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Offspring sex in mountain goat varies with adult sex ratio but only for mothers in good condition

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Abstract

Studies of mammals have often produced results inconsistent with theories predicting adaptive sex-ratio manipulation. Some apparently strong trends weaken or disappear over time, suggesting that multiple variables affect sex ratio and that their relative importance may change over time. Mountain goats are sexually dimorphic ungulates that satisfy all the assumptions of the Trivers-Willard hypothesis. Therefore, females able to provide high levels of maternal care are expected to produce an excess of sons. An earlier study found that older females, that are larger and dominant, produced more sons than did younger females, which are smaller and subordinate. Here, we show that, as sample size tripled, that trend disappeared. A large-scale climate index, reproductive status at conception, and population density had no consistent effects on offspring sex ratio. A composite measure of female condition at conception revealed that offspring sex ratio varied with adult sex ratio, as predicted by the homeostatic hypothesis, but only for mothers in good condition at conception. These females were dominant, heavier, and older. Their probability of producing a son decreased from about 80 to 20 % as the adult sex ratio became more male-biased. For mothers with a low condition index, however, adult sex ratio had no effect on offspring sex ratio. Our research suggests that offspring sex ratio is affected by complex interactions between individual condition and other variables, whose importance may vary over time and can only be elucidated by long-term studies.

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Acknowledgments

Our long-term research is primarily financed by the Natural Sciences and Engineering Research Council of Canada. We also obtained support from the Alberta Fish and Wildlife Division, the Rocky Mountain Goat Foundation, the Alberta Conservation Association, the Alberta Sport, Recreation, Parks and Wildlife Foundation, the Alberta Wildlife Enhancement Fund, Université de Sherbrooke and Université Laval. We are indebted to the many people who helped with fieldwork at Caw Ridge over the years, and to K. G. Smith for his role in the initiation of the study. We thank two anonymous reviewers for constructive comments on a previous version of this manuscript.

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Authors and Affiliations

  1. Department of Arctic and Marine Biology, UiT The Arctic University of Norway, 9037, Tromsø, Norway

    Sandra Hamel

  2. Département de biologie, and Centre d’études Nordiques, Université de Sherbrooke, Sherbrooke, QC, J1K 2R1, Canada

    Marco Festa-Bianchet

  3. Département de biologie, and Centre d’études Nordiques, Université Laval, Québec, QC, G1V 0A6, Canada

    Steeve D. Côté

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  1. Sandra Hamel
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  2. Marco Festa-Bianchet
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  3. Steeve D. Côté
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Correspondence to Sandra Hamel.

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The authors declare that they have no competing interests.

Compliance with ethical standards

All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.

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Communicated by K. E. Ruckstuhl

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Hamel, S., Festa-Bianchet, M. & Côté, S.D. Offspring sex in mountain goat varies with adult sex ratio but only for mothers in good condition. Behav Ecol Sociobiol 70, 123–132 (2016). https://doi.org/10.1007/s00265-015-2031-9

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