Why do female Columbian ground squirrels (Spermophilus columbianus) give an estrus call?

Female Columbian ground squirrels ( Spermophilus columbianus (Ord, 1815)) sometimes emit a repetitive vocalization after copulation. We examined two possible explanations for why sexual selection would favor expression of these “estrus calls”: to encourage sperm competition through mating with additional males and to increase mate guarding by the consort male as a mechanism of postcopulatory female mate choice. During three annual mating periods, we observed mating behaviour, estrus calls, and postcopulatory behavioural interactions of free-ranging individuals. Predictions of the advertisement hypothesis were supported, as females typically solicited courtship interactions with nonconsort males directly after emitting an estrus call. Thus, females that emitted an estrus call were more likely to acquire additional matings than noncalling females, particularly if calls were emitted after the female’s first mating. These results were not consistent with predictions of the postcopulatory female mate choice hypothesis, as calling females should initiate social contact with the consort male and stay proximate to the copulatory site after copulation if they are encouraging mate guarding. For reasons that remain unclear, the probability that an estrus call would follow mating increased linearly with the age of the consort male. However, our results taken together suggest that estrus advertisement is the most likely social context of female postcopulatory calling.

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 inter-colony 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.

The effects of capital on an income breeder: evidence from female Columbian ground squirrels

To maximize fitness, organisms must optimally allocate resources to reproduction, daily metabolic maintenance, and survival. We examined multiple years of live-trapping and observational data from a known-aged population of female Columbian ground squirrels, Spermophilus columbianus (Ord, 1815), to determine the influences of stored resources and daily resource income on the reproductive investments of females. We predicted that because yearling females were not fully grown structurally while producing their first litter, they would rely exclusively on income for reproduction, while reproductive investment in older females (≥2 years of age) would be influenced by both stored resources (capital) and daily income. Results from path analysis indicated that both yearlings and older females were income breeders. However, initial capital indirectly influenced investment in reproduction of yearling and older females. Females with the greatest initial capital maintained high body masses while investing relatively more income in reproduction. By considering influences of both capital and income, important relationships can be revealed between these resources and their influence on life histories.