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Most early primates lived in pairs and were not solitary creatures 

The discovery that early primates likely lived in pairs significantly alters our understanding of primate evolution and social behavior. This revelation comes from a study led by the University of Zurich

The research challenges the long-held belief that primates, particularly Strepsirrhines or “wet-nosed” primates, were primarily solitary creatures.

Study significance 

“Was the ancestor of all primates a solitary-living species? Did more social forms of primate societies evolve from this basic and simple society? Until now, the dogmatic answer was yes,” wrote the study authors. 

“We used a modern statistical analysis, including variations within species, to show that the ancestral primate social organization was most likely variable. Most lived in pairs, and only 10 to 20% of individuals were solitary.” 

Focus of the study 

Charlotte Olivier from the Hubert Curien Pluridisciplinary Institute collected detailed information on the social structures of primate populations in the wild. 

Over several years, the researchers created a database which included hundreds of populations from over 200 primate species. This extensive database, built from primary field studies, revealed a surprising diversity in primate social organizations.

Key insights

According to the researchers, over half of the primate species recorded in the database exhibited more than one form of social organization. 

“The most common social organization were groups in which multiple females and multiple males lived together, for example chimpanzees or macaques, followed by groups with only one male and multiple females – such as in gorillas or langurs. But one-quarter of all species lived in pairs,” explained study co-author Adrian Jaeggi. 

Complex statistical analysis 

The experts calculated the probability of different forms of social organization, including for our ancestors who lived some 70 million years ago. They accounted for variables such as body size, diet, and habitat. 

The calculations were based on complex statistical models developed by Jordan Martin at UZH’s Institute of Evolutionary Medicine. Fossil evidence, indicating that ancestral primates were small-bodied and arboreal, strongly supported the pair-living hypothesis.

Social organization of primates

“Our model shows that the ancestral social organization of primates was variable and that pair-living was by far the most likely form,” said Martin. He noted that only about 15% of our ancestors were solitary, suggesting that living in larger groups is a relatively recent development in primate history.

The significance of this finding extends to our understanding of human social structures. “Many, but by no means all of us, live in pairs while also being a part of extended families and larger groups and societies,” said Jaeggi. 

However, pair-living among early primates did not equate to sexual monogamy or cooperative infant care, noted Jaeggi. 

Study implications 

“It is more likely that a specific female and a specific male would be seen together for most of the time and share the same home range and sleeping site, which was more advantageous to them than solitary living,” said study co-author Carsten Schradin from the University of Strasbourg. For example, pair-living enabled them to fend off competitors or keep each other warm.

“Our results suggest that solitary living is a derived state and as such an adaptation to specific environmental conditions,” wrote the researchers. “Future studies will have to determine costs and benefits of solitary living as has been done for pair- and group-living.”

The study is published in the journal Proceedings of the National Academy of Sciences

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