Previously it was thought that these were just psychosomatic symptoms. For many such dads, however, it is likely that the changes help them to cope with the rigors of fatherhood once the baby is born.
This is particularly important for the doting, yet squirrel-sized, marmoset and tamarin dads, whose job includes toting around their often hefty babies.
"Males do most of the carrying of infants — usually two — once they are born," said Toni Ziegler, lead author of the study, which was published in a recent issue of
Biology Letters.
Ziegler, a senior scientist at the University of Wisconsin, Madison, added, "The males invest highly in infant care, even losing weight while carrying these heavy, multiple infants through the trees."
Ziegler and her team obtained monthly weight measurements for 29 male marmosets and 29 male cotton-top tamarins. Of these monkeys, 9 marmosets and 11 tamarins had pregnant mates.
The researchers determined that the expectant fathers experienced roughly a 10 percent increase in weight gain during their mate's pregnancy.
The extra poundage, which was not associated with copied eating habits, occurred gradually over the gestation periods, which are five months for marmosets and about six months for tamarins.
The extra padding is not the only pregnancy symptom that scientists have identified in the monkey dads. The males apparently are so in tune with their mates that they undergo other physiological changes.
"We have found for cotton-top tamarins, that males show hormonal changes around mid-pregnancy and these changes may help prepare them for their role in infant care," Ziegler told Animal Planet News. "They also detect when their mate is going to ovulate and have an increase in testosterone a few days before she ovulates, so I think they are getting signals for the female at important reproductive events."
Brian Craft, lead keeper at the Oakland Zoo in California, was not surprised by the findings.
Craft told Discovery News that most monogamous non-human primate dads have the following daily schedule: "feed, travel, rest and then more of the same, with just a little time in between for play."
He added, "The weight gain could serve as a decoy for predation, but it more likely fortifies males, who have less time for foraging once the offspring are born. The changes, therefore, may benefit the entire family."
