Spontaneous Emergence of a Relationship between Habitat Openness and Mean Group Size and its Possible Evolutionary Consequences in Large Herbivores

Group size generally increases with habitat openness in large mammalian herbivores, be it within species or across a given taxonomic family when considering the typical habitat and herd size of each species. This correlation is often considered to be the adaptive outcome of selective pressures arising from predation and from the intraspecific competition that communal foraging may entail. However, the increase of group size with habitat openness, when it occurs within a given species, might be the simple consequence of the instability of groupings and of the increasing possibility of perceiving congeners with environment openness. This is shown in the present paper through the outcomes of a simple model for group fusion and splitting-up, which assumes that the individuals behave independently of habitat openness. Some possible evolutionary consequences of the phenomenon are discussed. The spontaneous increase of group size with environment openness might in particular have initiated the evolutionary trend to large body sizes in the herbivore species which colonized open spaces.