Body fat and time of year interact to mediate dispersal behaviour in ground squirrels

Behaviour is often influenced by energy availability. Hibernation presents an energetic challenge to Belding's ground squirrels, Spermophilus beldingi, which depend entirely on stored energy for survival during their 8-9 month dormant period each year. Dispersal from the natal area may also place energetic demands on S. beldingi. In this study, we assessed the relationship between dispersal behaviour, body mass and body fat of juvenile male S. beldingi over 3 consecutive years, one of which had a substantially delayed juvenile active season due to heavy spring snowfall. We evaluated body fat using the non-destructive method of measuring electrical conductivity of the body. When the active season began late, acquisition of body mass and body fat were accelerated, and dispersal behaviour was inhibited. These results suggest that the ontogeny of mass and fat gain in juvenile male S. beldingi is influenced by a seasonal time-keeping mechanism. Energy allocation was hierarchical in S. beldingi, with pre-hibernation fattening taking precedence over dispersal. Physiological signals reflecting body fat content appear to interact with an endogenous timing mechanism in this species to regulate the dispersal behaviour of juvenile males. (C) 1998 The Association for the Study of Animal Behaviour.