Abstract: Factors related to dominance rank and the functions of aggression were studied in a herd of Sorraia horses, Equus caballus, under extensive management. Subjects were 10 adult mares 5-18 years old and a stallion introduced into the group for breeding. Dominance relationships among mares were clear, irrespective of rank difference, and remained stable after introduction of the stallion. The dominance hierarchy was significantly linear and rank was positively correlated with age and total aggressiveness. Higher-ranking mares received lower frequency and intensity of agonistic interactions. Nevertheless, higher-ranking dominants were not more likely to elicit submission from their subordinates than lower-ranking dominants. Neither close-ranking mares nor mares with less clear dominance relationships were more aggressive towards each other. Agonistic interactions seemed to be used more importantly in regulation of space than to obtain access to food or to reassert dominance relationships. Contexts of aggression were related to mare rank. The results suggest that dominance relationships based on age as a conventional criterion were established to reduce aggressiveness in a herd where the costs of aggression are likely to outweigh the benefits.

Abstract: This simulation study explores some conditions leading to transitivity within dominance orders. Combinations of three parameters were varied to study their consequences upon hierarchy formation and upon the degree of linearity of resultant structures. The factors studied were: (1) the importance of initial resource holding potentials (RHPs); (2) changes brought in RHPs by successive victories and defeats; and (3) accuracy of RHP assessment made by opponents. Results show that initial differences in RHP always lead to perfectly transitive chains whose rank order reflects the importance of initial differences. Even when simulated animals make important errors while assessing each other during round robin tournaments, emerging dominance structures are perfectly linear and ranks obtained in the structure are highly correlated with initial values in RHPs. Moreover, accumulated experiences of victory and/or defeat alone always lead to perfectly linear hierarchies. Their combination with initial individual differences in RHP led to the same conclusion. Even when assessment was far from being perfect, not only perfect chains were formed but initial values in RHPs significantly influenced rank order when the contribution of victory and defeat to RHP was relatively unimportant. The higher the importance of victory and defeat to RHP as compared to that of initial RHP values, the lower was the correlation between initial RHP values and the ranks order reached by individuals in the resultant hierarchies. In general also, the lower the variation within initial RHPs, the lower was the correlation between initial RHPs and ranks in the hierarchy. At a given level of initial RHP dispersion, increasing the contribution of victory and defeat to RHP diminished the correlation between initial RHP values and obtained ranks. In addition, inaccurate assessment reduced the overall correlation, especially when dispersion of initial RHP values was low and the contribution of victory and defeat relatively unimportant. These results shed some light on the controversy about the respective roles of initial individual attributes and that of patterns of resolution in the formation of animal hierarchies. We present the emergence of social order within closed systems as those simulated here as a case of self-organization.

Abstract: Gene expression affects social behavior only through changes in the excitabilities of neural circuits that govern the release of the relevant motor programs. In turn, social behavior affects gene expression only through patterns of sensory stimulation that produce significant activation of relevant portions of the nervous system. In crayfish, social interactions between pairs of animals lead to changes in behavior that mark the formation of a dominance hierarchy. Those changes in behavior result from changes in the excitability of specific neural circuits. In the new subordinate, circuits for offensive behavior become less excitable and those for defensive behavior become more excitable. Serotonin, which is implicated in mechanisms for social dominance in many animals, modulates circuits for escape and avoidance responses in crayfish. The modulatory effects of serotonin on the escape circuits have been found to change with social dominance, becoming excitatory in dominant crayfish and inhibitory in subordinates. These changes in serotonin's effects on escape affect the synaptic response to sensory input of a single cell, the lateral giant (LG) command neuron for escape. Moreover, these changes occur over a 2-week period and for the subordinate are reversible at any time following a reversal of the animal's status. The results have suggested that a persistent change in social status leads to a gradual change in the expression of serotonin receptors to a pattern that is more appropriate for the new status. To test that hypothesis, the expression patterns of crayfish serotonin receptors must be compared in dominant and subordinate animals. Two of potentially five serotonin receptors in crayfish have been cloned, sequenced, and pharmacologically characterized. Measurements of receptor expression in the whole CNS of dominant and subordinate crayfish have produced inconclusive results, probably because each receptor is widespread in the nervous system and is likely to experience opposite expression changes in different areas of the CNS. Both receptors have recently been found in identified neurons that mediate escape responses, and so the next step will be to measure their expression in these identified cells in dominant and subordinate animals.