PHILADELPHIA -- In 15 years of studying owl monkeys in Argentina, this was a first: Late last November, Eduardo Fernandez-Duque, assistant professor in the Anthropology Department at the University of Pennsylvania, got news from his field assistants that they had spotted a set of newborn twins.

The discovery of the large-eyed pair presented Fernandez-Duque with the opportunity to observe a unique twist on a common thread in his research — the division of care between male and female parents.

“This has really come as a wonderful natural experiment,” Fernandez-Duque said.

The owl monkey twins are featured in a video produced by National Geographic.

Through previous work, Fernandez-Duque and colleagues have demonstrated that the social habits of his study species are unusual in the world of monkeys. Owl monkeys are monogamous; adult males and females have exclusive sexual and, seemingly, emotional relationships.

Along with his colleagues, Fernandez-Duque conducted observations of the primate family, which lives in Argentina’s Chaco region. Owl monkeys, as their name implies, are nocturnal, so the researchers often employed night-vision goggles to watch the animals’ interactions.  

These observations have revealed that, not only are owl-monkey pairs committed, but males go above and beyond, shouldering the lion’s share of child-rearing.

“The most conspicuous behavior we notice is that the males are the main carriers of infants, carrying them up to 80 and 90 percent of the time,” Fernandez-Duque said. “That is a huge energy investment.”

With the special scenario of caring for twins, Fernandez-Duque hypothesized that the father might share the load a bit more, recruiting help from the mother or older siblings of the twins. But from what he’s seen so far, that hasn’t been happening. Instead, the father has done the majority of carrying, grooming, and playing with both infants.

That doesn’t mean the mother is lazy. Fernandez-Duque believes it’s possible that the father provides most of the care for his offspring in order to help the mother recover from pregnancy. And, he added, “we should not forget the large energetic burden of providing milk for two infants.“

Through ongoing studies in collaboration with fellow Penn anthropologists Theodore Schurr and Claudia Valeggia, Fernandez-Duque plans to analyze genetics, hormone levels and behavior to investigate the biological underpinnings of social relationships in owl monkeys and other pair-bonded monogamous primates.