A new study in owl monkeys (Aotus azarai) reveals that, when an owl monkey pair is severed by an intruding individual, the mate who takes up with a new partner produces fewer offspring than a monkey who sticks with its tried-and-true partner.
An owl monkey group (M. Corley / Owl Monkey Project)
The findings, published in the journal PLoS ONE, underscore how monogamy and pair-bonds – relatively rare social formations among mammals – can benefit certain individuals, with potential implications for understanding how human relationship patterns may have evolved.
Since 1997, a team of scientists led by Prof Eduardo Fernandez-Duque from the University of Pennsylvania 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.
The current study amasses data from 16 years of observation of 18 owl monkey groups, a total of 154 animals. 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.
“We have managed over the years to have quite significant sample sizes for a study of wild non-human primates,” Prof Fernandez-Duque said. “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.”
In 2008, the scientists discovered for the first time the presence of a so-called ‘floater’ individual, which attacked the male member of an owl monkey pair and essentially replaced him as a mate and infant-care provider. They now demonstrate that this usurping of mates by both male and female floaters is a common occurrence. The researchers documented 27 female and 23 male replacements in the groups they observed. The replacements often involved dramatic fights, some of which ended fatally for the evicted individual.
“These are high-stakes competitions for reproductive positions,” Prof Fernandez-Duque said. By following pairs and observing replacements, the scientists 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,” Prof 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. The finding helps explain previous research by the team, which has shown that male owl monkeys invest significantly in raising their offspring.
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Bibliographic information: Fernandez-Duque E, Huck M. 2013. Till Death (Or an Intruder) Do Us Part: Intrasexual-Competition in a Monogamous Primate. PLoS ONE 8(1): e53724; doi: 10.1371/journal.pone.0053724
