Abstract
Sex allocation theory and empirical evidence both suggest that natural selection should favour maternal control of offspring sex ratio in relation to their ability to invest in the offspring. Generalist parasites constitute a particularly interesting group to test this theory as different females commonly utilize different host species showing large variation in provisioning ability. The common cuckoo (Cuculus canorus) is a generalist brood parasite that lays its eggs in the nest of many different passerine birds, but each female tends to specialize on one particular host species giving rise to highly specialized host races. The different host species show large variation in their ability to invest in the parasitic offspring, presenting an opportunity for female cuckoos to bias offspring sex ratio in relation to host species quality. Here, we investigate host-race specific sex allocation controlling for maternal identity in the common cuckoo. We found no evidence of any significant relationship between host race and sex ratio in one sympatric population harbouring three different host races, or in a total of five geographically separated populations. There was also no significant association between host quality, as determined by species-specific female host body mass, and cuckoo sex ratio. Finally, we found no significant relationship between individual cuckoo maternal quality, as determined by her egg volume, and sex ratio within each host race. We conclude that the generalist brood-parasitic common cuckoo show no significant sex-ratio bias in relation to host race and discuss this finding in light of gene flow and host adaptations.
Highlights
Fisher [1] showed theoretically that a 1:1 sex ratio should be evolutionary stable as there otherwise would be a frequencydependent advantage to the rarer sex
There were no significant differences in sex ratio between any pair of the three cuckoo host races (Table 1, Table 2)
We tested whether individual maternal quality was associated with sex ratio variation within each host race
Summary
Fisher [1] showed theoretically that a 1:1 sex ratio should be evolutionary stable as there otherwise would be a frequencydependent advantage to the rarer sex. Hamilton [2] showed that females being able to control offspring sex towards the rarer one should be selected for by natural selection and Trivers and Willard [3] suggested that natural selection should favour maternal control of offspring sex ratio in relation to their ability to invest in the offspring. Females in good condition will benefit by producing high-quality sons over high-quality daughters, whereas females in poor condition will benefit by producing average-quality daughters over average-quality sons In support of this theory, several empirical studies have found an association between maternal condition at conception and sex ratio of their offspring in vertebrates, in ungulates [4,5,6]
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