Monkeys Suggest What It Takes to Be Monogamous

Coppery titi monkeys are a rare example of a completely monogamous mammal.

Posted Nov 23, 2020

Sofya Dolotovskaya, used with permission.
Coppery titi monkeys.
Source: Sofya Dolotovskaya, used with permission.

Many birds and mammals are socially monogamous—males and females pair up, live together, and often raise their young together. However, once methods for genetic paternity testing were introduced, it became clear that a great many socially monogamous animals are sexually unfaithful. In most species, there is some proportion of offspring that is not sired by the social father. In pair-living species, genetic monogamy is rare.

In a new study, scientists report that coppery titi monkeys living in the Amazon lowland rainforest appear to be an exception. Researchers from the German Primate Center (DPZ)-Leibniz Institute for Primate Research in Göttingen could not find evidence for extra-pair paternity in their study population in Peru.

Not-So-Perfect Pairs

Among pair-living mammals—which make up about 9 percent of mammal species—strict genetic monogamy has been reported for only seven species so far. A handful of other species can be considered “mostly” genetically monogamous, with the rate of extra-pair paternities (offspring sired by a male other than their social father) less than 10 percent.

According to Sofya Dolotovskaya, one of the new paper’s authors, some of the benefits of extra-pair matings are connected to the limited mate choice of pair-living animals.

“In socially monogamous animals, mate choice can be constrained,” she says. “Because of these constraints, they might end up with a mate who is not optimal—one who is genetically incompatible, closely related, or low-quality. By mating with other individuals, animals might pass superior genetic qualities or more genetic diversity to their offspring, increasing their overall fitness.”

Extra-pair matings allow pair-living animals to gain these indirect genetic benefits for their offspring while still taking advantage of the direct benefits provided by the social partner, such as a secure territory and a child-raising partner.

No Monkey Business

As a doctoral student at the German Primate Center, Dolotovskaya followed 14 groups of coppery titi monkeys (Plecturocebus cupreus) in northeastern Peru. These monkeys live in small family groups consisting of an adult male and female and their offspring, who defend a territory. The pair partners maintain a strong relationship, spending most of the day within a few meters of each other, sleeping together at night, and frequently engaging in joint visual and acoustic displays at the territorial borders.

In addition to months of observations, Dolotovskaya and her colleagues genotyped 41 wild titi monkeys from their fecal samples.

Sofya Dolotovskaya, used with permission.
Coppery titi monkey.
Source: Sofya Dolotovskaya, used with permission.

All of the 18 offspring examined by the researchers were sired by their social fathers. In other words, the monkeys in this population are genetically monogamous. Coppery titis are only the second primate and the seventh pair-living mammal with no evidence of extra-pair matings. Still, Dolotovskaya is hesitant to proclaim that this species is always genetically monogamous.

“In our small study population, they were genetically monogamous,” she says. “But if we sampled 100 or 200 family groups, there might be exceptions.”

In addition, the researchers discovered that mating partners were, on average, unrelated. Seeking out extra-pair matings would therefore not have provided much of a genetic advantage for the animals studied, making the risks of infidelity less necessary.

The researchers further found that when young titis come of age, both males and females disperse away from their parents’ territory opportunistically. It seems that young titis migrate far enough from their birth territory to find a suitable partner without incurring the risks of inbreeding. This, in turn, can render extra-pair matings less necessary, helping to maintain genetic monogamy.

Prerequisites for Monogamy

Dolotovskaya says that habitat characteristics are crucial for understanding how animals live together.

Sofya Dolotovskaya, used with permission.
Coppery titi monkey.
Source: Sofya Dolotovskaya, used with permission.

“For instance, our study population is living in a healthy, undisturbed forest habitat with plenty of places to disperse, and that seems to help maintain monogamy,” she says. “But in a situation where the forest is fragmented, you might have more monkeys living in crowded fragments, and the situation would be different. It would not be so easy for them to remain genetically monogamous.”

The distribution of animals within a habitat appears to be important for monogamy to work. If animals are scarcely distributed, they have more chances to stay monogamous or pair-living because they won’t encounter other animals often. And two young animals dispersing from home are not likely to encounter a potential mate to whom they are related.

Further studies are needed to see whether genetic monogamy also prevails in other populations of coppery titi monkeys, especially those living in fragmented habitats.

“We used to think of a mating system as a fixed characteristic of a species, but actually, mating systems can be quite flexible,” Dolotovskaya says. “Monogamy may be more feasible depending on the type of environment in which an animal lives.”

References

Dolotovskaya S, Roos C, and Heymann EW. 2020. Genetic monogamy and mate choice in a pair-living primate. Scientific Reports. Doi: 10.1038/s41598-020-77132-9.