For several decades, researchers have assumed that primates used to live in solitude. However, a new analysis sheds new light on this belief, suggesting that our mammalian ancestors were far more sociable than was previously thought.
Primate Social Group Composition
Primates, including humans, are known for having different forms of social organization. Many species of apes and monkeys live in groups, while nocturnal Strepsirrhines, believed to be solitary, live in pairs of males and females. Lemurs also have complex social lives as nocturnal and relatively solitary primates.
Being part of a large community helps provide protection against their enemies and protect scarce food resources. This is especially true for non-human primates, which rely on fruits for food. On the other hand, leaf-eaters tend to form smaller social groupings since there is little competition for food.
Meanwhile, primates that are more or less close to contact with other community members are usually tied to a particular locale. Their aloofness from other troops prevents high concentrations of individuals, which could otherwise result in rapid depletion of resources.
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Pair Living as a Survival Strategy
To better understand the social organization forms of primates, researchers at the Universities of Zurich and Strasbourg gathered detailed information on the composition of social units in primate populations in the wild. Scientists built a complex database from primary field studies led by Charlotte Olivier from the Hubert Curien Pluridisciplinary Institute. This database covered almost 500 populations from more than 200 primate species.
In the study "Primate social organization evolved from a flexible pair-living ancestor," the researchers suggest that over half of the primate species recorded in the database demonstrated more than one form of social organization. The most common social organization was groups where multiple males and multiple females lived together, followed by groups with only one male and numerous females. From this database, one-quarter of all species lived in pairs.
The researchers calculated the probability of various forms of social organization of our ancestors who lived 70 million years ago. They considered several socioecological and life history variables like diet, body size, or habitat. Meanwhile, the experts relied on fossils to reconstruct the ancestral state of primates. These fossils suggest that early primates were relatively small-bodies and arboreal, correlating with pair-living.
The model shows that the ancestral social organization of primates varied. Pair-living is the most likely form, while only about 15% were solitary. From this result, researchers assumed that living in larger groups could have only evolved later in the history of primates.
This revelation could indicate that early primates' social structure was probably more similar to that of modern humans than previously thought. Many of us live in pairs while being part of extended families, larger organizations, and societies.
However, this tendency to live in pairs did not equate to sexual monogamy or cooperative infant care, according to study co-author Adrian Jaeggi. It is more likely that a specific female and a specific male would be seen together most of the time, sharing the same home, which is more advantageous than solitude. This allowed them to ward off competitors or to keep each other warm.
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