Testing Fisher and Zahavi's theories of sexual selection

Both theories view sexual selection as a process of female choice, and the preference has to be open-ended.

An absolute preference: 'mate preferentially with males whose tails are 30 cm long'.

An open-ended preference: 'mate preferentially with the male who has the longest tail you can find'.

In Fisher's theory there can there be an equilibrium with a costly male character only if the preference is open-ended. Likewise, in the handicap theory, females must prefer males with the most costly handicaps. If we find evidence for such preferences, it suggests the male character is indeed maintained by female choice; but it does not tell us whether Fisher's runaway, or Zahavi's handicap, theory is at work.

The prediction has been tested in more than one species. One example to illustrate the procedure is Miller's study of barn swallows (Hirundo rustic ). The two sexes are similar in the swallow, except for the outermost tail-feather which is about 16% longer in the male than in the female. Miller tested whether females open-endedly prefer males with longer tails by experimentally shortening the tails of some males, by cutting them off with a pair of scissors, and elongating the tails of others, by sticking those severed tail feathers on to other intact males, with superglue that hardened in less than 1 second. He then measured how long it took the different males to find a mate: males with elongated tails mated faster, resulting in higher reproductive success.

Further evidence for Zahavi's argument comes from the observation that many forms of sexual display show the blood color of the male, which is a good indication of an organism's health. The display of this anole lizard opposite, for example, shows the quality of its blood.

W.D. Hamilton explains why he thinks sexual displays, such as the peacock's tail, are a means of indicating health.