Group dynamics and local population density dependence of group size in the Pyrenean chamois, Rupicapra pyrenaica

Several models have recently confirmed the hypothesis that mean group size should increase with population density in animal species in which groups are unstable. Based upon different assumptions with respect to the mechanisms of groups merging and splitting up, these models, however, predict quantitatively different relations between mean group size and population density. Taking advantage of a quasiexperimental situation, we studied the diurnal group dynamics of the Pyrenean chamois within a 58-ha open pasture in a protected area during winter. Mean group size scaled with local population density raised to the power of 2/3 within the pasture, which implies that the mean number of groups was related to the cube root of local population density. On the basis of the fate of groups that included tagged individuals, we found that: (1) the rate at which groups joined increased as the number of groups within the pasture increased; (2) the rate at which groups split up increased with their size; and (3) the rate at which groups were involved in both fusion and fission events increased as the local population density increased. We discuss the possible role of population spatial structure and mother–kid association in both the dynamics of group formation and the relations between mean group size and population density.