The social organization of primates is more flexible than previously thought. While early primates likely lived in pairs, only 15% of individuals lived alone, according to a new study from the University of Zurich.

Primates (including humans) are considered highly social animals. Many monkey species live in groups. In contrast, lemurs and other strepsirrhines, often called “wet-nosed” primates, have long been considered solitary creatures, and other forms of social organization are assumed to have evolved later. Therefore, previous studies have tried to explain how and when partnered life emerged in primates.

However, recent research shows that many nocturnal strepsirrhines, which are more difficult to study, are not actually solitary but live in male and female pairs. So what does this mean for the forms of social organization of the ancestors of all primates? And why do some monkey species live in groups, while others live in pairs or singly?

Different forms of social organization

Researchers at the University of Zurich and Strasbourg are currently investigating these questions. For her research, Charlotte Olivier of the Hubert Curien Multidisciplinary Institute collected detailed information on the composition of social units in primate populations in the wild. Over several years, researchers have built a detailed database covering nearly 500 populations of more than 200 primate species through primary field studies. The study was published in the journal Proceedings of the National Academy of Sciences .

More than half of the primate species recorded in the database exhibited more than one form of social organization.

“The most common social organization was groups in which several females and a few males lived together, such as in chimpanzees or macaques, followed by groups in which there was only one male and a few females, such as in gorillas or langurs,” says the second. University of Zurich. “But a quarter of all species lived in pairs.”

Little ancestors united

By taking into account a variety of socioecological and lifestyle variables, such as body size, diet or habitat, researchers calculated the likelihood of different forms of social organization, including among our ancestors who lived about 70 million years ago. The calculations were based on complex statistical models developed by Jordan Martin at the UZH Institute of Evolutionary Medicine.

To reconstruct the ancestral state of primates, researchers relied on fossils showing that primate ancestors were relatively small and lived in trees; these factors were strongly linked to mating.

“Our model shows that the social organization of primate ancestors was variable and that pair life was by far the most likely form,” says Martin. He adds that only 15 percent of our ancestors were single. “Therefore, life in large groups did not develop until later in primate history.”

Couples with benefits

In other words, the social structure of early primates was probably more similar to modern humans than previously thought. “Most, if not all, of us live as couples while also being part of extended families, larger groups and communities,” says Jeggy. However, he adds, couple life among early primates did not imply sexual monogamy or joint infant care.

“A particular woman and a particular man are more likely to be seen together most of the time, sharing the same house and sleeping space, which will be more beneficial for them than living alone,” explains the late writer Karsten Shradin from Strasbourg. This allowed them to fend off opponents or warm each other up, for example.