Weighty subject: why does dad put on pounds when a woman is pregnant?© Getty

Male monkeys gain weight during their partner's pregnancy, and this finding hints at a biological basis for expectant fathers' expanding waistlines.

Men commonly mirror symptoms of pregnancy, such as weight gain, nausea and backache. But the phenomenon, sometimes called couvade syndrome, is often dismissed as psychosomatic, with no real physical explanation.

Now, primate researcher Toni Ziegler at the University of Wisconsin-Madison and her colleagues have shown that two types of male monkeys experience one aspect of sympathetic pregnancy too.

They weighed 14 male common marmosets (Callithrix jacchus) and 11 cottontop tamarins (Saguinus oedipus) during their partners' pregnancies of five and six months, respectively. They chose these animals because the males are monogamous and take on as much or more of the childcare as the mothers - just like some human dads.

The animals gained as much as 20% of their original body weight, the team reports in Biology Letters¹.

Beefing up

Fattening themselves in this way may help the male monkeys get through the gruelling few weeks after the baby arrives, Ziegler says, and so help ensure that their offspring survive. "We think it's preparing them," she says.

The study raises the possibility that a human father who gains weight during his partner's pregnancy might also do so partly because his body is naturally stocking up for the exhausting days and sleepless nights ahead. "A long time ago it might have been advantageous," Ziegler says.

A handful of other studies have shown that expectant fathers experience swings in hormones such as prolactin, testosterone and the stress hormone cortisol. It is possible that these hormonal changes could drive some gain in weight but there have been few studies to test this idea.

Manly behaviour

Of course, men could pad out during pregnancy simply because they are mirroring the behaviour of their burdened partners, who tend to eat and rest more.

And in food-rich Western societies that already tend heavily towards the obese, it is difficult to say whether gaining weight is an advantage for a new dad.

There are also many other reasons that a man's hormone levels could go askew during pregnancy and birth, such as changes in routine and stress, points out Katherine Wynne-Edwards who has studied hormone changes in expectant fathers at Queen's University in Ontario, Canada.

"The arrival of one's mother-in-law might change the hormones of any man," she says.

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• References

  1. Ziegler T.E., et al. Biol. Lett. doi:10.1098/rsbl.2005.0426 (2006).