Parent Cannibalism of Offspring and Egg Raiding as a Courtship Strategy

In fish, and possibly other groups, egg-guarding males may benefit from the consumption of some of their own genetic offspring by remaining in good condition for subsequent brood cycles. This parasitism of the female's great cytoplasmic commitment to reproduction generates conflicts of interest which provide an evolutionary explanation of several previously enigmatic behavioral patterns. The fundamental conflict obtains from females trying to minimize their egg loss by laying for males that will receive multiple clutches; this means males without eggs are handicapped in courtship. Egg raiding in sticklebacks, wherein eggs are stolen and deposited in the thief's nest, is interpreted as a courtship strategy, partly deceptive and partly honest, designed to convince the female that loss of her eggs to the fasting male will be minimal. The canopied nests of sticklebacks may conceal the absence of eggs and make courtship fanning serve to deceptively entice females to proceed with courtship, because fanning normally indicates that the male has eggs. The shift from the courtship to the parental phase by guardian males occurs because females should refuse to lay for males with perceptibly advanced eggs yet will benefit from eating such eggs. Finally, differences among species in the relative importance of filial cannibalism and heterocannibalism may explain patterns of sexual size dimorphism in fish with parental care. Where heterocannibalism is more important, females should prefer larger males as better brood defenders; thus, epigamic and agonistic sexual selection are reinforcing. But where filial cannibalism is more important, females should prefer small males because they need to consume fewer eggs for their own maintenance; thus, intramale selection for larger body size is countered by female choice of small males.