True monogamy（一夫一妻制） is rare in the animal kingdom. Even in species that appear to "mate for life," genetic maternity and paternity tests have revealed that philandering（玩弄女性，调情） often takes place. Yet a new study by University of Pennsylvania researchers shows that Azara's owl monkeys (Aotus azarae) are unusually faithful. The investigation of 35 offspring born to 17 owl monkey pairs turned up no evidence of cheating; the male and female monkeys that cared for the young were the infants' true biological parents.

An additional analysis of 15 pair-living mammals by the Penn team found a strong connection between a species' faithfulness and significant involvement of males in caring for their young.

"Our study is the first of any primate species, and only the fourth for a pair-living mammal, to show genetic monogamy, or real faithfulness, between partners," said study author Eduardo Fernandez-Duque, an associate professor in Penn Arts and Sciences' Department of Anthropology. "Paternal care in owl monkeys now makes sense. The males are making a huge investment in their own offspring."

Fernandez-Duque collaborated on the work with lead author Maren Huck, who completed a postdoctoral fellowship in his lab and is now a lecturer at the University of Derby, as well as professor Theodore Schurr of Penn's Department of Anthropology and Paul Babb, who completed his Ph.D. with Fernandez-Duque and Schurr and is now a postdoctoral research at Penn's Perelman School of Medicine. The study will be published in Proceedings of the Royal Society B: Biological Sciences.

As part of the Owl Monkey Project, the Penn evolutionary anthropologists have been studying a population of these primates in Argentina's Chaco region for 18 years. Previous work had shown that male and female owl monkeys form strong pair-bonds and that males contribute significantly to raising young by carrying them on their bodies, playing with them and feeding them solid foods.

Though the species was known to be socially monogamous, no one had tested whether the species was genetically monogamous -- in other words, whether there were any cases of females reproducing with a male other than her mate, a behavior known as extra-pair paternity.

To test this, the researchers paired behavioral field observations with genetic tests to see whether the "social" mothers and fathers of infant monkeys were the biological parents. They collected samples from 128 individual monkeys living in 29 groups or as solitary "floaters." This set included genetic samples from 35 infants born to 17 reproducing pairs.

By examining 14 different regions of the genome, the research team's analysis strongly suggested that owl monkeys were completely faithful. They found no evidence of extra-pair paternity.

"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 said. "So in that sense we were not very much surprised by our results. But true genetic monogamy is very rare. We would not have been surprised if there had been at least one non-pair infant, but there were none."