Why are some primates living in pairs?

Humans are not the only species to form bonded pairs— many other animals do, including a large number of non-human primates. But why did pair bonding evolve in the first place? New research is exploring this issue.

While many primates live in pairs, the reasons behind this evolutionary step remained unclear.
While many primates live in pairs, the reasons behind this evolutionary step remained unclear.

Social relations and mating behaviors are as diverse among animal species as the species itself. Several features have proven to be more difficult to researchers than others, and one of them is pair-bonding.

Humans are not unique in their tendency to find a mate for a long time— sometimes for a lifetime.

Among the ranks of our distant cousins, the non-human primates, about 1 in 5 live in pairs, according to Prof. Peter Kappeler, of the German Primate Center– Leibniz Institute for Primate Research, Göttingen, Germany, and Assistant Professor Luca Pozzi, Ph.D., of the University of Texas, San Antonio.

In a new study paper — now featured in Science Advances— the two scientists tried to elucidate the mystery of what led some primates to a largely monogamous lifestyle.

“Living as a pair is an evolutionary jigsaw in the evolution of mammalian social systems because males could achieve higher reproductive rates if they were not linked to a single female,” says Pozzi.

Female spacing and paternal care

Throughout their analysis, the researchers clarify that some primates live mainly throughout pairs, others are solitary— usually males who occasionally associate with groups of females— and the remainder live in hierarchical groups of both sexes.

But what made this complex set of social arrangements possible?

Pair-living, writes Prof. Kappeler and Pozzi, may play a crucial role in understanding the evolution of the complex social structures of non-human primates.

“[ The term ‘ pair-living’] refers to a social organization in which one adult male and one female stay together and coordinate their activities,” the researchers write. They’re moving on to clarify that this relationship isn’t just about monogamous marriage.

However, they note that many primates live in pairs are not monogamous— some prefer reproductive partners outside of a couple. So, what caused them to live in pairs?

In an attempt to answer this query, Pozzi and Prof. Kappeler studied the genetic data and standard behavior of 362 different primate species and modeled how their social relations may have evolved over time.

“Of the several explanations given for the evolution of mammalian couple-living,[ so far] only two have received repeated empiric support: female spacing and paternal care hypotheses,” the researchers write.

The female spacing hypothesis refers to the idea that when females spread across the area in search of resources, males become less likely to interact with larger groups of females and, in response, engage in pair-bonding.

The paternal care theory indicates that, in the case of some non-human primates, males may benefit from taking greater care of their offspring, providing them with food and shielding them from threats that may be more suited to a couple-bonding background.

To date, most scholars have assumed that the two theories are mutually exclusive. That’s not the case, according to Pozzi and Prof. Kappeler.

Their model— which gives an idea of how primates adapted to respond to environmental changes — suggests that both the two hypotheses regarding pair-living evolution could be right.

The paternal care hypothesis suggests that, in the case of some non-human primates, males may benefit from taking greater care of their offspring, providing them with food and protecting them from threats that may be more suited to a couple-like context.

To date, most researchers have concluded that both hypotheses are mutually exclusive. This is not the case, according to Pozzi and Prof. Kappeler.

Their model— which gives an idea of how primates evolved to react to environmental changes — suggests that both the two theories about pair-living evolution could be right.

‘Challenging and exciting’ research

Simultaneously, Pozzi and Prof. Kappeler warn that their description of the tendency of primates to form bonded pairs most certainly does not explain why humans do so.

“With our findings, the pair bond characteristic of humans inside larger social units can not be clarified, since none of our recent ancestors lived solitary,” warns Prof. Kappeler.

“However, the effects of parental care may also have led to pair-living consolidation in humans,” he goes on to suggest.

The researchers are pleased for the moment to have worked to elucidate a small part of the mystery of the social evolution of mammals.

“The evolution of complex social systems in mammals, and more specifically in primates, is a challenging and exciting area of research. Our study shows that pair-living — although rare — might have played a critical role in it.”

– Luca Pozzi, Ph.D.

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