Causes and consequences of sex-biased dispersal in Columbian ground squirrel, Spermophilus columbianus

Dispersal is a fundamental process with wide ranging evolutionary consequences. In birds and mammals, members of one sex typically disperse more frequently, sooner and/or further than members of the other sex. The aim of this study was to examine factors affecting dispersal by yearling male and female Columbian ground squirrels (Spermophilus columbianus), and to determine whether inbreeding avoidance, competition or other factors can explain why intercolony dispersal is more common in males than in females. Males who stayed in their natal colony as adults emerged as yearlings heavier and later from hibernation than males who disappeared in their yearling year, whereas for females, this was not the case. Males who had sisters emerging as yearlings in the same colony were not more likely to leave the colony than males who were alone or with brothers. Further, there was no significant difference in the probability that females would mate with an immigrant compared to a natal male. Finally, three-year-old males who stayed in the colony moved significantly further away from their natal burrow than females of the same age. I conclude that sex differences in inter-colony dispersal, while promoting outbreeding, are not directly due to inbreeding avoidance, but that inbreeding avoidance may play a role in governing intra-colony dispersal distances. Further, resource competition seems to play a minor role. Aggression by adults against yearling males and the acceptance of yearling females by their mothers may be the proximate cause for male bias in inter-colony dispersal in Columbian ground squirrels. To conclusively demonstrate this effect, however, we need to look at aggression in more detail.