Open habitats promote female group formation in a solitary ungulate: the Japanese serow

Abstract

Understanding the factors that drive the evolution of sociality is one of the central issues in behavioural ecology. In ungulates, open habitats, such as alpine meadows, are thought to have promoted the evolution of various gregarious societies from solitary territorial lifestyles. However, the transition from a solitary territorial lifestyle to a gregarious society and the ultimate factors underlying this change remain unclear. The Japanese serow is a primal solitary ungulate. Females in forest-dwelling populations have solitary territorial lifestyles, but the social relationships among females in open-land dwelling populations have not been explored. We evaluated the spatial organisation and social interactions among female Japanese serows in an area with alpine meadows and subalpine forests and examined the effects of alpine meadows on female social relationships through within-population comparisons. We confirmed the presence of both spatially solitary animals (little home range overlap with other individuals) and spatial groups (large home range overlap with two to three individuals), having exclusive territories. Females with more alpine meadows in their home range formed group territories, whereas those with more subalpine forests formed solitary territories. Accordingly, we show, for the first time, that open habitats promote the formation of female spatial groups in a primal solitary ungulate. We suggest that homogenous food distribution and abundant food supply in alpine meadows promote spatial group formation and that the mitigation of exclusive relationships and home range overlapping is a transitional phase for the development of gregarious societies. We found spatial group formations among females in a primal solitary ungulate, the Japanese serow, which were promoted by open habitats, such as alpine meadows. Our findings provide evidence for an evolutionary pattern in ungulates, whereby open habitat structures promote the evolution from solitary, territorial lifestyles, to gregarious societies. This indicates that the mitigation of exclusive relationships (i.e. territoriality) and the overlapping of home ranges among females is a transitional phase for the development of gregarious societies. Homogenous food distribution and an abundant food supply in the alpine meadows appear to promote the formation of spatial groups, suggesting that food abundance and distribution are among the important ultimate factors that potentially contribute to the evolution of gregarious societies.