Confusion within ‘mating balls’ of garter snakes: does misdirected courtship impose selection on male tactics?

Abstract

Although courting males are under intense selection to recognize the sex of potential partners, mistakes sometimes occur. Using descriptive and experimental data on garter snakes, Thamnophis sirtalis parietalis, from courting aggregations around a communal den in Manitoba, we tested two previously proposed hypotheses that suggest evolutionary significance to such mistakes. One idea, that female mimicry enables a ‘she-male’ to confuse his rivals within a mating ball, predicts that many mating balls will concurrently contain both she-males and females; where both types of sexual targets are present, males will frequently align their bodies with she-males rather than females. The second idea, that small body size confers a selective advantage to males because it facilitates sex recognition and thus reduces misdirected courtship by other males, predicts that larger males will receive more courtship than their smaller rivals within mating balls. Our results falsify these predictions. Natural courting groups rarely contained both females and female-mimicking males. When both potential sexual targets were present, males essentially ignored she-males. Similarly, male snakes rarely attracted courtship even when they were larger than females. The sensitive chemosensory apparatus of male garter snakes enables these animals to focus their courtship on females, ignoring males that resemble females either physically (body size) or chemically (pheromones). The degree to which a male garter snake resembles females thus has little or no significance for his mating success within a communal mating ball; further work is needed to evaluate the generality of this conclusion for other snake species. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.