Why do male snakes have longer tails than females?

In most snake species, males have longer tails than females of the same bod y length. The adaptive significance of this widespread dimorphism has attra cted much speculation, but few tests. We took advantage of huge mating aggr egations of red-sided gartersnakes (Thhamnophis sirtalis parietalis) in sou thern Manitoba to test two (non-exclusive) hypotheses about the selective f orces responsible for this dimorphism. Our data support both hypotheses. Fi rst, relative tail length affects the size of the male copulatory organs (h emipenes). Males with longer tails relative to body length have longer hemi penes, presumably because of the additional space available (the hemipenes are housed inside the tail base). Second, relative tail length affects male mating success. Males with partial tail loss (due to predation or misadven ture) experienced a threefold reduction in mating success. Among males with intact tails, we detected strong stabilizing selection on relative tail le ngth in one of the two years of our study. Thus, our data support the notio n that sex divergence in tail length relative to body length in snakes refl ects the action of sexual selection for male mating success.