Powerful female primates are more common than imagined – 02/26/2024 – Science

A detailed analysis of the power relations between the sexes in primate species relativizes the idea that the closest relatives of human beings would always have been ruled by males. According to the new study, 42% of the societies formed by these species do not have male supremacy, and everything indicates that this characteristic was also not present in the common ancestor of current primates.

The conclusions were recently published in an article in the specialized journal Animals. The work was coordinated by Rebecca Lewis and Christopher Kirk, from the University of Texas at Austin (United States).

The researchers used statistical methods to examine social and biological characteristics of 79 current primate species, to estimate not only how common male dominance is but also the factors behind this type of social structure.

In general, primate societies tend to be seen from the perspective of male dominance, especially in the case of larger and better-known species, such as chimpanzees, gorillas and baboons (in addition, of course, to Homo sapiens).

The main exception are lemurs, members of the group with the most primitive anatomy and exclusive to the African island of Madagascar, in the Indian Ocean. Among lemurs, females tend to occupy higher hierarchical positions than males. The same goes for bonobos or pygmy chimpanzees (pan paniscus), the only species of African great apes –those most closely related to humans– in which this occurs.

To verify whether male dominance is indeed so widespread, the researchers first compiled data on primate species whose natural history is well known enough to establish how interaction between the sexes works in their societies.

The species were separated into two groups: those characterized by male-dominated hierarchies, on the one hand, and those with mixed or female-dominated hierarchies, on the other. The study authors highlight that the classification is provisional because not all primatologists clearly define what corresponds to each category.

In addition, the researchers compiled factors known to be associated with power asymmetries between the sexes. The main ones are the level of sexual dimorphism (anatomical differences between males and females, especially in size), the proportion between the sexes in the population, differences in the size of canines (related to aggression and disputes between individuals) and the joint occurrence or not of estrus (the popular “heat”) among females.

This last fact may seem curious, but it influences disputes over partners, and this, in turn, has effects on the social structure. There are indications that the simultaneous estrus of females, by facilitating the search for mating – as there are more females sexually available in the same period – could dilute their bargaining power in the group and, thus, tip the balance in favor of males, and the opposite can happen if estrus is not simultaneous.

Putting all these factors in the balance, the researchers set up a statistical and evolutionary analysis, trying to estimate not only which characteristics influence the balance of power between the sexes today but also the origin of this tug of war in the evolution of primates.

Based on this, in addition to concluding that the power of males does not seem to have been the “ancestral condition” of primates, the researchers assess that this social structure became predominant with the emergence of catarrhines, or Old World monkeys (the group that goes from baboons and mandrills to orangutans, chimpanzees and humans).

Even among the catarrhines, however, more egalitarian situations between the sexes evolved again four times: among the bonobos, the gibbons (great Asian apes of a more modest size and much more arboreal than the others) and two monkeys with tails, the talapoins ( found in Angola) and red monkeys (also African).

As expected, species with proportionally larger males and more threatening canines tend to have more male dominance. On the other hand, in species in which there is a smaller proportion of females in the population or in which they are not in heat at the same time, the chances of them being dominant or staying on the same level as males increase. In these cases, they have greater bargaining power and are able to use this to their advantage.