Differences in the size of males and females are common in nature, and the simple shape of snakes makes them excellent subjects for studies of the phenomenon. In many species, males have longer tails than females, and this dimorphism seems to have arisen early in snake evolution. But why? From studies in Manitoba, Canada, R. Shine and colleagues provide evidence that long-tailed males make better lovers than their shorter rivals (Proc. R. Soc. Lond. B 266, 2147–2151; 1999).
Mating of red-sided gartersnakes (Thamnophis sirtalis parietalis) takes place just after they emerge from over-wintering, when they form mating balls (see picture). Shine et al. found, first, that longer-tailed males tended to be more successful in copulating. Second, they took advantage of an event in which, due to the enthusiasm of males to get in on the act, over 100 had suffocated at the bottom of a huge ball. From these dead specimens, the authors found that the length of the tail and the size of a male's reproductive organ, the hemipenis, are correlated. Finally, snakes with shortened tails, due to predation or misadventure, obtained far fewer matings than their intact compatriots.
These results are only correlations, and it is unclear how natural selection has influenced the evolution of this trait. Females may select mates using tail length as an indicator of sexual potency, or perhaps a longer tail makes it easier for males to hold their partner in a loving embrace.