The ecological and evolutionary consequences of brood sex ratio variation

PhD ceremony: Mr. R. Radersma, 12.45 uur, Aula Academiegebouw, Broerstraat 5, Groningen

Dissertation: The ecological and evolutionary consequences of brood sex ratio variation

Promotor(s): prof. J. Komdeur, prof. J.M. Tinbergen

Faculty: Mathematics and Natural Sciences

Offspring that was raised in broods with sex ratios at parity had the highest fecundity, concluded Reinder Radersma. To reveal the adaptive significance of sex allocation to avian life-history, he conducted an experimental study on great tits.

Sex allocation theory is successful in predicting sex ratio variation in some taxa, but often not in birds and mammals. An important reason may be that most theoretical models do not account for the complexities of avian and mammalian life-history. Radersma investigated the effect of manipulated brood sex ratios on long-term fitness benefits. Since offspring recruitment, parental survival and parental future fecundity were unaffected, there was stabilizing selection for producing brood sex ratios at parity. To gain insight in the underlying mechanism he focused on different aspects of the offspring’s first year. Brood sex ratio did not affect fledging behaviour, but there were some effects on social behaviour in winter, which did not correlate to fitness. As adults, individuals raised as the rare sex in a brood had larger tarsi than individuals of the abundant sex. This relationship was neither present in the nestling phase nor caused by selective disappearance. This suggests a sex-specific directional effect of brood sex ratio on the late development of the offspring, which potentially links brood sex ratio to fecundity. Stabilizing selection for brood sex ratios at parity might provide an explanation why sex allocation in birds and mammals is often subtle. Higher benefits for equal brood sex ratios counteract selection for facultative sex allocation to extreme values.