Male primates gain weight while their mates are pregnant.

Marmosets at the National Primate Research Center. A red tag indicates a female monkey of breeding age; blue indicates a male monkey of breeding age. Credit: Jeff Miller, March 2005

Goldie the tamarin is expecting a child. During the pregnancy, Goldie gains weight and experiences increased levels of several hormones, including estrogen. Goldie’s situation seems pretty unremarkable except for one small detail: Goldie’s a dude.

In a recent study published in Biology Letters, researchers from the University of Wisconsin-Madison showed that male tamarins and marmosets—two monkey species known for their long tails and multicolored coats of fine fur—gain weight while their mates are pregnant. The researchers believe these monkeys put on the pounds to prime themselves for their future fatherly duties.

“Males most likely gain weight to prepare them for the energetic costs of carrying infants for several months following birth,” said lead researcher Toni Ziegler via e-mail. “It’s quite a load, especially for males who don’t have other offspring in the family to help with the carrying.”

Ziegler said she and her team initially noticed the weight gain while studying hormonal changes in male marmosets and tamarins when their mates were pregnant. To test their suspicions, they decided to weigh the males monthly during their mates’ gestation periods—five months for marmosets, six months for tamarins.

All the males gained weight during a mate’s pregnancy. Although the amount varied from monkey to monkey, the average gain was around a 10% increase in girth. Ziegler added that the male monkeys also showed increased levels of hormones such as prolactin, estrogen and testosterone.

Ziegler said both of these species—the common marmoset and the cotton-top tamarin—are known for their monogamy and devotion to parenthood. This emphasis on male participation in child-rearing means that future energy consumption might be responsible for the increase in weight.

“The parents of both species are monogamous and they form special attachments to each other,” she said. “Not all marmosets and tamarins species are strickly monogamous but these two species maintain their social bonds with their mate. The males stay with the female and their offspring and are there to do most of the infant care.”

Ziegler said she hopes to study whether the fathers who show the largest gains in weight and hormone levels are the same ones who are most responsive to their infants. This might yield insight into what makes a devoted father, the researchers said.

Human males also complain of weight gain during a partner’s pregnancy. However, according to Ziegler, it is unclear whether the weight gain in humans serves the same biological purpose—storing energy for carrying and caring for infants—as it does in monkeys.

Perhaps the men of our species should demonstrate their devotion through love handles and start storing up the energy they’ll need to change diapers throughout the night.

Originally published February 6, 2006


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