Developmental and social constraints on early reproduction in an asocial ground squirrel

For short-lived species, selection for early reproduction should be strong, yet females often delay their first reproductive bout. Delay in age of first reproduction due to developmental constraints, such as food availability, or social constraints, such as the inhibitory presence of breeding adults, has been documented for social mammals, but effects on asocial species are less well known. We evaluated the influence of developmental and social factors on early reproduction in a short-lived, asocial species, the golden-mantled ground squirrel (Callospermophilus lateralis). We found that females who reproduced as yearlings had been weaned earlier in their natal summer and experienced early snow melt during their yearling spring, suggesting the importance of access to high-quality food at critical stages. Females were more likely to reproduce as yearlings when there were more adult males present during the breeding season, possibly because exposure to males accelerates reproductive maturity. Maternal presence had no effect on yearling reproduction, but yearlings with a littermate sister present were only 22% as likely to reproduce as females without a sister present, suggesting the effect of sibling competition well past weaning. Furthermore, the negative effect of a sister's presence, but not the presence of other females, suggests that relatedness affects reproductive competition in this asocial species.