Energetic and Endocrine Mediation of Natal Dispersal Behavior in Belding's Ground Squirrels

Abstract

Natal dispersal, the permanent departure of an individual from its birth site, is sex biased in most mammals, with males dispersing at higher rates or over greater distances than do female conspecifics. Because dispersal movements may be energetically expensive, their occurrence should theoretically be influenced by energy availability. Moreover, the male bias typical of mammalian dispersal suggests that this behavior might be mediated by gonadal androgens. Using free-living Belding's ground squirrels (Spermophilus beldingi) as subjects, we provisioned juveniles with extra food to evaluate energetic influences on male dispersal behavior. Provisioning increased body mass and body fat of juvenile males and caused them to disperse at younger ages, but did not affect blood glucose levels. Dispersing males were fatter than same-aged males that had not yet dispersed. Moreover, body fat of provisioned and unprovisioned males did not differ when evaluated relative to the week during which they dispersed, suggesting that there may be a fat threshold for dispersal. In a second experiment, we measured plasma concentrations of testosterone (T) in provisioned and unprovisioned, free-living juveniles to evaluate the hypothesis that male dispersal behavior is activated by concurrent high levels of T. We observed no increase in plasma T associated with dispersal by juvenile males, no sex differences in circulating T among juveniles, and no effects of food provisioning on juvenile T levels. In a third experiment with free-livingS. beldingi,we concurrently altered early androgen exposure by treating females with T at birth and manipulated energy availability by food provisioning. Perinatal T-treatment increased the likelihood of dispersal among juvenile females. Provisioning increased body mass and body fat of juveniles and caused males and T-treated females to disperse at significantly younger ages than either their unprovisioned counterparts or the few provisioned control females that dispersed. These results suggest that early T exposure in this species determines the probability of dispersal, whereas the amount of energy an individual has stored as fat strongly influences the timing of dispersal. Early T exposure also appears to cause the timing of dispersal to respond to energy availability and body fat in a male-typical way, possibly by organizing masculine mass and fat thresholds for dispersal.