Feeding competition in wild female Assamese macaques (Macaca assamensis)

Food is one of the most important resources animals compete over and assessing the way individuals compete over such limiting resources is fundamental to our understanding of the adaptations of living organisms. In mammals, females are predicted to compete more intensively for access to food resources than males as females generally have lower potential reproductive rates and female fertility and reproduction are more strongly influenced by nutritional status. In addition to affecting individual survival and reproduction, socio-ecological models propose that food resource characteristics influence feeding competition and social interactions between individuals and groups, leading to predictable differences in female social structure. The validity of these models is currently under debate due to the many discrepancies found between model predictions and empirical data. However, it is argued that these mismatches mainly result from inaccurate or incomplete testing of model predictions and from ignoring important factors such as competition reducing mechanisms and competition for other limiting resources. The overall aim of this thesis was to broaden our understanding of the relationships between food characteristics, proximate mechanisms of feeding competition and female fitness by linking food characteristics to aggression, energy intake and reproduction in a group of wild Assamese macaques (Macaca assamensis). To achieve this I incorporated several factors that have been suggested to improve the predictive value of socio-ecological models, including measures of energy intake and fitness, measures of feeding competition at the level of individual food patches, competition reducing mechanisms and competition for safety. I investigated the ultimate consequences of feeding competition by examining the influence of food availability on female energy intake and reproduction as well as the influence of female rank on energy intake, activity and reproduction. To gain insight into the proximate mechanisms of feeding competition I then examined feeding competition at the level of individual food patches, investigating the impact of social and ecological factors on rates of food related aggression. Additionally, I examined strategies females may use to avoid direct conflicts over food. Finally, I related group activity and ecological conditions to inter-individual spacing behaviour to assess how group cohesiveness may be adjusted to mediate the costs and benefits of group living and I investigated how dominance rank and reproductive state affect a female’s distance from the centre of the group. Data for this study were collected on a group of wild Assamese macaques at Phu Khieo Wildlife Sanctuary in north-eastern Thailand over two 1-year sampling periods (period 1: October 2007–September 2008; period 2: May 2010–April 2011). During these sampling periods the group consisted of a total of 53 individuals, 12 of which were adult females (period 1) and 49 individuals, 15 of which were adult females (period 2). Focal data were collected on all adult females, resulting in over 2,100 hours of data on feeding behaviour, energy intake, and activity budgets which were combined with data on resource characteristics, female reproduction, physical condition and spatial position. In addition, I conducted 355 focal tree observations which allowed for the simultaneous observation of all individuals feeding in a food patch, proving detailed information on resource characteristics, aggression, and feeding party composition. My study revealed that the study population has a highly frugivorous diet and fruit availability affected both female energy intake and conception rates, indicating that food is indeed a limiting resource. The influence of nutritional status on female fitness was especially apparent during the lactation period which is considered to be one of the most energetically costly parts of the reproductive cycle in female mammals. During this period female Assamese macaques followed an energy conserving strategy, trading-off an increase in time spent resting for reduced feeding time. The degree to which females were able to compensate for the costs of lactation affected whether they were able to attain a high enough physical condition to conceive again the following mating season. This suggests that food availability during energetically demanding periods such as peak lactation can constrain female reproduction. Surprisingly, despite their highly frugivorous diet and the fact that females exhibited strict linear dominance hierarchies, females did not display a rank related skew in energy intake, activity budgets or reproduction. In addition, the relationships between female rank and either energy intake or activity did not change depending on overall fruit abundance or the nutritional quality, patch size or density of the most consumed food items. The frequency of food related aggression in feeding patches increased with increasing feeding group size and decreasing number of feeding sites inside patches (patch size) while it was not affected by ecological variables representing resource value. Interestingly, females appear to employ several mechanisms to avoid direct conflicts, including the use of alternative feeding sites within food patches, storing food in cheek pouches and co-feeding with closely bonded individuals. Inter-individual distances were greater when the group was feeding compared to when the main group activity was resting, engaging in social behaviour or moving. This is likely due to the high costs of proximity associated with feeding competition. Among adult females, higher ranking individuals occupied more central positions within the group while lower ranking females were more peripheral. In sum, this thesis unites information on the relationships between resource characteristics, behavioural mechanisms of feeding competition and fitness consequences in female Assamese macaques, creating a detailed picture of the social and ecological influences on female feeding competition and fitness. Based on resource characteristics, females were predicted to exhibit within-group contest competition over food. Although females did exhibit food related aggression, aggression rates were lower than predicted. This was probably due to the fact that females employ conflict reducing mechanisms and that aggression rates may be more strongly affected by patch size and feeding group size than by food density, quality and distribution per se. Females did not exhibit the rank related skew in energy intake and fitness that is associated with within-group contest competition. Here, again, competition reducing mechanisms are important as they can enable low ranking females to enhance their access to food resources although this may come at a cost. For example, low ranking females occupy more peripheral positions which are likely to reduce feeding interference but which are also associated with increased predation risk. This suggests that competition for safe positions can interact with feeding competition to affect individual fitness. For true tests of the internal consistency of socio-ecological models more studies are needed which incorporate detailed measurements of resource characteristics as well as energetic and fitness measures and which take into account conflict reducing adaptations and competition for limiting resources other than food.