Evol Ecol Res 7: 143-149 (2005)     Full PDF if your library subscribes.

Condition dependence of testis size in small mammals

Albrecht I. Schulte-Hostedde,1* John S. Millar2 and Graham J. Hickling3

1Department of Biology, Laurentian University, Sudbury, Ontario P3E 2C6, Canada,  2Department of Biology, University of Western Ontario, London, Ontario N6A 5B7, Canada and  3Department of Fisheries and Wildlife, Michigan State University, Lansing, MI 48824-1222, USA

Author to whom all correspondence should be addressed.
e-mail: aschultehostedde@laurentian.ca


Sperm competition is a common phenomenon across the animal kingdom and is recognized as a major factor in the sexual selection of males. Intraspecific variation in testis size (ejaculate investment) has been implicated as an important factor in male reproductive success because larger testes produce higher quality ejaculates. Sexual selection theory predicts that traits associated with male–male competition or female mate choice must be costly and heritable to confer an evolutionary advantage to those individuals carrying such traits. Because ejaculate production can be costly, we tested the prediction that testis size is condition dependent using morphological data from three species of small mammals. In addition, subordinate males, which are less likely to copulate with a female at the optimal time for achieving fertilization, should invest more heavily in ejaculates than dominant males. Thus we also predicted that small males should invest more heavily in ejaculates than large males. In all three species, testis size was positively related to body condition (size-corrected body mass) and independent of body length. These results suggest that males in good condition are capable of investing more in ejaculates than males in poor condition, and are consistent with the costly nature of ejaculate production. In addition, the results are consistent with other traits that are condition dependent and that serve as honest signals of male quality, whether in the context of male–male competition or female mate choice.

Keywords: body condition, ejaculates, rodents, sexual selection, sperm competition.

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