Ecological and Social Determinants of Group Cohesiveness and Within-Group Spatial Position in Wild Assamese Macaques

Abstract

Individual spatial positioning plays an important role in mediating the costs and benefits of group living and, as such, shapes different aspects of animal social systems including group structure and cohesiveness. Here, we aim to quantify variation in individual spacing behaviour and its correlates in a group of wild Assamese macaques (Macaca assamensis) in north-eastern Thailand, which experiences both predation pressure and within-group feeding competition. Data on individual spatial positions were collected during group scans using GPS devices and results suggest that both group cohesiveness and individual spatial positions within a group can be adjusted to mediate the costs and benefits of group living. Individuals had greater nearest neighbour distances and lower numbers of close neighbours when the group was feeding, compared to when the main group activity was resting/social or moving. This is likely due to the high costs of proximity associated with feeding competition. Immature individuals and females with young infants which are more vulnerable to predation were located closer to the centre of the group than both adult males and females without infants. This indicates the importance of predation risk in driving individual spatial position. Among adults, higher ranking individuals occupied more central positions within the group while lower ranking individuals were more peripheral. It appears that low ranking individuals trade reduced feeding interference and improved feeding success for increased predation risk. This could help explain why females in the study population do not display a rank related skew in energy intake.