Abstract
The processes that underlie the formation of the dominance hierarchy in a group are since long under debate. Models of self-organisation suggest that dominance hierarchies develop by the self-reinforcing effects of winning and losing fights (the so-called winner-loser effect), but according to ‘the prior attribute hypothesis’, dominance hierarchies develop from pre-existing individual differences, such as in body mass. In the present paper, we investigate the relevance of each of these two theories for the degree of female dominance over males. We investigate this in a correlative study in which we compare female dominance between groups of 22 species throughout the primate order. In our study female dominance may range from 0 (no female dominance) to 1 (complete female dominance). As regards ‘the prior attribute hypothesis’, we expected a negative correlation between female dominance over males and species-specific sexual dimorphism in body mass. However, to our surprise we found none (we use the method of independent contrasts). Instead, we confirm the self-organisation hypothesis: our model based on the winner-loser effect predicts that female dominance over males increases with the percentage of males in the group. We confirm this pattern at several levels in empirical data (among groups of a single species and between species of the same genus and of different ones). Since the winner-loser effect has been shown to work in many taxa including humans, these results may have broad implications.
Highlights
Dominance hierarchies based on agonistic interactions are observed in many group-living animals and in humans [1]
Our results indicate that the degree of female dominance over males depends on group composition rather than on effects of sexual dimorphism
In that case a higher number of males leads to a higher number of interactions with males (N = 360, Tau = 0.148, P = 0.0001), and to a sufficiently higher average intensity of aggressive interactions, so that more female dominance develops via a stronger hierarchical differentiation per sex [11,12,38]
Summary
Dominance hierarchies based on agonistic interactions are observed in many group-living animals and in humans [1]. According to the self-organisation hypothesis based on several models, a dominance hierarchy emerges in a group of individuals even in the absence of any pre-existing differences, through the self-reinforcing effects of winning and losing fights [4,5,6,7,8,9,10,11,12,13]. This so-called winner-loser effect implies that, after losing, an individual is more likely to lose again and vice versa after winning [14,15]. In individuals that have recently lost, levels of corticosteroids are often increased and plasma testosterone levels are reduced [20], but the effects of winning experiences are less clear [19]
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