When the balance of the sexes is skewed towards one gender, parents are more likely to split up, leaving the father to care for the offspring, says a study from an international team of scientists studying bird populations.

The researchers, including scientists from the Milner Centre for Evolution at the University of Bath, studied six different populations of plovers located across Africa, Asia and Latin America.

Whilst three populations of plover had a balance in the sex ratio of males to females and shared parenting of their offspring, the scientists found that in populations where there were more males than females, or vice versa, the parenting roles shifted leaving the males to look after the chicks.

Professor Tamás Székely, Professor of Biodiversity and Royal Society Wolfson Merit Award Holder at the Milner Centre for Evolution at the University of Bath said: “When there are more males in the population, the females have more opportunities to find partners and so they are more likely to leave the family and mate with multiple partners in the breeding season, leaving their male partner to look after the chicks.

“Conversely, it is harder for males to get another partner and so they are more likely to stay monogamous and be the primary parent and invest time in raising their offspring.

“By influencing mate availability, adult sex ratio bias can alter social behaviour with divorce, infidelity, and parental antagonism being more common in sex-biased populations.

“Moreover, in human societies, adult sex ratio variation is shown to be linked to economic decisions, community violence, and disease prevalence. So we wanted to look at what factors were related to imbalances in adult sex ratio of plover populations.”

The study, published in Nature Communications, took data collected over 10 years from six wild shorebird populations that were closely related but displayed different parental strategies.

Dr Luke Eberhart-Phillips, from the Max Planck Institute for Ornithology (Germany), was first author on the paper. He said: “We found that the chicks had a 50:50 sex ratio at hatching in all these species, and that the skewed adult sex ratios were caused by a difference in survival of male and female juveniles, although it’s still unclear why this happens.

“Our study highlights the knock-on effects that differences in survival rates between the sexes can have on population dynamics and social behaviour.”

The researchers hope in the future to investigate the effects of sex ratio on social behaviour in other animal populations including humans.

The research was jointly funded by the Natural Environment Research Council, Biotechnology and Biological Sciences Research Council, The Leverhulme Trust, CONACYT (Mexico) and Deutsche Forschungsgemeinschaft (Germany).

"Demographic causes of adult sex ratio variation and their consequences for parental cooperation” Luke Eberhart-Phillips, Clemens Küpper, María Carmona-Isunza, Orsolya Vincze, Sama Zefania, Medardo López, András Kosztolányi, Tom Miller, Zoltán Barta, Innes Cuthill, Terry Burke, Tamas Szekely, Joseph Hoffman, and Oliver Krueger (2018) is published in Nature Communications, DOI: 10.1038/s41467-018-03833-5