skip to main content

Attention:

The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 5:00 PM ET until 11:00 PM ET on Friday, June 21 due to maintenance. We apologize for the inconvenience.


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

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.

 
more » « less
Award ID(s):
1552185
NSF-PAR ID:
10461653
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Ecology and Evolution
Volume:
9
Issue:
9
ISSN:
2045-7758
Page Range / eLocation ID:
p. 5248-5259
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract Objectives

    Several theories have been proposed to explain the impact of ecological conditions on differences in life history variables within and between species. Here we compare female life history parameters of one western lowland gorilla population(Gorilla gorilla gorilla) and two mountain gorilla populations(Gorilla beringei beringei).

    Materials and Methods

    We compared the age of natal dispersal, age of first birth, interbirth interval, and birth rates using long‐term demographic datasets from Mbeli Bai (western gorillas), Bwindi Impenetrable National Park and the Virunga Massif (mountain gorillas).

    Results

    The Mbeli western gorillas had the latest age at first birth, longest interbirth interval, and slowest surviving birth rate compared to the Virunga mountain gorillas. Bwindi mountain gorillas were intermediate in their life history patterns.

    Discussion

    These patterns are consistent with differences in feeding ecology across sites. However, it is not possible to determine the evolutionary mechanisms responsible for these differences, whether a consequence of genetic adaptation to fluctuating food supplies (“ecological risk aversion hypothesis”) or phenotypic plasticity in response to the abundance of food (“energy balance hypothesis”). Our results do not seem consistent with the extrinsic mortality risks at each site, but current conditions for mountain gorillas are unlikely to match their evolutionary history. Not all traits fell along the expected fast‐slow continuum, which illustrates that they can vary independently from each other (“modularity model”). Thus, the life history traits of each gorilla population may reflect a complex interplay of multiple ecological influences that are operating through both genetic adaptations and phenotypic plasticity.

     
    more » « less
  2. Abstract Objectives

    Weaning is a key life history milestone for mammals that represents both the end of nutritional investment from the perspective of mothers and the start of complete nutritional independence for the infants. The age at weaning may vary depending on ecological, social, and demographic factors experienced by the mother and infant. Bwindi mountain gorillas live in different environmental conditions and have longer interbirth intervals than their counterparts in the Virunga Volcanoes, yet other life history characteristics of this population remain less well known. We use long‐term data from Bwindi Impenetrable National Park, Uganda to examine factors related to weaning age.

    Materials and methods

    We analyzed data on infants born in four mountain gorilla groups in Bwindi to quantify their age of weaning (defined as last nipple contact) and to test if the sex of offspring, parity, and dominance rank of mother influences age of weaning. We also compared the age at weaning and time to conception after resumption of mating in Bwindi and Virunga gorillas.

    Results

    Bwindi gorillas were weaned at an average age of 57.5 months. No difference was found between age of weaning for primiparous and multiparous mothers, nor did maternal dominance rank influence age of weaning, but sons were weaned at a later age than daughters. The majority of Bwindi mothers were still suckling when they resumed mating and mothers generally conceived before they weaned their previous offspring. The age of weaning was significantly later in Bwindi than in Virunga gorillas. After mothers resumed mating, the time to conceiving the next offspring was not significantly longer for Bwindi females than Virungas females (6 vs. 4 months).

    Discussion

    Later weaning age for sons than daughters is similar to findings of other studies of great apes. Bwindi mountain gorillas are weaned at approximately the same age as western gorillas and chimpanzees, which is more than a year later than Virunga mountain gorillas. The results of this study suggest that variation in ecological conditions of populations living in close geographic proximity can result in variation in life history patterns, which has implications for understanding the evolution of the unique life history patterns of humans.

     
    more » « less
  3. Abstract Objectives

    Availability of fruit is an important factor influencing variation in great ape foraging strategies and activity patterns. This study aims to quantify how frugivory influences activity budgets across age‐sex classes of mountain gorillas in Bwindi Impenetrable National Park, Uganda.

    Materials and methods

    Daily proportions of fruit‐feeding and activity budgets were calculated using 6 years of observational data on four habituated groups. We fitted generalized linear mixed models to test for age‐sex differences in the amount of fruit‐feeding, and to test whether these factors influence the proportion of time spent feeding, resting, and traveling.

    Results

    Bwindi mountain gorillas spent on average 15% of feeding time consuming fruit, with monthly variation ranging from 0 to 70%. Greater amounts of fruit‐feeding were associated with more time feeding and traveling, and less time resting. Immatures tended to spend more feeding time on fruit than adults, but less overall time feeding and more time traveling. There were no significant differences in the amount of fruit‐feeding and overall feeding time between adult females and silverback males, despite differences in body size.

    Discussion

    This study confirms that gorillas are frugivorous, and only the Virunga mountain gorilla population can be characterized as highly folivorous. Along with other frugivorous great apes, Bwindi mountain gorillas alter their activity patterns in response to varying amounts of fruit in their diet. A better understanding of how variable ecological conditions can drive diversity even within a subspecies has important implications for understanding relationships between ecology, body size, and foraging strategies in great apes.

     
    more » « less
  4. Illegal activities pose challenges to the conservation of mountain gorillas (Gorilla beringei beringei) across the Virunga Landscape (VL). This paper investigates the relationship between household livelihood security (HLS) and the perceived prevalence of illegal activities across the VL. Results from a survey of 223 residents of areas adjacent to the VL in Uganda and Rwanda reveal varied links between human livelihoods and illegal activities threatening wildlife. For example, while poaching appears to be negatively associated with health and financial security among residents, it is positively associated with education security, indicating that education may be contributing to illegal activities threatening wildlife. Food security constraints were also found to be significantly associated with poaching. Finally, findings suggest that although HLS investments are essential in improving local community livelihoods, only food and financial security are the most effective means of reducing illegal activities in Virunga. 
    more » « less
  5. Abstract In humans and chimpanzees, most intraspecific killing occurs during coalitionary intergroup conflict. In the closely related genus Gorilla, such behavior has not been described. We report three cases of multi-male, multi-female wild mountain gorilla ( G. beringei ) groups attacking extra-group males. The behavior was strikingly similar to reports in chimpanzees, but was never observed in gorillas until after a demographic transition left ~25% of the population living in large social groups with multiple (3+) males. Resource competition is generally considered a motivator of great apes’ (including humans) violent intergroup conflict, but mountain gorillas are non-territorial herbivores with low feeding competition. While adult male gorillas have a defensible resource (i.e. females) and nursing/pregnant females are likely motivated to drive off potentially infanticidal intruders, the participation of others (e.g. juveniles, sub-adults, cycling females) is harder to explain. We speculate that the potential for severe group disruption when current alpha males are severely injured or killed may provide sufficient motivation when the costs to participants are low. These observations suggest that the gorilla population’s recent increase in multi-male groups facilitated the emergence of such behavior, and indicates social structure is a key predictor of coalitionary aggression even in the absence of meaningful resource stress. 
    more » « less