Games of power: Scientists decode behavioral patterns of dominance between the sexes in mammals
image: In female-dominated species such as spotted hyenas, animals of both sexes rely less often on aggression and more often on submissive signals and gestures. view more
Credit: Photo by Oliver Höner/Leibniz-IZW
The stronger and more aggressive sex dominates the weaker sex. This simplistic view of male-female dominance relationships is common but falls short of the complexity of how dominance hierarchies are established in animal societies. A team of scientists with participation of the Leibniz Institute for Zoo and Wildlife Research in Berlin (Leibniz-IZW) now compared intersexual dominance hierarchies of nine group-living mammals using a set of standardised methods and behaviours. They found that the species ranged from being strictly male to strictly female dominated, and that hierarchies were robust with respect to the method applied to construct them. They also found that in female-dominated societies, animals mostly relied on submissive signals and gestures to establish and maintain dominance, whereas in male-dominated societies, they mostly used aggressive behaviours. The results were published in the open access journal “Frontiers in Ecology and Evolution”.
Group-living animals commonly form dominance hierarchies to avoid physical contest and injury. These hierarchies can be derived from the outcome of agonistic interactions between group members. In many species, males and females compete over resources and frequently interact with each other, but scientists often create separate hierarchies for each sex and use sex-specific theoretical frameworks to study social dominance. This is because dominance is commonly assumed to be driven by physical strength, and males and females often differ in size and strength.
This traditional approach ignores that dominance can be driven by traits that are unrelated to physical attributes, such as social support, and are independent of the sex. “The binary view of either female or male dominated societies is also too simplistic because we now know that dominance varies along a gradient that includes societies where males and females share power”, says co-lead author Prof Peter Kappeler from the German Primate Center – Leibniz Institute for Primate Research (DPZ). Research on intersexual dominance relationships was further hindered by scientists applying different methods to construct hierarchies. “We urgently need standardised methodological approaches and a broader theoretical framework to study intersexual dominance relationships”, says Prof Kappeler.
In a first step towards this goal, the team of scientists compiled behavioural observations and established the intra- and intersexual dominance hierarchies in nine mammals (spotted hyenas, rock hyraxes, and seven primates: Verreaux’s sifakas, redfronted lemurs, chacma baboons, crested macaques, mandrills, vervet monkeys and bonobos) using a set of commonly applied methods. They calculated the degree of female dominance over males using five different indices. They then studied whether there was a relationship between the degree of female dominance in a species and how dominance was established and maintained, specifically how strongly the animals relied on aggressive acts (e.g., lunging and biting) versus submissive signals (e.g., having the ears flat and the bum low).
The team found that the rank order of individuals in the hierarchy was the same irrespective of the methods used and that all indices of female dominance were well correlated with each other. “This shows that the methods to construct intersexual hierarchies are comparable and that the associated measures of the degree of female dominance are robust”, says co-author Dr Oliver Höner from the Leibniz-IZW. The scientists further confirmed that intersexual dominance varies along a continuum from strict female dominance to strict male dominance and showed that this is independent of the method used.
The study additionally revealed a striking difference in dominance style between male- and female-dominated societies. “The higher the degree of female dominance was in a species, the less frequently the animals used aggression to establish and maintain their dominance relationships”, explains co-lead author Dr Elise Huchard from the Institute of Evolutionary Sciences of Montpellier (ISEM), France. “In strongly female-dominated species, such as spotted hyenas, animals of both sexes more often display submissive signals and gestures and less often use aggressive acts compared to male-dominated species, such as Chacma baboons, where aggression predominates.” These results suggest that in societies where dominance is biased toward females, signals are particularly important for structuring social life and likely limit the use of direct aggression during conflicts.
“The fact that scientists working on different species often use different tools, methods and behaviours to study dominance contributed to limit advances in the field”, according to Dr Höner. “With this work, we show that we have robust methodological tools to study intersexual relationships in group-living species in a standardised way.”
This study provides important foundations for future studies of broader interest that aim to uncover the ecological and evolutionary causes of variation in intersexual dominance within and across animal societies.