Eduardo Fernandez-Duque, an associate professor in the Department of Anthropology in Penn Arts & Sciences, has been studying owl monkeys in Argentina for nearly 20 years. From observing their interactions day in and day out, he knew that males and females formed strong bonds.

“In the 18 years of the Owl Monkey Project, we never witnessed a little sneaky copulation with a neighbor, or that one partner dashed off for some time,” Fernandez-Duque says.

But a new study by Fernandez-Duque and collaborators shows that owl monkey couples are perhaps even more faithful than the researchers previously believed. Moreover, signs suggest that the involvement of fathers in caring for infants plays a role in this loyalty.

To see if owl monkeys practiced so-called “genetic monogamy”—in other words, if males and females didn’t cheat on their mates—the Penn team analyzed genetic samples from 128 monkeys living in 29 groups or as solitary floaters. These samples included 35 infants born to 17 reproducing pairs.

According to the “paternity tests,” owl monkeys don’t cheat. The biological fathers of each infant indeed appeared to be the biological mother’s mate. Fernandez-Duque’s research marks the first time genetic monogamy has been demonstrated in a primate species, and only the fourth time it has been shown in a pair-living mammal species.

“True genetic monogamy is very rare,” says Fernandez-Duque. “We would not have been surprised if there had been at least one non-pair infant, but there were none.”

Expanding on their findings, the researchers evaluated the results of similar experiments that had been conducted on 14 other mammal species that are socially monogamous, but not necessarily genetically monogamous. Their analysis identified a strong connection between genetic monogamy and intensive infant care by males.

Fernandez-Duque says this finding makes sense on an evolutionary level: A male is more likely to invest energy in raising an infant if he is confident the infant is carrying his genes. That connection can shed light on the monogamy and pair bonding that is present in other species—humans included.

“Pair bonding—love, if you want—is prevalent in all human societies, whereas fathering is much more variable,” Fernandez-Duque says. “The owl monkey story is suggesting that, under very specific ecological settings, this preference for each other leads to the pair spending a lot of time in close proximity, thus facilitating paternal care and increasing paternity certainty. Genetic monogamy is the result.”