Mating Is Correlated with a Reduced Risk of Predation in Female Red-Sided Garter Snakes, Thamnophis Sirtalis Parietalis

By O'Donnell, Ryan P.; Mason, Robert T. | The American Midland Naturalist, January 2007 | Go to article overview

Mating Is Correlated with a Reduced Risk of Predation in Female Red-Sided Garter Snakes, Thamnophis Sirtalis Parietalis


O'Donnell, Ryan P., Mason, Robert T., The American Midland Naturalist


Abstract.-

We took advantage of unique aspects of the red-sided garter snake's breeding biology at northern latitudes to investigate whether mating was correlated with a reduced risk of predation. We scored the number of mated vs. unmated and predated vs. surviving female snakes at four dens in Manitoba, Canada. Predation intensity did not significantly vary through time during the course of the study, nor did it vary among dens. Female mating significantly decreased the odds that a female would be the victim of predation. This is likely a result of reduced courtship of females after mating, which reduces female conspicuousness and facilitates the ability to escape. These results provide support for the hypothesis that the copulatory pheromone, which communicates female mating status to males and thus reduces male courtship, benefits females by indirectly reducing their risk of predation after mating.

INTRODUCTION

The costs and benefits of mating are important in the understanding of life-history theory (Trivers, 1972; Andersson, 1994). One of the costs often associated with courtship and mating is an increased exposure to predators (e.g., Berglund et al., 1986; Herberstein et al., 2002). Reduced exposure to predation following mating is an often-invoked but rarely demonstrated benefit of mating for females.

Red-sided garter snakes (Thamnophis sirtalis parietalis) in many northern populations are exposed to heavy predation during the breeding season (Aleksiuk, 1977; Shine et al., 2001b). After emerging from their underground hibernacula, they form large breeding aggregations on the surface of the den and in its immediate vicinity (hencefordi, "at the den"). These large breeding aggregations consist of many conspicuous mating balls that generally consist of 1 to 100s of males courting a single female.

Red-sided garter snakes have a pheromone that communicates female mating status to males (Devine, 1977; O'Donnell et al., 2004). This copulatory pheromone has been hypothesized to benefit females because it drastically reduces courtship of a female by subsequent males after she has mated. Reduced courtship by males, in turn, reduces females' energy expenditure (Shine et al., 2000a, 2004) and is thought to reduce their risk of predation (Shine et al., 2000a). Reduced courtship has been predicted to reduce a female's risk of predation because courting males in mating balls are thought to attract the attention of predators, but evidence supporting this hypothesis is lacking (Shine et al., 2000a, 2001b). The reverse also seems plausible, that courting males physically shield females from predatory attacks, in which case reduced courtship would increase the risk of predation. In either case, documented effects of mating status on susceptibility to predation are exceedingly rare.

We took advantage of unique aspects of the biology of red-sided garter snakes in the northern part of their range to test the hypothesis that female mating, and its consequent reduction in attractiveness to males, reduces female risk of predation. The duration of the copulatory pheromone which reduces male courtship is approximately equivalent to the duration of the mating plug (O'Donnell et al., 2004). The mating plug is externally visible in the cloaca and, thus, makes it possible to score mated (unattractive) and unmated (attractive) females at the den. Because of the high concentration of sexually active males at the den, all unmated females are courted, regardless of their sexual maturity or receptivity (Shine et al., 2000a). Predation at the dens is very common, thus we are provided a unique opportunity to score many predation events in the field over a relatively short time span (Aleksiuk, 1977; Shine et al., 2001b).

If mating does reduce the risk of predation for female garter snakes, then there should be a correlation between mating status and odds of being a victim of predation. This prediction is based on the hypothesis that recently mated females experience a reduced risk of predation due to reduced courtship by males. …

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