Ground squirrels manipulate offspring sex in response to local population density

Background/Question/Methods

The formation of matrilines is thought to be key to the evolution of sociality in several mammalian taxa, including primates, carnivores, and ground-dwelling squirrels.  In ground squirrels, matrilines are formed through retention of daughters in social and spatial proximity. Retaining daughters is thought to enhance maternal fitness, and may be more possible in years of low female density when competition is reduced. We used 18 years of demographic and pedigree data from a population of golden-mantled ground squirrels (Callospermophilus lateralis) to investigate whether females produce more daughters in years of low female density. We evaluated litter sex ratio, corrected for the role of chance in varying litter sizes, as a function of maternal condition, environmental conditions, and female density.

Results/Conclusions

We found that female golden-mantled ground squirrels show predictable changes in litter sex-ratio in response to female density: squirrels produced more daughters when female density was low, and fewer daughters when female density was high. By contrast, litter sex ratio did not vary by maternal age or body condition: maternal mass at spring emergence was positively correlated with litter size, but not litter sex ratio. Litter sex ratio also did not vary with food availability. Finally, females that produced female-biased litters were more likely to retain a daughter in the matriline in later years. Our results support local resource enhancement theory of offspring sex allocation, and are consistent with physiological mechanisms that bias offspring sex in female ground squirrels.