Aggression and dominance in matched groups of subadult Icelandic horses (Equus caballus)

Abstract

We studied sex differences in the nature of aggression and dominance behaviour in two newly formed groups of 1-year-old Icelandic horses. One herd contained nine geldings, the other nine mares. The groups were matched with regard to dominancedetermining traits such as age, weaning age, composition of native herd, social experience, genetic origin, body condition and maternal dominance status. Highranking individuals of both sexes were more aggressive, high-ranking males were older, and high-ranking females had a better body condition. Frequencies of aggressions were similar in both groups. The mares reacted significantly more by showing submission upon an aggression rather than by not responding or by escalating the aggression. For the geldings, this difference was not observed due to a lower tendency to submit. A linear dominance hierarchy was found in both groups. David's scores provided additional information regarding cardinal rank distances and were used to calculate steepness of hierarchies. The female hierarchy was somewhat steeper compared to the male hierarchy, suggesting somewhat lower despotism among males. This was mainly a consequence of the lower unidirectionality in male submission. Male contests occurred predominantly between dyads at top and mid positions, suggesting a low degree of acceptance of the hierarchy.

Dominance relationships and patterns of aggression in a bachelor group of Sorraia horses (Equus caballus)

“…These findings are consistent with the sex differences observed in 1-year-old Icelandic horses by Vervaecke et al (2007), and suggest that males contest dominance relationships more frequently than females. Males may contest dominance relationships more often than females because benefits of high rank are higher for polygynous males than for females (Trivers 1972;Vervaecke et al 2007). Dominance relationships among males were also less stable, as changes occurred after the two breeding stallions returned to the group.…”

“…The frequency of agonistic interactions given per individual in this study is within the range reported for bachelor groups of Przewalski horses (Feh 1988: 0.76 h -1 ;Christensen et al 2002: 1.46 h -1 ) and domestic horses (Christensen et al 2002: 0.63 h -1 ). Consistent with the findings of other studies on horse bachelor groups (Feh 1988;Christensen et al 2002;Vervaecke et al 2007), breeding groups (e.g. Houpt and Keiper 1982;Keiper and Sambraus 1986;Keiper and Receveur 1992;Heitor et al 2006) and feral stallions in multi-male bands (Miller 1981), intensity of agonistic interactions was low, consisting mostly of threats.…”

“…Frequency of agonistic interactions was also higher in a breeding group than in a bachelor group of Przewalski horses, although this difference was not significant (Feh 1988). Vervaecke et al (2007) found no significant difference in aggressiveness between a male group and a female group of 1-year-old Icelandic horses. However, the mean frequency of agonistic interactions given per individual per partner among Sorraia males (0.16 h -1 ) was slightly higher than in the breeding group (mares: 0.09 h -1 ; adults: 0.11 h -1 ).…”

“…The dominance hierarchy was significantly linear, such as reported for other bachelor groups (Feist and McCullough 1976;Feh 1988;Tilson et al 1988;Vervaecke et al 2007) and breeding groups (e.g. Tyler 1972;Wells and von Goldschmidt-Rothschild 1979;Keiper and Receveur 1992;Van Dierendonck et al 1995;Heitor et al 2006).…”

Social feedback and the emergence of rank in animal society

“…Dominance hierarchies are found in primates [10,11], social carnivores [12,13], ungulates [14,15], birds [16][17][18][19], fish [20], and even crustaceans [21,22] and insects [23]. These group-level social structures form and stabilize on the basis of perceptions and actions necessarily made at the individual level [24].…”