Sexual selection and the role of parasites.

Six years ago, William Hamilton and Marlene Zuk(1) proposed a novel and - on the face of it - bizarre explanation for the evolution of secondary sexual characters. Contrary to most theories of sexual selection through female choice(2), Hamilton and Zuk suggested that the bright colours and elaborate ornaments of many animal species have a utilitarian function: they allow females to assess a potential mate's ability to resist parasites. Recently, several studies have begun to test the hypothesis directly, and the idea has been discussed at two meetings(3,4). Here I review the work to date, and discuss the difficulties involved in gathering and interpreting the evidence still required.