Female cannibalism and male courtship tactics in threespine sticklebacks

Summary Female threespine sticklebacks (Gasterosteus aculeatus) frequently raid male nests and eat all the eggs therein. We tested the hypothesis of Vickery et al. (1988) that females prefer to raid nests containing large numbers of eggs than ones with smaller numbers of eggs. This hypothesis is based on the finding that females spawning in nests containing many eggs will have reduced hatching success because of egg crowding. By consuming the male's eggs and forcing him to rebuild his nest, raiding females might obtain a new opportunity to spawn under better conditions. Our results were consistent with the first prediction of this hypothesis that females were more likely to spawn in nests containing fewer eggs than in nests with many eggs. However, this may be the result of males becoming less receptive to females as the number of eggs in their nests increases. Prediction 2 was that females should raid those nests containing the most eggs. Contrary to this prediction, males defending only one clutch were as likely to have their nests raided by groups of females as males defending several clutches of eggs. Female cannibalism is therefore unlikely to have evolved as a means of gaining access to a male defending a small number of eggs. We also examined the tactics used by males to counter female raids. Most raids occur when the male is courting, and nests are more vulnerable to shoals of females than to single females. Therefore, we hypothesized that males with eggs preferentially court a single female rather than large groups of females, and that males without eggs court both groups indiscriminately. We also predicted that males restrict the number of females they mate with when risk of having their nest raided is high. Our results indicate that: (1) both males with eggs and those without eggs minimize the risk of female cannibalism by courting solitary females rather than groups of females and (2) males limit the number of females that lay eggs in their nest when several potentially raiding females are present. Offprint requests to: G.J. FitzGerald