If females were freed of baby-rearing duties, would they turn promiscuous?

This question has been nagging me since I read evolutionary biologist Robert Trivers’ theory. He says males get maximum reproductive advantage with minimal investment, while females invest a great deal of energy and effort for few offspring. This biological cost-benefit imbalance pushes females to be cautious in their choice of mates while males philander.

What would happen if these sexual roles were switched? If males were left holding the babies, would they be faithful to one partner? Freed of responsibilities, would females become promiscuous and brawny?

Male sea horses incubate their young in belly pouches. I expected female sea horses to solicit sex from other males. But I was wrong. Males are apparently so eager to get pregnant, they compete with each other by snapping their heads at each other and wrestling with their tails while females coyly watch the outcome of these masculine brawls.

Although they can produce eggs within a matter of hours of meeting males, females play hard to get, demanding several days of courtship, says Heather Masonjones of University of Tampa, Florida. They deposit eggs in males’ pouches where they are fertilised and incubated. Pregnant fathers pump oxygen, nutrition, and sea water for the growing embryos. Females do nothing further than check on their mates every day, but they appear to be devoted to their mates. The unusual incubating arrangement doesn’t seem to make any difference to sex roles.

If Trivers’ theory is right, male sea horses ought to be choosy and play hard to get, and females would be promiscuous. Was Trivers wrong then?

Masonjones measured the amount of oxygen the genders burn up, to calculate energy consumption.

Despite all the hard work, males spend only half the energy that females do. Females exhausted from making one male pregnant aren’t in a hurry to check out other males. Although sea horses swap incubation duties, they appeared to be monogamous — some through one breeding season, others for life.

But pipefish, close relatives of sea horses, behaved differently. They share the same incubating arrangement as sea horses, yet female pipefish are promiscuous while males are choosy. Why did one fit into Trivers’ theory so neatly, while other behaved contrary to expectations? I looked more closely through scientific literature on sea horses.

Some are monogamous, while many others don’t fall into any particular pattern. They live in groups, forge short-term and long-term relationships, and both males and females switch partners. Biologists continue to puzzle over other factors that may influence sea horses’ sexual roles so I switched my attention to birds.

Sex role reversal is found in about two per cent of bird species. These are not weird or rare creatures, but common ones like buttonquails, emus, and some species of jacanas, plovers, and coucals.

Usually male birds are prettier, sporting flamboyant tails, colours, and crests. But in sex reversed species, females are generally larger and more attractive. They court males, lay eggs, and leave their mates to incubate them and rear babies, while they go off to find other males.

Why would males that have it easy give up their freedom to take on maternal duties while freeing females to play the field? Several theories have been suggested: the nature of the creature’s life history, availability of food, predation of offspring, and traveling between breeding sites.

In 2008, Hanna Kokko of University of Helsinki, Finland, and Michael Jennions of The Australian National University came up with another theory: Adult sex ratio influences mating system. In March this year, András Liker of the University of Sheffield, U.K., and his colleagues showed the theory had merit among a range of shorebirds.

If adult females outnumbered adult males in a species, then females were the child rearers. If males were numerous, they raised young while females gallivanted.

All this leads me to question whether caring and nurturing tendencies are innately feminine. They seem to be dictated by numbers, and supply and demand.


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