A flurry of

studies followed, confirming that females of some species actively sought extra-pair opportunities (e.g. Kempenaers et al., 1992) and raising the question that had so challenged Darwin about the possible benefits that females gained by choosing to copulate with a particular male. The issue was a thorny one in terms of extra-pair behaviour because the only benefits that females can gain are genetic, because (assuming extra-pair males provide no parental care – and typically they do not), the only thing they obtained from males was semen. The situation Selleck CHIR99021 was very similar to that among the females of lekking species, where the only benefits of mate choice were indirect (genetic). Accordingly, this became known as the paradox of the lek (Kirkpatrick & Ryan, 1991).

In fact, there was one direct benefit females could gain by copulating with more than one male – insurance against a partner being infertile. The idea among researchers that infertility might drive infidelity in non-humans is undoubtedly RO4929097 manufacturer coloured by the situation in humans, where women are known to seek extra-pair partners if they are having trouble conceiving (Jequier, 1985). In birds at least, true infertility, that is, males having inadequate sperm supplies to fertilize a females’ eggs, seems to be extremely rare (Birkhead et al., 2008). Cases where we might expect temporary infertility as a result of sperm depletion, as in the case of polygynous species with particularly large harems (e.g. Gray, 1996), 4-Aminobutyrate aminotransferase still need to be critically examined. The lek paradox revolves around the maintenance of additive genetic variation in traits subject to strong, directional sexual selection (Fisher, 1930; Kirkpatrick & Ryan, 1991). If females prefer males with particular traits why have these traits not gone to fixation? Several solutions have been suggested, including fluctuating selection, such as that which would occur through

host–parasite co-evolution (Hamilton & Zuk, 1982), and ‘genic capture’ (Houle, 1992; Rowe & Houle, 1996), which is based on the idea of mutation–selection balance, where male quality of condition is determined by so many alleles that mutations occur as quickly as selection removes them. Testing these ideas has been problematic, for many reasons, but particularly because it has proved difficult to define and measure male quality. It has also been suggested that the idea of females initiating extra-pair copulations in birds may have been overplayed (Westneat and Stewart, 2003). With no consensus over possible female benefits to promiscuity, it is possible (see Griffith, 2007) that for many birds, extra-pair copulation carries relatively little benefit, but also little cost, especially in those species where the incidence of extra-pair paternity is relatively low.