Monogamy Pays

Breaking up is hard to do — and can be detrimental to one’s reproductive fitness, according to a new University of Pennsylvania study.

Nocturnal Azara’s owl monkeys (
Aotus azarae), are considered a socially monogamous species— a relatively rare social structure among mammals (Figure 1). Owl monkeys live in monogamous groups consisting of an adult male, an adult female and their offspring. The juveniles disperse from the group around age 3 or 4. When a monogamous pair is severed by an intruding individual, the one who takes up with a new partner produces fewer offspring than the monkey who sticks with its tried-and-true partner.

Owl monkey for release
Figure 1. A group of Azara’s owl monkeys. (Credit: Photo: M. Corley/Owl Monkey Project)

The findings underscore how monogamy and pair-bonds can benefit certain individuals, with potential implications for understanding how human relationship patterns may have evolved.

Eduardo Fernandez-Duque and Maren Huck report on the research in PLOS ONE. Since 1997, Fernandez-Duque and colleagues have monitored an owl monkey population in a portion of Argentina’s Chaco region. Their behavioral observations, demographic data and physiological sampling have provided a wealth of information on the animals.

“These findings are possible because we have intense demographic monitoring year-round that allows us to notice when a male is missing, when a female is missing or when there’s a new adult in the group. We couple this with intense behavioral monitoring that allows us to document the details of fights or the whole process of mate replacement,” said Fernandez-Duque. The current study amasses data from 16 years of observation of 18 owl monkey groups, a total of 154 animals (Figure 2).

owl monkey
Figure 2. Number of infants per decade of tenure length (infants/10ytl). The boxplot show medians (solid line), means (dotted line) and interquartile ranges for individuals with one or with two or more partners during their tenure. (From Fernandez-Duque et al. , 2013)

Owl monkey groups may include a “floater” individual. In one case, a floater male attacked the male member of an owl monkey pair and essentially replaced him as a mate and infant-care provider. This usurping of mates is practiced by both male and female floaters; there were 27 female and 23 male replacements in the groups observed. The replacements often involved dramatic fights, some of which ended fatally for the evicted individual. “These are high-stakes competitions for reproductive positions,” Fernandez-Duque said.
By following pairs and observing replacements, Fernandez-Duque and Huck show that having a partner evicted harms the reproductive success of the remaining mate. Owl monkeys with one partner produced 25 percent more offspring per decade than those with two or more partners.

“What we’re showing is that if you manage to stay with the same partner you produce more infants than if you’re forced to change partners,” Fernandez-Duque said. The reason for this significant impact on the reproductive success of the remaining partner is not yet completely clear, but the researchers surmise that it may have to do with a delay in reproduction due to the fact that female owl monkeys in Argentina typically only conceive between March and May. It’s also possible the delay occurs because the two individuals take time to assess one another before reproducing, given the significant commitment to infant care that both males and females make.
The results demonstrate that, for owl monkeys, long-term monogamy and pair-bonding improves reproductive fitness.

“Monogamy makes sense for these primates, because the male who sticks to a female is certain about the paternity of the young, and so he invests in their care,” he said. “The female benefits from shared provisioning of care which may help her reduce the burden of pregnancy and lactation.”


Source: Modified from materials provided by the University of Pennsylvania.

Reference

Eduardo Fernandez-Duque, Maren Huck. Till Death (Or an Intruder) Do Us Part: Intrasexual-Competition in a Monogamous Primate.
PLoS ONE, 2013; 8 (1): e53724 DOI: 10.1371/journal.pone.0053724