Social structure of a semi-free ranging group of mandrills (Mandrillus sphinx): a social network analysis

Abstract

The difficulty involved in following mandrills in the wild means that very little is known about social structure in this species. Most studies initially considered mandrill groups to be an aggregation of one-male/multifemale units, with males occupying central positions in a structure similar to those observed in the majority of baboon species. However, a recent study hypothesized that mandrills form stable groups with only two or three permanent males, and that females occupy more central positions than males within these groups. We used social network analysis methods to examine how a semi-free ranging group of 19 mandrills is structured. We recorded all dyads of individuals that were in contact as a measure of association. The betweenness and the eigenvector centrality for each individual were calculated and correlated to kinship, age and dominance. Finally, we performed a resilience analysis by simulating the removal of individuals displaying the highest betweenness and eigenvector centrality values. We found that related dyads were more frequently associated than unrelated dyads. Moreover, our results showed that the cumulative distribution of individual betweenness and eigenvector centrality followed a power function, which is characteristic of scale-free networks. This property showed that some group members, mostly females, occupied a highly central position. Finally, the resilience analysis showed that the removal of the two most central females split the network into small subgroups and increased the network diameter. Critically, this study confirms that females appear to occupy more central positions than males in mandrill groups. Consequently, these females appear to be crucial for group cohesion and probably play a pivotal role in this species.

Figure 1. Number of associations observed per day corrected by the number of scans recorded per day.

The solid line represents the trend followed by the distribution over time. The slope of this linear curve was not statistically different from 0, meaning that the distribution of the number of associations corrected by the number of scans did not significantly evolve over time.

Figure 2. Representation of the contact network.

Nodes represent individuals, and the size of nodes is related to the individual's betweenness (A) and the individual's eigenvector centrality (B), with bigger nodes corresponding to more central individuals. White nodes correspond to females and dark gray nodes correspond to males. Widths of lines represent the strength of association between two individuals.

Figure 3. The cumulative distribution of centrality values for the contact network.

(A) Betweenness values, and (B) eigenvector centrality values. Solid lines represent the power function fitted by the distributions.

Figure 4. Changes in network characteristics after the removal of central individuals (targeted condition) and randomly chosen individuals (random condition).

(A) Network fragmentation, and (B) network diameter. In all figures, dark columns represent the initial network, grey columns represent random condition and white columns represent targeted condition.