Female kin density affects offspring sex ratio in an asocial mammal, the golden-mantled ground squirrel, Callospermophilus lateralis

Abstract

Evolutionary theory predicts that parents should bias investment in offspring towards the sex that will yield the higher fitness returns, and one outcome may be biased offspring sex ratios. Bias in offspring sex ratios has been proposed to vary with specific social, maternal and environmental conditions that alter the fitness returns of each sex, but empirical evidence is conflicting both within and across taxa. We used 18 years of pedigree and demographic data to investigate variation in offspring sex ratios in a population of golden-mantled ground squirrels. Despite overall parity in offspring sex ratio, we found predictable changes in offspring sex ratio in response to female density at the population level: squirrels produced more daughters when female density was low and more sons when female density was high. At the individual level, females adjusted litter sex ratios similarly, but only in response to high densities of kin, and not in response to density of nonkin. This effect was reversed for older females (≥3 years) in dense kin neighbourhoods, which were more likely to produce daughters, perhaps because older females had the behavioural dominance necessary to recruit daughters at high densities. By contrast, litter sex ratios did not vary with maternal condition or food availability. Our results support the local resource competition theory of offspring sex allocation for this species. Knowledge of female relatedness revealed patterns of sex ratio adjustment that otherwise would have been obscured, highlighting the importance of interactions with kin in an asocial species.