Common marmosets and cotton-top tamarins—both squirrel-sized primates known for monogamous lifestyles—have pregnancies lasting five and six months, respectively. During this time, the moms' weight increases about 20 percent.
The dads, meanwhile, fatten up by about 10 percent.
The fathers-to-be likely add on the extra bulk to make sure they have enough energy to carry their young, study leader Toni Ziegler of the University of Wisconsin-Madison told LiveScience.
Once the female monkeys start to show they're with child, the males take notice and go through changes of their own.
"The males somehow cue in to the cascade of hormonal changes going on in their pregnant mates," Ziegler said.
This triggers changes in their own hormonal levels. Estrogen and testosterone levels rise, but the weight increase is most likely related to increases in the lactation-inducing hormone prolactin.
The study is detailed this week in the journal Biology Letters.
The weight doesn't stay on long—they shed the pounds while slinging the little ones around.
Human males also gain weight and show other symptoms of pregnancy, researchers have noticed, although the phenomenon has never been systematically studied in men. However, most scientists believe these sympathetic responses—known as the "couvades effect"—to be strictly emotional, and not beneficial to fatherhood.