Demography influences parental care in plovers
In populations with an unbalanced adult sex ratio, male plovers take care of the young
The ratio of adult males and females is an important demographic trait in wild populations. In plovers, closely related populations express strikingly different adult sex ratios, according to researchers of the Max Planck Institute for Ornithology and colleagues. This variation is mainly driven by sex differences in the survival of juveniles. Families in populations with biased adult sex ratios were predominantly tended by a single parent (typically the father), whereas in balanced populations, generally both parents take care of the young—suggesting that parental cooperation breaks down under an unbalanced sex ratio.
A biased adult sex ratio has consequences for the availability of mating partners in a population and hence social behaviour, with divorce, infidelity, and parental antagonism being more frequent. Researchers of the Max Planck Institute for Ornithology in Seewiesen and colleagues mainly from the Universities of Bath and Bielefeld examined during which developmental stage biases in adult sex ratio evolved. They then explored the association of adult sex ratio variation with patterns of parental care. To accomplish this, they monitored the survival, fecundity, and breeding behaviour of over 6000 individually marked birds from six populations of five closely related species of plovers worldwide.
Origins of biased adult sex ratios
In all six populations, the hatching sex ratio was balanced. However, the survival rate of male and female juveniles and adults varied among and within species. The researchers found that mortality during the juvenile stage contributed the most to sex ratio bias of the adult population: “Sex biases in juvenile survival was on average eight times more important than sex biases in adult survival and 327 times more important than sex biases at hatching”, says Luke Eberhart-Phillips, first author of the study. “We don’t know what causes one sex to survive better than the other during the juvenile stage, but understanding this intriguing variation will be the next step of our research,” he added.
Consequences for parental behaviour
The researchers found that the adult sex ratio in a population was tightly linked to parental cooperation: In unbiased populations, it was more likely that both parents worked together to care for the young, whereas in male- or female-biased populations, there were higher rates of single parent care. The researchers expected that the more abundant sex would be the one to provide most of the care due to limited opportunities for other mating partners. Indeed, they found that when there were more males, fathers were more likely to raise the young. However, when the population was female-biased, fathers were still more prone to care. “One possibility for this surprising result is that during the phylogenetic history of plovers, males were generally the sex providing most of the care”, says Clemens Küpper, co-author of the article. The study demonstrates that breeding strategies may respond flexibly to local mating opportunities provided by a bias in adult sex ratio—a result with profound implications for population dynamics and the evolution of social behaviour in animals.