New findings by researchers at the University of Montpellier, the Max Planck Institute for Evolutionary Anthropology in Leipzig, and the German Primate Center in Göttingen resolve why male-female power asymmetries vary across primate societies. Bringing together detailed observations of male-female aggression from 253 populations across 121 primate species, the study shows that clear cut dominance by either sex is rare, and highlights the conditions under which females have become socially dominant over males in primate evolutionary history.
Intersexual conflicts are common
The comprehensive review of available data on intersexual aggression revealed that contests between males and females are surprisingly common. On average, almost half of the aggressive interactions in social groups involved a male and a female. Previous research mostly focused on fights among individuals of the same sex, because existing theories in social evolution assume that males and females compete over different resources. "The observation that in most populations any given individual is more likely to be involved in a fight against another individual of the opposite rather than the same sex highlights that the battle of the sexes is common in other animals", says Dieter Lukas from the Max Planck Institute for Evolutionary Anthropology. The study goes on to analyse the outcome of such contests: are they most often won by males or by females, and does this proportion vary across species?
It was long assumed that power tends to be biased towards males in primates, and that the few emblematic female-dominant species, like ring-tailed lemurs or bonobos, represented an exception requiring special explanation. Instead, the new study reveals the complexity and variability of sex biases in dominance relations in primate societies. As Peter Kappeler, from the German Primate Center, notes: "Recent research started to challenge the traditional views of male dominance being the default status, and our study now provides a more comprehensive exploration of variation in intersexual dominance relationships." In their sample, clear dominance of males, where they win more than 90 percent of contests over females, is observed in only 25 of the 151 populations with quantitative data. Clear female dominance is observed in 16 populations, leaving 70 percent of the populations with moderate or no sex-biases in power.
Implications for understanding gender relationships
The research team tested five hypotheses to explain sex biases in dominance relations, and found that female-biased dominance is associated with several key factors. Female power is mostly seen in species in which females are monogamous, of similar size as males, or primarily forage in trees - all situations in which females have more choice in whether or not to mate with a particular male. In addition, female dominance is also facilitated in situations where females face intense competition over resources, such as in solitary or pair-living species, as well as when male-female conflicts are less risky for their dependent offspring, for example because mothers park offspring when they forage instead of carrying them around with them.
Conversely, male-biased dominance prevails in species that are terrestrial, where males have larger bodies or weapons than females, and where males mate with multiple females. "Critically, while primate males gain power via physical force and coercion, female empowerment relies on alternative pathways, such as reproductive strategies to gain control over matings", explains Elise Huchard from the University of Montpellier.
The study's findings have important implications for our understanding of the evolution of social behaviour across animals, and may also shed some light on our understanding of human social dynamics. The finding that it is almost as likely that females dominate males than the reverse, and that most primate societies do not have clear-cut sex-biases in power, challenges traditional views of the natural origins of gender roles. Humans do not share all the traits that characterise species where males strictly dominate females. Instead, the set of human traits places them closer to species showing more nuanced relationships, where individuals of either sex can become dominant. Accordingly, arguments presenting human patriarchy as a primate legacy appear misguided, and gender relations should be considered in relation to their social and ecological contexts.
