Cues that females use to select potential mates have attracted substantial research effort, but the criteria for male mate choice remain very poorly known. Red-sided garter snakes (Thamnophis sirtalis parietalis) court and mate in large aggregations around overwintering dens in southern Manitoba, Canada. Both courtship and mating are size-assortative: small male snakes court small as well as large females, whereas larger males court only large females. This system provides a unique opportunity to assess the cues that males use in selecting mates, and in particular the mechanisms that generate a size-related shift in mate preference. Experiments in which we manipulated body sizes and scents showed that both vision and scent (sex pheromones) were important. Large males directed intense courtship only when the stimulus provided both visual and chemical (skin lipid) evidence of large body size. Small males were much less discriminating in both respects. Thus, size-assortative mating in this system is generated not by larger males excluding their smaller rivals from the largest females (as has been reported in other reptile species), but by a size-related shift in the visual and pheromonal cues that elicit courtship. Males of some species may thus show complex patterns of mate choice, with the cues that stimulate courtship differing even among males within a single population based on traits such as age or body size.
Bibliographical noteFunding Information:
Acknowledgements We thank D. Roberts, A. and G. Johnson, and R. Nesbitt for help and encouragement. Financial support was provided by the Australian Research Council (to RS), and by a National Science Foundation National Young Investigator Award (IBN-9357245), and the Whitehall Foundation (W95–04) to RTM. Research was conducted under the authority of Oregon State University Institutional Animal Care and Use Committee Protocol No. LAR-1848B. All research was conducted in accord with all relevant Canadian regulations, and with the US Public Health Service Policy on Humane Care and Use of Laboratory Animals and the National Institutes of Health Guide to the Care and Use of Laboratory Animals.
- Body size
- Male mate choice
- Thamnophis sirtalis parietalis