; Judge et al., 1994; Ruehlmann et al., 1988) highlight the important observation that aggression does not directly correlate with wounding. Thus to make truly informed husbandry and management decisions to minimize wounding, direct observations of wounds are necessary.

The purpose of this study was to describe patterns of wounding in mixed-sex groups of western lowland gorillas living in North American zoos. Wounding rates were evaluated in relation to eight specific variables: age, sex, presence of a neighboring gorilla group, presence of young offspring, group size, number of adult females, number of non-adults and silverback age. These variables were evaluated as they have been associated with varying rates of aggressive behaviour in previous studies in zoos and in nature. However, because of the indirect relationship between aggression and wounding, a null hypothesis was adopted for all questions.

Section snippets

Study subjects

Study participants were 131 (nmale = 41; nfemale = 70) western lowland gorillas living in 26 social groups at 18 North American zoos as part of the Association of Zoos and Aquariums’ Species Survival Plan® (SSP®). Ages ranged from 1 to 58 years. Group size averaged 5.04 individuals (range: 2–10). All groups contained one adult male (≥17 years of age) except for one containing two adult males (22 and 27 years of age). Feedback from participating institutions suggested identification of wounds

Results

Low-level severity wounds (Severity 1, 2 or 3; Table 1) accounted for 98.7 % (n = 683) of all wounds. Given the rarity of high-level severity wounds (n = 9), no analysis was done to differentiate patterns of wounding severity. An initial model was run with sex as the only fixed factor to determine if sex was a significant predictor of wounding. Adult males received significantly fewer wounds (WGMX = 0.21, SE = 0.08) than adult females (WGM X = 0.38, SE = 0.04; F1,2060 = 12.326, P < 0.001; Fig. 1

Discussion

In nature, mixed-sex gorilla groups are generally described as tolerant of neighboring groups (Cooksey et al., 2020; Magliocca and Gautier-Hion, 2004), however, conflict within groups still occurs (Breuer et al., 2016; Stokes, 2004). How these patterns of conflict occur within zoos has yet to be evaluated. Here patterns of wounding were measured as a proxy for understanding intragroup conflict within a large subset of the North American population. We found that, on average, adult female

Conclusions

Incidences of wounding were evaluated longitudinally over a 26-month period in 131 gorillas living in mixed-sex groups. Sex differences were observed with females receiving significantly more wounds than males. Wound locations were also found to be non-random with the majority of wounds occurring on the trunk of adults and face of immatures. These data are informative to guiding husbandry practices and population management of gorillas living in zoos.

Declaration of Competing Interest

None.

Acknowledgements

The authors would like to thank the animal care staff from the 18 AZA-accredited institutions who collected data reported here. Thanks also go to Brittany Neier, Sarah Boyer, Kaitlin Grell, Becca Evey and Cassie Giarrusso for their assistance with data entry and management.

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