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When sex roles reverse


Why don’t men do more to help raise their children?

This question isn’t taken from a women’s weekly, it’s an evolutionary teaser given to undergraduate biology students. Humans are monogamous, on the whole, forming long-term relationships in which males contribute to the care and protection of their offspring. But they could still do more, and I don’t mean just changing the baby’s diapers at 3 a.m. Men have nipples, for example, but they don’t breast-feed. Why not?

Because most of the energy that males of almost all species allocate for reproduction gets spent on attracting and winning mates, that’s why not. Most of the energy that females spend on reproduction goes toward nurturing and caring for their offspring. Maybe it’s not worth it, evolutionarily speaking, for men to suckle their offspring.

Some men might protest this representation of their sexual roles. There are more and more househusbands, they could say, who look after the children while the wife works. But how many would be willing to go further, to take on pregnancy and nurture their unborn offspring while the wife struts and carouses with other women? This is sex-role reversal gone too far, they would probably say.

In that case, they should take a look at what goes on in pipefish.

Copulation in these fish (a group that includes sea horses) consists of the transfer of eggs from the female to a special brood pouch in the male. Instead of sperm going to the female, the eggs go to the male, but that’s not the only thing that is reversed. The male fertilizes the eggs and carries and protects them until they hatch. After that, the young fish are on their own.

Evolutionary theoreticians have long predicted that male pregnancy, when it occurred, would lead to extreme sex-role reversal, meaning that females would be free to do the things that males usually do. And only this week it wasconfirmed that female pipefish really do behave like males in “normal” species: They compete just as hard, and they have courtship displays to try to attract males. And, yes, they sleep around.

Adam Jones, a researcher at Oregon State University’s department of zoology, and colleagues from the University of Georgia conducted a genetic analysis of wild Gulf pipefish off the Atlantic coast of Florida. The scientists investigated the strength of sexual selection in females using three measures: how often males and females mated; how successful they were; and how sexually successful and unsuccessful females differed.

In species with conventional mating systems, males tend to mate more than females, because sperm are cheaper to make than eggs and because, with internal fertilization, females are often left “holding the baby.” But in Gulf pipefish, males only mate once per breeding season, whereas females will mate several times.

Moreover, this sets up conditions in the pipefish world whereby females face intense competition for mates. If females are mating with more than one male, but males are only mating once, it’s easy to see that there won’t be enough males to go round. The variation in mating success — the second measure of sexual selection — is larger in females than it is in males. Some females do very well and some very badly, but all males do OK.

The high level of competition has led to the evolution in female pipefish of traits that increase attractiveness or, perhaps, their ability to fight. Familiar examples from the males of other species would be the peacock’s tail or the stag’s antlers. In Gulf pipefish, the successful individuals were larger and had more elaborate ornaments (brilliantly colored striped bodies).

Jones and co-workers conclude that the strength of sexual selection in pipefish rivals that in any other animal. “The Gulf pipefish,” they write in the journal Proceedings of the Royal Society, “exhibits the most extreme form of classical polyandry yet described.”

But how did it evolve?

Sticklebacks (close relatives of pipefish) have a habit of building and defending nests in which females lay their eggs. Male pregnancy might have evolved from that. “It is a small leap from eggs that are stuck together and are sometimes transported by males to eggs that are stuck directly on the surface of the male,” said Jones in an e-mail interview. “Male pregnancy might have increased a male’s ability to aerate or protect the eggs . . . or prevented fertilization by rivals through sneaking [i.e., sex outside the pairbond].”

The pipefish family, the authors say, “represents a largely untapped resource for the study of the evolutionary causes and consequences of differences between the sexes.”

Even for the differences between men and women. Work on sexual selection in other species might help us understand why men don’t breast-feed their young. In rare cases, lactation does occur in male mammals. Males of a certain bat species have been reported to lactate in the wild, and milk production is seen occasionally in human males, too.

“With our complex social systems, almost any form of sexual selection is conceivable,” said Jones. So could sex-role reversal in humans progress further than an increased frequency of house husbands?

“Given the pace at which technology has advanced over the last few decades, it is hard to say what might be possible in the future. An elaboration of male parental care is certainly within the realm of possibility.”