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Questions: How does average male investment in ejaculates vary with changing population density (and thus with the risk of sperm competition) in a promiscuous species? Do individual male investment strategies vary with population density?

Data studied: Total testicular mass, somatic mass and annual population density for wild-caught male deer mice, Peromyscus maniculatus, collected by snap-trapping over a 23-year period in Algonquin Provincial Park, Ontario, Canada.

Search methods: We analysed the relation between mean testicular mass and mouse population densities across years. To investigate individual investment patterns, we compared the relation between total testicular mass and somatic mass among males for years differing in population density.

Conclusions: Average investment in the testes was positively correlated with annual population density. An individual’s investment in testes depended on both the abundance of rival males and on relative body size, a trait associated with social rank.


This article was originally published in Evolutionary Ecology, 8(2): 345-356. © 2006 Tristan A.F. Long