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

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Abstract

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).

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Data availability

Ausband DE (2022b) Wolf pup sex ratios and covariates, Idaho, USA, 2008–2018 [Data set]. Zenodo, https://doi.org/10.5281/zenodo.7035926

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Acknowledgements

I thank M.A. Hurley, J.S. Husseman, S.B. Roberts, C.R. Stansbury, J.L. Stenglein, J.L. Struthers, J. Adams, and L. Waits for their assistance and S.B. Bassing for the manuscript review. I also thank field technicians for the perseverance in the field while collecting these data. Lastly, I thank anonymous reviewers for their thoughtful and thorough reviews. During the data collection, I received financial support from the Regina Bauer Frankenberg Foundation for Animal Welfare, Bernice Barbour Foundation, Coypu Foundation, Eppley Foundation for Research, Idaho Department of Fish and Game, Kampe Foundation, Leonard X. Bosack and Bette M. Kruger Charitable Foundation, Nancy Carroll Draper Foundation, Nez Perce Tribe, Oregon Zoo Future for Wildlife grants, Shikar Safari Club International Foundation, Steven Leuthold Family Foundation, The Mountaineers Foundation, U.S. Fish and Wildlife Service, Wilburforce Foundation, Wolf Recovery Foundation, University of Idaho College of Natural Resources, and University of Idaho Environmental Science Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This paper was subject to USGS Fundamental Science Practices (https://pubs.usgs.gov/circ/1367).

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  1. U.S. Geological Survey, Idaho Cooperative Fish and Wildlife Research Unit, University of Idaho, 975 West 6th Street, ID, 83844, Moscow, USA

    David E. Ausband

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Correspondence to David E. Ausband.

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Field sampling was conducted under the University of Montana IACUC (Animal Use Protocol 008-09MMMCWRU). All applicable international, national, and/or institutional guidelines for the use of animals were followed.

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The author declares no competing interests.

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Communicated by C. Soulsbury

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Ausband, D.E. Offspring sex ratios are male-biased reflecting sex-biased dispersal in Idaho, USA, wolves. Behav Ecol Sociobiol 76, 134 (2022). https://doi.org/10.1007/s00265-022-03243-0

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