Flexible group cohesion and coordination, but robust leader-follower roles, in a wild social primate using urban space

Abstract

Collective behaviour has a critical influence on group social structure and organization, individual fitness and social evolution, but we know little about whether and how it changes in anthropogenic environments. Here, we show multiple and varying effects of urban space-use upon group-level processes in a primate generalist-the chacma baboon (Papio ursinus)-within a managed wild population living at the urban edge in the City of Cape Town, South Africa. In natural space, we observe baboon-typical patterns of collective behaviour. By contrast, in urban space (where there are increased risks, but increased potential for high-quality food rewards), baboons show extreme flexibility in collective behaviour, with changes in spatial cohesion and association networks, travel speeds and group coordination. However, leader-follower roles remain robust across natural and urban space, with adult males having a disproportionate influence on the movement of group members. Their important role in the group's collective behaviour complements existing research and supports the management tactic employed by field rangers of curbing the movements of adult males, which indirectly deters the majority of the group from urban space. Our findings highlight both flexibility and robustness in collective behaviour when groups are presented with novel resources and heightened risks.

Keywords: cohesion; coordination; flexibility; leadership; urban space-use.

Figure 1.

Reduced baboon group cohesion in urban compared to natural space. (a) Frequency density plot for mean inter-baboon distances (natural log scale); (b) frequency density plot for number of baboons observed within 150 m of each other; (c) frequency density plot for baboon group convex hull perimeter (natural log scale). In (a–c), data are shown for baboons in natural (green) and urban (grey) space. Panels (d) and (e) show satellite images of baboons in a suburb of Cape Town, and in the natural space surrounding this suburb, respectively. Baboon locations (identified by coloured dots) are overlaid, and white circles of 150 m diameter (as shown in (b)) illustrate differences in group cohesion at these example moments. (Online version in colour.)

Figure 2.

Interrupted networks in urban space, but retention of leader–follower roles. (a) Baboon association network (undirected) and (b) leader–follower network (directed) in natural space. (c) Baboon association network (undirected) and (d) leadership network (directed) in urban space. In (a–d), high to low eigenvector centrality is represented by large to small circle sizes, and high to low dominance rank from dark to light colours. (e) Relationship (not statistically significant) between association network eigenvector centrality (undirected) for baboons when in natural and urban space. (f) Relationship (statistically significant) between leadership network eigenvector centrality (directed) for baboons when in natural and urban space. (g) Trend for higher dominance-ranked baboons being more central in the association network (undirected) in natural space, but not in urban space. (h) Higher-ranked baboons are more central in the leadership network (directed) in both urban and natural space. (i) Five baboons: the orange baboon is the blue baboon's closest spatial neighbour, within a 5 m radius (shown by the light blue circle). If two baboons are often observed within 5 m of each other they will have a strong link in (a) and (c). If the blue baboon (initiator) moves greater than 5 m away from a group member who then follows by greater than 5 m (orange baboon, follower), then this is considered a local leadership event, where one baboon influences the movement of another. Baboons with greatest influence on others' movements will have high eigenvector centrality in (b) and (d). (Online version in colour.)