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Male-biased sex ratios in New Zealand fur seal pups relative to environmental variation

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Abstract

A number of models have been proposed to provide adaptive explanations of sex-ratio variation in mammals. Two models have been applied commonly to primates and ungulates with varying success—the Trivers-Willard (TW) hypothesis, and the local resource competition (LRC) hypothesis. For polygynous, sexually dimorphic mammals, where males are larger and disperse more readily, these models predict opposite outcomes of sex-ratio adjustment within the same environmental context (high-resource years: TW—more sons; LRC—more daughters). However, many of the predictions of these two models can vary depending on factors influencing resource availability, such as environmental stochasticity, resource predictability, and population density. The New Zealand fur seal (Arctocephalus forsteri) is a polygynous mammal showing marked sexual dimorphism (larger males), with higher variation in male reproductive success expected. We provide clear evidence of male-biased sex ratios from a large sample of A. forsteri pups captured around South Island, New Zealand during 1996/1998, even after accounting for a sex bias in capture probability. The extent of the bias depended upon year and, in 1998, strong climatic perturbations (El Niño/Southern Oscillation, ENSO) probably reduced food availability. Significant male-biased sex ratios were found in all years; however, there was a significant decline in the male bias in 1998. There was no relationship between sex ratio and population density. We suggest that the sex-ratio bias resulted from the production of relatively more male pups. Under the density-independent scenario, the strong male bias in A. forsteri sex ratios is support for the TW model within an environment of high resource predictability. We suggest that some plasticity in the determination of pup sex among years is a mechanism by which A. forsteri females in New Zealand, and perhaps other otariid seals, can maximise fitness benefits when living in regions of high, yet apparently predictable, environmental variability. We also suggest that much of the inconsistency in the reported sex ratios for otariid seals results from the complex interaction of population density and environmental stochasticity influencing relative food availability over time.

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Acknowledgements

Research was funded by the University of Otago. We also thank Allflex New Zealand (Palmerston North) for providing the plastic tags used to identify fur seal pups, Combined Rural Traders Society (Otago) for providing field equipment, and Next Stop Backpackers (Dunedin) for providing accommodation for volunteers. We particularly thank the Department of Zoology (University of Otago), New Zealand Department of Conservation, Landcare Research New Zealand, and New Zealand Sea Adventures (Kaikoura) for logistic support. We thank A. Anderson, G. Anderson, C. Bevers, K. Barton, C. Duncan, C. Littnan, N. McNally, I. Rasmussen, B. Thomas, M. Wright and the many volunteers who assisted with data collection. We thank R.J. Barker for providing statistical advice, and the editor, F. Trillmich, and two anonymous referees for greatly improving the manuscript. All animal treatment procedures were approved by the University of Otago Animal Ethics Committee (no. 83-95) and licensed under a New Zealand Department of Conservation Permit to Take Marine Mammals (30 July 1996).

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  1. Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, G.P.O. Box 252-05, Hobart, Tasmania 7001, Australia

    Corey J. A. Bradshaw

  2. Marine Mammal Research Group, Graduate School of the Environment, Macquarie University, Sydney, New South Wales 2109, Australia

    Robert G. Harcourt

  3. Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand

    Lloyd S. Davis

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  1. Corey J. A. Bradshaw
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Bradshaw, C.J.A., Harcourt, R.G. & Davis, L.S. Male-biased sex ratios in New Zealand fur seal pups relative to environmental variation. Behav Ecol Sociobiol 53, 297–307 (2003). https://doi.org/10.1007/s00265-003-0580-9

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