MUTUAL MATE CHOICE IN STICKLEBACKS - ATTRACTIVE MALES CHOOSE BIG FEMALES, WHICH LAY BIG EGGS

Brighter red three-spined stickleback, Gasterostetus aculeatus, males have been shown to be preferred by females in the laboratory but in th e field, these males did not receive more eggs. Instead, they had heav ier eggs in their brood. We investigated the hypothesis that sexual se lection for red coloration in male sticklebacks acts through mate choi ce by preferred males, who can afford to be choosy, for high-quality f emales which lay heavier eggs. We assume here that heavier eggs provid e a direct fitness advantage. In simultaneous choice tests males were presented with two females differing in size. The number of zigzags di rected to and the time spent orienting to each female were measured. A fter the test the females laid eggs, which we counted and weighed. Big ger (i.e longer and heavier) females laid significantly more and heavi er eggs than smaller females. For all 23 males pooled together, the pr eferred female was the bigger of the two in 17 cases, laid more eggs i n 18 cases,but laid heavier eggs in only 13 cases. When bright and dul l males were analysed separately, we found that bright but not dull ma les spent more time oriented to the bigger female, and to the female t hat laid more eggs. Females preferred by bright males tended to lay he avier eggs than nonpreferred females, although this result was not qui te significant. We conclude that in nature this preference for bigger females results in brighter males receiving on average heavier eggs. A ssuming higher survival of bigger offspring, we propose that this can explain how brightness can be sexually selected in spite of brighter m ales not receiving more eggs. (C) 1998 The Association for the Study o f Animal Behaviour