In group-living animals, males are assumed to be dominant over females when they are larger than females. Despite this, females have sometimes been proven to be dominant over some males possibly via the winner-loser effect, which becomes clearer when the intensity of aggression in the group is higher. To test whether the winner-loser effect can lead to (partial) female dominance in a species with a pronounced sexual dimorphism, we studied the hierarchy in 12 rat colonies (Rattus norvegicus) in which the rats could freely interact with their group members within a spacious area. To investigate the underlying mechanisms, we compared the empirical data to hypotheses generated by the agent-based model ‘DomWorld’. We show that females dominated on average 55% of the males, and occupied the alpha position in four colonies, in three of them they shared it with one or several males. Moreover, in line with the predictions of the computational model, females dominated a higher percentage of males when the intensity of aggression of the colony was higher. This shows that although females are only half as heavy as males, they dominate part of the males probably through the winner-loser effect. We suggest that this effect may be widespread in many other species and can be tested experimentally.

Puentes-Escamilla, M.A., Schweinfurth, M.K. & Hemelrijk, C.K. (2024). When are females dominant over males in rats (Rattus norvegicus)? Behavioral Ecology and Sociobiology 78: 56.