The influence of natural selection and sexual selection on the tails of sea-snakes (Laticauda colubrina)

Many phenotypic traits perform more than one function, and so can influence organismal fitness in more than one way. Sexually dimorphic traits offer a n exceptional opportunity to clarify such complexity, especially if the tra it involved is subject to natural as well as sexual selection, and if the s exes differ in ecology as well as reproductive behaviour. Relative tail len gth in sea-snakes fulfils these conditions. Our field studies on a Fijian p opulation of yellow-lipped sea kraits (Laticauda colubrina) show that relat ive tail lengths in male sea kraits have strong consequences for individual fitness, both via natural and sexual selection. Males have much longer tai ls (relative to snout-vent length) than do females. Mark-recapture studies revealed a trade-off between growth and survival: males with relatively lon ger tails grew more slowly, but were more likely to survive, than were shor ter-tailed males. A male snake's tail length relative to body length influe nced not only his growth rate and probability of survival, but also his loc omotor ability and mating success. Relative tail length in male sea kraits was thus under a complex combination of selective forces. These forces incl uded directional natural selection (through effects on survival, growth and swimming speed) as well as stabilizing natural selection (males with avera ge-length tails swam faster) and stabilizing sexual selection (males with a verage-length tails obtained more matings). In contrast, our study did not detect significant selection on relative tail length in females. This sex d ifference may reflect the fact that females use their tails primarily for s wimming, whereas males also must frequently use the tail in terrestrial loc omotion and in courtship as well as for swimming. (C) 2001 The Linnean Soci ety of London.