How to measure intersexual dominance?

Intersexual dominance (dominance between the sexes) is often assumed to be binary with species categorised as either male- or female-dominant. Yet in many species, the degree of intersexual dominance falls somewhere in the middle of these two extremes. There are several measures of intersexual dominance, but in empirical studies it is not possible to evaluate which is best because the real degree of intersexual dominance is unknown. This evaluation is possible, however, in the agent-based model, DomWorld because individuals have internal dominance values that drive their agonistic behaviour. In the present study, we examined the accuracy of measures of intersexual dominance in DomWorld by the strength of the correlation between the degree of intersexual dominance based on their internal dominance values and the degree of intersexual dominance based on observations of their competitive interactions (similar to observations in empirical studies). We examined the four measures that have been most commonly used in the literature: the proportion of intersexual conflicts won and initiated, the Female Dominance Index (FDI) and the proportion of female-dominant dyads. The FDI was highly accurate, possibly because it was based on intra- and intersexual fights, both of which influence an individual’s dominance. The proportion of intersexual conflicts initiated was similar in its accuracy to FDI and it was the only measure to be unaffected by missing data. Measures were more accurate when groups were smaller, or the intensity of aggression was higher, but their accuracy did not depend on the degree of sexual dimorphism. To best represent dominance relations between the sexes, we recommend reporting both FDI and the proportion of intersexual conflicts initiated

Interrelationship among spatial cohesion, aggression rate, counter-aggression and female dominance in three lemur species

Abstract: How social and ecological factors are associated with variation in dominance style across species of animals has been studied frequently, but the underlying processes are often not addressed. Theoretical research indicates that stronger spatial cohesion among individuals in a group causes a higher frequency of fighting and, thus, through the self-reinforcing effects of winning and losing fights, a stronger differentiation of the dominance hierarchy and dominance of females over more males. Our aim in the present paper is to study whether the same interrelationship among processes may underlie differences in dominance style among three species of lemur that differ in their degree of despotism: Lemur catta, Propithecus verreauxi and Eulemur rufifrons. We investigated their agonistic interactions and spatial cohesion based on 2752 h of observational data of 20 wild groups of these three species. We determined dominance style using the proportion of counter-aggression, with a lower proportion indicating a more despotic dominance style. We found that stronger spatial cohesion among individuals is associated with a higher rate of aggression, stronger despotism and dominance of females over more males. The results of our study emphasise the general importance of spatial cohesion in determining dominance style. Significance statement: Theoretical studies have shown that the spatial configuration of individuals in a group influences the dominance style. In an agent-based model, DomWorld, individuals are guided by simple rules of grouping and fighting and emergent patterns of behaviour switch between resembling those of despotic or egalitarian primates depending on the degree of cohesion in groups. Yet this link has seldom been studied empirically. We, therefore, examine the relevance of spatial cohesion on patterns of behaviour of individuals in groups of three species of lemur. We confirm the predictions from the model and show that stronger spatial cohesion results in more frequent aggression, a more despotic dominance style and stronger female dominance over males. In light of this, we urge future research of animal dominance to include measures of cohesion

Female emancipation in a male dominant, sexually dimorphic primate under natural conditions

In most group-living animals, a dominance hierarchy reduces the costs of competition for limited resources. Dominance ranks may reflect prior attributes, such as body size, related to fighting ability or reflect the history of self-reinforcing effects of winning and losing a conflict (the winner-loser effect), or both. As to prior attributes, in sexually dimorphic species, where males are larger than females, males are assumed to be dominant over females. As to the winner-loser effect, the computational model DomWorld has shown that despite the female’s lower initial fighting ability, females achieve some degree of dominance of females over males. In the model, this degree of female dominance increases with the proportion of males in a group. This increase was supposed to emerge from the higher fraction of fights of males among themselves. These correlations were confirmed in despotic macaques, vervet monkeys, and in humans. Here, we first investigate this hypothesis in DomWorld and next in long-term data of 9,300 observation hours on six wild groups of robust capuchin monkeys (Sapajus libidinosus; S. nigritus, and S. xanthosternos) in three Brazilian sites. We test whether both the proportion of males and degree of female dominance over males are indeed associated with a higher relative frequency of aggression among males and a higher relative frequency of aggression of females to males. We confirm these correlations in DomWorld. Next, we confirm in empirical data of capuchin monkeys that with the proportion of males in the group there is indeed an increase in female dominance over males, and in the relative frequency of both male-male aggression and aggression of females to males and that the female dominance index is significantly positively associated with male male aggression. Our results reveal that adult sex ratio influences the power relation between the sexes beyond predictions from socioecological models.</p

Sex and dominance: How to assess and interpret intersexual dominance relationships in mammalian societies

The causes and consequences of being in a particular dominance position have been illuminated in various animal species, and new methods to assess dominance relationships and to describe the structure of dominance hierarchies have been developed in recent years. Most research has focused on same-sex relationships, however, so that intersexual dominance relationships and hierarchies including both sexes have remained much less studied. In particular, different methods continue to be employed to rank males and females along a dominance hierarchy, and sex biases in dominance are still widely regarded as simple byproducts of sexual size dimorphism. However, males and females regularly compete over similar resources when living in the same group, and sexual conflict takes a variety of forms across societies. These processes affect the fitness of both sexes, and are mitigated by intersexual hierarchies. In this study, we draw on data from free-ranging populations of nine species of mammals that vary in the degree to which members of one sex dominate members of the other sex to explore the consequences of using different criteria and procedures for describing intra- and intersexual dominance relationships in these societies. Our analyses confirmed a continuum in patterns of intersexual dominance, from strictly male-dominated species to strictly female-dominated species. All indices of the degree of female dominance were well correlated with each other. The rank order among same-sex individuals was highly correlated between the intra- and intersexual hierarchies, and such correlation was not affected by the degree of female dominance. The relative prevalence of aggression and submission was sensitive to variation in the degree of female dominance across species, with more submissive signals and fewer aggressive acts being used in societies where female dominance prevails. Thus, this study provides important insights and key methodological tools to study intersexual dominance relationships in mammals

Interrelationship among spatial cohesion, aggression rate, counter-aggression and female dominance in three lemur species

Abstract: How social and ecological factors are associated with variation in dominance style across species of animals has been studied frequently, but the underlying processes are often not addressed. Theoretical research indicates that stronger spatial cohesion among individuals in a group causes a higher frequency of fighting and, thus, through the self-reinforcing effects of winning and losing fights, a stronger differentiation of the dominance hierarchy and dominance of females over more males. Our aim in the present paper is to study whether the same interrelationship among processes may underlie differences in dominance style among three species of lemur that differ in their degree of despotism: Lemur catta, Propithecus verreauxi and Eulemur rufifrons. We investigated their agonistic interactions and spatial cohesion based on 2752 h of observational data of 20 wild groups of these three species. We determined dominance style using the proportion of counter-aggression, with a lower proportion indicating a more despotic dominance style. We found that stronger spatial cohesion among individuals is associated with a higher rate of aggression, stronger despotism and dominance of females over more males. The results of our study emphasise the general importance of spatial cohesion in determining dominance style. Significance statement: Theoretical studies have shown that the spatial configuration of individuals in a group influences the dominance style. In an agent-based model, DomWorld, individuals are guided by simple rules of grouping and fighting and emergent patterns of behaviour switch between resembling those of despotic or egalitarian primates depending on the degree of cohesion in groups. Yet this link has seldom been studied empirically. We, therefore, examine the relevance of spatial cohesion on patterns of behaviour of individuals in groups of three species of lemur. We confirm the predictions from the model and show that stronger spatial cohesion results in more frequent aggression, a more despotic dominance style and stronger female dominance over males. In light of this, we urge future research of animal dominance to include measures of cohesion

Data_Sheet_1_How to measure intersexual dominance?.pdf

Intersexual dominance (dominance between the sexes) is often assumed to be binary with species categorized as either male- or female-dominant. Yet in many species, the degree of intersexual dominance falls somewhere in the middle of these two extremes. There are several measures of intersexual dominance, but in empirical studies, it is not possible to evaluate which is best because the real degree of intersexual dominance is unknown. This evaluation is possible, however, in the agent-based model, DomWorld, because individuals have internal dominance values that drive their agonistic behavior. In the present study, we defined the accuracy of measures of intersexual dominance in DomWorld by the strength of the correlation between the degree of intersexual dominance based on A) their internal dominance values and B) observations of their competitive interactions (similar to observations in empirical studies). We examined the four measures that have been most commonly used in the literature: the proportion of intersexual conflicts won and initiated, the Female Dominance Index, and the proportion of female-dominant dyads. The Female Dominance Index was highly accurate, possibly because it was based on the outcomes of intra- and intersexual conflicts, both of which influence an individual's dominance. The proportion of intersexual conflicts initiated was similar in its accuracy to the Female Dominance Index and it was the only measure to be unaffected by missing data. Measures were more accurate when groups were smaller, or the intensity of aggression was higher, but their accuracy did not depend on the degree of sexual dimorphism. To best represent dominance relations between the sexes, we recommend reporting both the Female Dominance Index and the proportion of intersexual conflicts initiated.</p

Replication Data for: How to measure intersexual dominance?

This dataset contains the output from DomWorld for 40 runs per each of the 36 different parameters (36 data files in three zipped folders), and five R-scripts to manipulate and analyse data and summary data files (output from scripts 1:4) that can be used to replicate the analysis from the paper

Table_1_How to measure intersexual dominance?.XLSX

Intersexual dominance (dominance between the sexes) is often assumed to be binary with species categorized as either male- or female-dominant. Yet in many species, the degree of intersexual dominance falls somewhere in the middle of these two extremes. There are several measures of intersexual dominance, but in empirical studies, it is not possible to evaluate which is best because the real degree of intersexual dominance is unknown. This evaluation is possible, however, in the agent-based model, DomWorld, because individuals have internal dominance values that drive their agonistic behavior. In the present study, we defined the accuracy of measures of intersexual dominance in DomWorld by the strength of the correlation between the degree of intersexual dominance based on A) their internal dominance values and B) observations of their competitive interactions (similar to observations in empirical studies). We examined the four measures that have been most commonly used in the literature: the proportion of intersexual conflicts won and initiated, the Female Dominance Index, and the proportion of female-dominant dyads. The Female Dominance Index was highly accurate, possibly because it was based on the outcomes of intra- and intersexual conflicts, both of which influence an individual's dominance. The proportion of intersexual conflicts initiated was similar in its accuracy to the Female Dominance Index and it was the only measure to be unaffected by missing data. Measures were more accurate when groups were smaller, or the intensity of aggression was higher, but their accuracy did not depend on the degree of sexual dimorphism. To best represent dominance relations between the sexes, we recommend reporting both the Female Dominance Index and the proportion of intersexual conflicts initiated.</p