The role reversal in species such as sea horses and some others has evolved because of an imbalance in the numbers of males relative to females, the said.
In 1871, Charles Darwin observed that in most animals, it is the females that spend most time looking after the young while males focus on competing with each other for females.
However, in some species -- such as seahorses -- the sex roles are reversed, with the females producing the eggs before leaving it to their male mates to rear their offspring.
Scientists at the Universities of Sheffield and Bath in Britain, along with Hungarian colleagues, said an ongoing higher ratio of males to females in the population of a species sometimes leads to sex role reversal when it comes to raising young.
"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," Bristol researcher Tamas Szekely said.
The role reversal isn't usually seen in mammals, the researchers said, because males can't produce milk, making it difficult for them to take over the parenting completely.
"Sex-role reversal has been a formidable puzzle for evolutionary biologists ever since Darwin," Sheffield researcher Andras Liker said. "Our study is the first supporting the idea that sex ratio plays an important part in the evolution of role reversal."