Why some fathers are left holding the baby
European scientists have found the answer to why, for some animals, it's the father rather than the mother that takes care of their young
The role reversal was caused by an imbalance in the numbers of males relative to females, according to researchers.
Darwin noted in 1871 that in most animals, it is the females that spend most time looking after the young, whilst males focus on competing with each other for females.
However, in some species, such as seahorses, the sex roles are reversed where the females produce the eggs but then leave it to their male mates to care for 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: since males can't produce milk it's not as easy for them to take over the parenting completely.
The study has been published in the journal Nature Communications.