Offspring sex ratios are male-biased reflecting sex-biased dispersal in Idaho, USA, wolves

Offspring sex ratios can vary widely across species, and the reasons for such variation have long intrigued ecologists. For group-living animals, predicting offspring sex ratios as a function of group and environmental characteristics can be challenging. Additionally, mortality of group members can upend traditional theory used to explain offspring sex ratios observed in populations. Gray wolves (Canis lupus) in Idaho, USA, are an excellent study species for asking questions about offspring sex ratios given their group-living behavior and persistent exposure to human-caused mortality. I hypothesized that offspring sex ratios would be influenced by the characteristics of individuals, groups, and populations. I generated genotypes for 419 adult and 400 pup wolves during 2008-2018. There was a significant male-bias in litters of wolf pups with nearly 12% more male pups born than females. The individual, group, and population variables I considered did not have significant associations with offspring sex ratios. Local resource competition helped explain offspring sex ratios in wolves in my study system, but not local resource enhancement theory. Although female helpers have been shown to help slightly more than males, offspring sex ratios did not favor the helping sex suggesting that the overall benefit of female helpers may have been negligible in wolf groups during my study. Three wolf groups consistently overproduced males, the dispersing sex, suggesting that habitat quality was poor in their territories. The male-biased offspring sex ratios observed throughout this population reflect sex-biased dispersal in wolves in Idaho. Such a pattern suggests breeding females may be reducing local resource competition (e.g., mates and successful reproduction) by producing more males than females. Significance statement Natural selection can favor biased offspring sex ratios in some species. This may be particularly true for animals that live and breed in groups such as gray wolves. Using genetic sampling, I show that offspring sex ratios in wolves are male-biased and reflect sex-biased dispersal in wolves. Breeding females may be reducing future local resource competition for mates by producing significantly more offspring of the dispersing sex (males).

Automated summary

Offspring sex ratios in gray wolves in Idaho are male-biased, reflecting sex-biased dispersal in the population. This may be a strategy used by breeding females to reduce future local resource competition. Individual, group, and population characteristics were not significantly associated with offspring sex ratios.