LAUSR

Habitat structure and population density have long been considered key factors in determining intraspecific variation in the group size of social ungulates. However, the effect of interaction between habitat structure and population density on group size is poorly understood, and whether the underlying causes of these relationships are biological adaptation, emergent properties, or both is still debated. Therefore, over 10 years, we examined group size of the sika deer ( Cervus nippon ) at different population densities and habitat types (open habitats vs closed forests). We found that deer group size was larger in open habitats than closed forests and increased with population density, and population density and habitat type had a synergistic effect on group size. There was greater density effect in open habitats, while the density effect was very weak in closed forests. Our findings in open habitat support emergent properties, because both population density and visibility enhanced the frequency of group fusion. Alternatively, our findings in closed forests support biological adaptation; the scarce food supply in closed forests may increase the cost of conspecific feeding competition, which consequently limits group size even under high densities. We suggest that both emergent properties and biological adaptation determine group size variability of sika deer, and emergent properties may be the mechanism that contributes to group size increase, whereas biological adaptation may be the mechanism that contributes to group size restriction.