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Intraspecific variation of testis size and sperm length in the yellow-pine chipmunk (Tamias amoenus): implications for sperm competition and reproductive success

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Abstract

Comparative analyses have found that relative testis size is a strong predictor of the prevalence of sperm competition for many taxa, including mammals, yet underlying this pattern is the assumption that intraspecific variation in testis size is related to individual fitness. Because intraspecific variation in ejaculate investment underlies interspecific patterns, it is critical to understand the causes and consequences of intraspecific variation in ejaculate investment. We examined relationships between ejaculate investment (testis size and sperm length) and reproductive success, body size and condition in the yellow-pine chipmunk (Tamias amoenus), a small ground squirrel in which sperm competition occurs. We examined genetic estimates of male reproductive success from a wild population of yellow-pine chipmunks and determined that males with large testes had higher annual reproductive success than males with small testes. This result provides empirical support for the numerous comparative studies that indicate testis size is associated with the intensity of sperm competition. In addition, males in good condition had relatively larger testes than males in poor condition, but there was no evidence of sperm length being dependent on condition. Finally, contrary to many predictions, males that invested more in sperm production (relatively heavy testes) produced shorter sperm, not longer sperm, than males that invested less.

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Acknowledgements

We thank Dr. K. Goodrowe and D. Ryckman of the Toronto Zoo, and Prof. C. Platz of the International Canine Semen Bank for advice on the electroejaculation technique, and Dr. R. Cooper of the Calgary Zoo for the loan of the electroejaculation equipment. Dr. R.D. Montgomerie provided facilities for the measurement of sperm morphology. The University of Calgary’s Kananaskis Field Station is gratefully acknowledged for logistical assistance. We are indebted to H.L. Gibbs and L. DeSousa for facilities and expertises in the genetic analysis of male reproductive success. Comments from G. Burness, S. Casselman and two anonymous reviewers greatly improved the manuscript. This study was supported by an Ontario Graduate Scholarship, Natural Sciences and Engineering Research Council of Canada (NSERC) post-graduate scholarship and two grants-in-aid of research from the American Society of Mammalogists to A.I.S.-H., and an NSERC operating grant to J.S.M. All procedures described herein were approved by the Canada Council on Animal Care.

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  1. Albrecht I. Schulte-Hostedde

    Present address: Department of Biology, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada

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  1. Department of Biology, University of Western Ontario, N6A 5B7, London, Ontario, Canada

    Albrecht I. Schulte-Hostedde & John S. Millar

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  1. Albrecht I. Schulte-Hostedde
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Correspondence to Albrecht I. Schulte-Hostedde.

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Communicated by P.M. Kappeler

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Schulte-Hostedde, A.I., Millar, J.S. Intraspecific variation of testis size and sperm length in the yellow-pine chipmunk (Tamias amoenus): implications for sperm competition and reproductive success. Behav Ecol Sociobiol 55, 272–277 (2004). https://doi.org/10.1007/s00265-003-0707-z

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