COS 113-6 - Paternal contribution to offspring sex ratio is resource-dependent but not heritable in a wild mammal

Background/Question/Methods

The evolution and maintenance of population sex ratios is thought to be dependent on natural selection acting on the relative costs and benefits to an individual allocating resources towards their offspring, and assumes offspring sex ratios are heritable traits. In mammals, males are the heterogametic sex and thus contribute to offspring chromosomal sex determination; however, much of the work examining contributions to and fitness benefits of variation in biasing offspring sex allocation in vertebrates has focused on maternal contributions and condition-dependent effects. Here, we explore genetic and environmental components of paternal contributions to offspring sex ratios in a population of North American red squirrels (Tamiasciurus hudsonicus). Red squirrels rely on cached conifer cones for overwinter survival, and, particularly for males, reproductive success. We determined whether annual offspring sex ratios by sires are repeatable within individuals, heritable, and condition-dependent. Using a pedigreed long-term dataset, we estimated heritability using an animal model approach. We determined whether individual-level resource availability (i.e., a sire’s cache size from the previous autumn) corresponded to annual offspring sex ratio. We also examined whether sires experienced a fitness benefit of biasing their offspring sex ratio by examining offspring survival to 200 days of age (i.e., recruitment).

Results/Conclusions

Using pedigreed breeding records for 329 unique males who sired at least one pup within a given breeding season yielding 580 siring records, we found both within-individual and between-individual variation in the annual sex ratios of offspring produced by sires, marked by moderately high repeatability (0.215, 95% CI .039, 0.843) and low estimated heritability (0.004; 95% CI 0.001, 0.095). We found that offspring sex ratio (expressed as proportion male) increased when sires had larger food caches amassed in the autumn preceding the breeding season. However, we did not find a annual offspring sex ratio to be associated with recruitment when controlling for number of offspring sired. This suggests that while offspring sex ratio may be dependent on sire condition, biasing offspring sex ratio does not necessarily yield higher fitness. With a wide range of variation in offspring sex ratios in the population, but without a strong heritable component to variation in the trait, observed population-level maintenance of overall parity of males:females in this system is not likely to be maintained by natural selection acting on offspring sex ratios as determined by sire contributions.