Monogamy is rare in mammals, but there’s a group that researchers widely consider to be socially monogamous: owl monkeys. However, a new study of these canopy-dwelling, nocturnal primates, which live in areas that stretch from Panama to Argentina, suggests that sometimes even long-bonded pairs endure break-ups that have consequences for their future reproductive success.

Eduardo Fernandez-Duque, an associate professor in the Department of Anthropology in the School of Arts and Sciences, led the research with Maren Huck, who completed a postdoctoral fellowship at Penn. Their findings were reported online in the journal PLOS ONE.

Fernandez-Duque, along with colleagues and students, has been studying owl monkeys in the Chaco region of Argentina since 1997, regularly monitoring their behavior and biology. Until recently, the researchers believed the primates formed pair-bonds that lasted indefinitely.

However, more recent observations have shown that these pairs-bonds can be volatile. Break-ups do not arise because of a falling-out or loss of interest, but due to interloping “floater” individuals, who initiate fights and frequently succeed at evicting one of the paired mates from its group.

Fernandez-Duque and Huck’s study documents 50 instances of mate replacements, 27 by females and 23 by males. The individual kicked out of the group often died from the wounds he or she suffered.

The scientists’ long-term tracking of individual monkeys also revealed that these replacements come with a reproductive cost: Monkeys who remained with one partner produced 25 percent more offspring per decade than monkeys with two or more partners.

“The newly paired male and females are not reproducing right away,” says Fernandez-Duque. “They may be assessing each other for a long time and that delays reproduction.”  

Fernandez-Duque says that this finding—that staying in a monogamous relationship appears to improve a monkey’s chances of passing on its genes to the next generation—could have implications for humans as well.

“There’s some consensus among anthropologists that pairs-bonds must have played an important role in the origin of human societies,” Fernandez-Duque says. “Call it love, call it friendship, call it marriage, there is something in our biology that leads to this enduring, emotional bond between two individuals that is widespread among human societies.”