A study of shorebirds has helped shed light on why some species reverse the roles of the sexes, with males carrying out the parental duties.
A team of European researchers found that an imbalance between the number of males and females triggered the change.
They reported the switch occurred when there was a higher ration of males to females, making it beneficial for males to stay with their mate.
The findings have been published in the journal Nature Communications.
Adult sex ratio
It had been argued that the conventional sex roles were widespread because females invested considerable energy in producing eggs so the survival of the offspring was a priority, therefore it made sense for the female to oversee the care of the young.
"Although a lot of research has investigated the reasons for why animals have many contrasting types of breeding behaviour, we are still far from the full understanding of this question," explained co-author Andras Liker from the University of Sheffield.
"A simple possibility is that, among other things, the opportunity to find a new partner can influence mating and parenting decisions, hence the number of males and females in a population - the adult sex ratio (ASR) - may be important."
Prof Liker added that this theory had been suggested by several mathematical models, but had not been systematically studied.
He explained that the team compared ASR between shorebirds with the non-conventional sex roles, collecting data from published literature on the sex ratio, mating and parental behaviour of these species.
Prof Liker said that many of the known examples of role reversal were found in shorebirds, hence the reason for these birds being selected for the study.
"Sex role reversal also occurs in some other groups of birds, such as kiwis and tinamous, but - in general - it is rare (in birds). It also occurs in frogs and fish, like seahorses and pipefish," he told BBC News.
"[It] has been a formidable puzzle for evolutionary biologists ever since Darwin," he explained.
"Our study is the first supporting the idea that sex ratio plays an important part in the evolution of role reversal."
'Holding the baby'
Fellow co-author Tamas Szekely from the University of Bath said the research group had been investigating sex role reversal for more than two decades, so it was "extremely pleasing to see such a clear-cut result".
"When there are lots of males in a population, it's harder to find females so it benefits males to stay with their mate and look after the young," Prof Szekely observed.
"However, the females often take advantage of this and leave the male holding the baby while they go and find another mate."
Prof Liker said that he hoped the team's findings would lead to further research on the significance of population sex ratios.
"For example, it would be very interesting to know what factors generate sex ratio differences between different species or populations," he said.
"This may originate either from the differential production of male and female offspring, or may be the result of the different mortality of adult males and females.
He added: "Sex differences in body size, migration, or other behaviour can expose the males and females to different causes of mortality, and if one sex survives better than the other the ASR becomes biased."
A separate study by researchers from the University of East Anglia showed that extensive shell fishing and sewerage discharge in estuaries could have differing consequences for male and female Icelandic black-tailed godwits.
The research, published in the journal Ecology and Evolution, revealed very different winter feeding habits between the sexes of these wading shorebirds.
The study showed that females, which were bigger and had longer bills, were able to peck deeper into the silt for prey than the shorter-billed males. As a result, the males were more exposed to environmental factors that affected food resources.