We’ve all heard the phrase “Mr. Mom” as a descriptor for involved fathers, but men may be more like their female counterparts when it comes to nurturing than we expect.

Most animals are risk-averse and tend to avoid danger, but parents can be a different story. Certain species of mammals will even risk their own lives to save their offspring.

This extreme bond between parent and child has its roots in a biological phenomenon known as “bonding.” When children are born, their mothers experience a rush of hormones designed to facilitate the bonding experience, including oxytocin, estrogen, progesterone and others.

Perhaps the most interesting hormone released during this process is prolactin, which stimulates lactation in new mothers to feed their offspring. It is intriguing not only because of its physical effect on mothers, but because it also appears in (and influences) their mates as well.

Researchers at the Wisconsin National Primate Research Center (WiNPRC) at the University of Wisconsin (UW) have discovered male tamarins and marmosets—which live in family units like those of humans—display certain physical characteristics when their respective mates become pregnant.

“The father is critical to the survival of the offspring,” Toni Ziegler, distinguished scientist at the WiNPRC, said of the animals during an interview on a recent BBC Earth podcast.  “And what we know about them from our studies, is the father is picking up on cues from the mother that she’s pregnant. And the father actually starts gaining weight.”

Ziegler noted this is consistent with human males, who frequently report gaining “sympathy weight” when their partners become pregnant.

This discovery could potentially lead to a better understanding of the bonding process and what can be done to nurture the infant-parent relationship from birth into early childhood.