Pre- and postcopulatory sexual selection in Threespine Stickleback.

The Threespine Stickleback male with its conspicuous sexual traits, is a well-known model for pre-copulatory sexual selection. Threespine Stickleback are also often engaged in sneak fertilizations, inducing sperm competition and post-copulatory sexual selection.

Questions: Is there functional variation in sperm traits (sperm morphology, swimming speed) as expected from sperm competition theory? How do pre- and post-copulatory sexual selection interact?

Recent advances have been made in understanding functional variation in Stickleback sperm traits. Sperm with a longer tail and a smaller head-to-tail-length ratio swam faster but did not live as long. In sperm competition experiments with equal sperm numbers of rival males, head-to-tail-length ratio of a male’s sperm determined paternity. When one of the two rivals ejaculated more sperm or ejaculated before the other male, his paternity share increased. Relatedness only influenced pre-copulatory sexual selection. Sperm traits correlated with breeding coloration. There are some intriguing links between pre- and post-copulatory sexual selection. Attractive males produced relatively more sons than duller males. Under food restriction, sperm swimming speed and coloration were traded off.

I will start my presentation by discussing a more fundamental level of sexual selection and present an experiment on environmental sex reversal in Threespine Stickleback.