The functional significance of sexual display: stone carrying in the black wheatear

Male black wheatears, Oenanthe leucura, carry on average almost 2 kg of stones to cavities inside caves before each clutch is laid. This costly behaviour may play a role in post-mating sexual selection, if females adjust their reproduction to the amount of stones carried by their mates, an hypothesis tested in two field experiments. First, breeding sites with many new stones also contained many old stones from previous breeding seasons. In a stone removal experiment, in which all old stones were removed from half of the territories, but not from control territories, males carried the same amount of stones following the two treatments. Reproductive success was similar in the two treatments, and the number of old stones carried was thus an unimportant feature of the display. Second, either the number of new stones present or those carried could be the important cue of the display; this was tested by manipulating stones during the display period. When the number of new stones was doubled every second day, males reduced their stone-carrying activity, and when the number of new stones was halved every second day, males compensated for the removal. Laying date and annual reproductive success were positively affected by the treatment and the number of stones carried, but not by the number of stones present, suggesting that females pay attention only to the number of stones carried. ? 1996 The Association for the Study of Animal Behaviour Sexual selection, particularly pre-mating sexual selection, has been the subject of intense study in recent years (reviews by Andersson 1994; Møller 1994a), but post-mating sexual selection has been relatively neglected. Under social monogamy, however, the quality of parental care is a very important determinant of reproductive success (Clutton-Brock 1990), and females that are able to acquire mates with high parenting ability will experience a considerable selective advantage (Heywood 1989; Hoelzer 1989; Price et al. 1993). Theoretical treatments of direct fitness benefits such as parental care have traditionally assumed that sexual displays directly reflect the ability of signallers to provide parental care (Heywood 1989; Hoelzer 1989; Price et al. 1993). However, Burley (1986, 1988) has shown that males may experience sexual selection advantages through access to mates of high parenting ability because of either differential access or differential parental investment. Differential female access applies when attractive males acquire mates of high fecundity or parenting ability, because such females start breeding earlier, or are better able to compete for access to attractive males, than other females. This was clearly the situation envisaged by Darwin (1871) and Fisher (1930) in their models of sexual selection under monogamy, and several studies have provided evidence for this mechanism (e.g. Burley 1986; Møller 1994a). Correspondence: A. P. Møller, Department of Population Biology, Copenhagen University, Universitetsparken 15, DK 2100 Copenhagen, Denmark (email [email protected]). M. Soler is at the Depto de Biología Animal, Universidad de Granada, E-18001 Granada, Spain. J. Moreno is at the Museo Nacional de Ciencias Naturales, J. Gutierrez Abascal 2, E-28006 Madrid, Spain. M. Lindén is at the Department of Animal Ecology, Swedish Agricultural University, S-901 83 Umeå, Sweden. 0003–3472/96/020247+08 $12.00/0 ? 1996 The Association for the Study of Animal Behaviour