RESUMEN
Stress is the body's response to cope with the environment and generally better survive unless too much chronic stress persists. While some studies suggest that it would be more stressful to be the dominant individual of the group, others support the opposite hypothesis. Several variables can actually affect this relationship, or even cancel it. This study therefore aims to make the link between social status and the basal level of stress of 14 wild European bison (Bison bonasus, L. 1758) living together. We collected faeces and measured the faecal glucocorticoid metabolites (FGM). We showed that FGM is linked to different variables of social status of European bison, specifically age, dominance rank, eigenvector centrality but also to interactions between the variables. Preferential leaders in bison, i.e., the older and more dominant individuals which are more central ones, are less stressed compared to other group members. Measurement of such variables could thus be a valuable tool to follow and improve the conservation of species by collecting data on FGM and other social variables and adapt group composition or environmental conditions (e.g., supplement in food) according to the FGM concentration of herd individuals.
RESUMEN
The use of social network analysis to better understand animal behaviour has led to an increase in the number of studies on this topic, particularly for species management and conservation as well as the welfare of captive animals. The current knowledge of social organization in the European bison (Bison bonasus) is still largely based on descriptions of group compositions, most of which were obtained during opportunistic field observations. However, the numerous conservation programs for this species can only implement effective management decisions after the analysis of its social structures. For instance, the removal of a key individual can disturb the social stability of a group and thus indirectly affect the fitness of group members. This is the first study that uses social network analysis to examine the social structure of a semi-free ranging bison herd (N = 14). Closest neighbour associations are used to calculate the eigenvector centrality and individual social strength of each animal. These two indices are combined after a principal component analysis (PCA) to form the Sociability variable, which was then tested for correlations with age, dominance rank, number of matrilines and reproductive status (lactating vs non-lactating). Results reveal strong group cohesion, with a close association of individuals belonging to the same matriline within the herd. The cumulative distribution of the eigenvector centralities follows a linear function, meaning that no individual has a highly central position compared to its congeners. Finally, a high Sociability value was not correlated to age, dominance rank or number of matrilines, but was dependent on the reproductive status of females. These results indicate the necessity to consider the social welfare of animals, for instance by taking matrilineal relationships into account when making bison groups management decisions.
