Even the first primates probably lived in pairs

Image by Andy Holmes on Unsplash
Image by Andy Holmes on Unsplash
Primates exhibit more flexible forms of cohabitation than previously assumed. The first primates probably lived in pairs - only around 15 percent were solitary, as a study led by UZH shows.

Primates, to which we humans belong, are considered to be highly social animals. Monkeys and apes often live in groups. Lemurs and other so-called wet-nosed primates, on the other hand, were thought to be rather solitary and this also represented the original way of life of all primates. Earlier studies therefore attempted to explain how and when pair living arose in the evolution of primates.

However, recent studies show that many of the nocturnal and therefore difficult to study wet-nosed primates do not live individually, but in pairs of females and males. But what does this mean for the lifestyle of the ancestors of all primates? And why do some monkey species live in groups, others in pairs and still others alone?

Researchers from the Universities of Zurich and Strasbourg investigated these questions. For the study, Charlotte Olivier from the Institut Pluridisciplinaire Hubert Curien collected detailed information on the composition of wild primate groups, all of which came from field studies. Over several years, this resulted in a database of almost 500 populations from over 200 primate species.

More than half of the primate species recorded in this way exhibited more than one form of social organization. ’The most common were groups in which several females and several males lived together, such as in chimpanzees or macaques. The second most common were groups with only one male and several females - as in gorillas or langurs,’ explains last author Adrian Jäggi from the University of Zurich. ’However, a quarter of all species also organized themselves in pairs.

Taking into account socio-ecological and life-history variables such as body size, diet and habitat, the researchers used complex statistical models created by Jordan Martin from the Institute of Evolutionary Medicine at UZH to calculate the probability of individual forms of social organization - including for our ancestors almost 70 million years ago.

To reconstruct their way of life, the scientists relied on fossil finds, which showed that they were smaller than many species living today and lived in trees. These factors correlate strongly with pair living. ’Our model shows that primates also lived together variably in the past and that pairs were by far the most likely form of social organization of our last common ancestors,’ explains Martin. Only about 15 percent of our ancestors were solitary. ’Life in larger groups therefore only emerged later in the history of primates.

This means that the social structure of the first primates was probably more similar to that of modern humans than previously assumed. ’We too often, but by no means always, live as pairs and are also embedded in extended families and larger groups and societies,’ says Jäggi. However, living in pairs did not mean that our predecessors were sexually monogamous or raised their young together. ’It was more likely that a particular female and a particular male were together most of the time and shared their foraging area and sleeping place, which gave them advantages over a solitary lifestyle,’ explains last author Carsten Schradin from Strasbourg. For example, they were able to keep competitors away or keep each other warm.

Charlotte-Anaïs Olivier, Jordan Martin, et al. Primate Social Organization Evolved from a Flexible Pair-Living Ancestor, Proceedings of the National Academy of Sciences, December 28, 2023. doi: 10.1073/pnas.2215401120