Mutual grooming and preferred associate relationships in a band of free-ranging horses

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Abstract

Preferred associate (nearest neighbour) and mutual grooming relationships among mares, in an isolated family band of free-ranging horses (Equus caballus), were studied, and the structural and functional differences between these two relationships were examined. The frequent partners accompanying the mare were not the same in both these relationships (p<0.05) and mares changed their partners during the study period between 1988–1990. Individual horses of similar rank tended to remain closer together in both winter and summer. Aggressive–submissive behaviour was so infrequent during spring that rank determinations could not be made; however, in fall, although rank could be determined, rank was not correlated with nearest neighbour. Three subgroups, based on preferred associate relationships in summer, fall and winter, directly reflected the age and social rank of the mares in the group. Individual horses of higher rank tended to have many partners in winter, while individuals of lower rank had fewer. There was no significant correlation between the frequency of mutual grooming and individual rank. The mutual grooming relationship was strongly influenced by seasonal changes as the relative amount of grazing time per day increased. Thus, the frequency of mutual grooming was lowest in winter and highest in summer. The mutual grooming relationship was based on the bonds between individual horses, which were little influenced by social rank. Lower ranking individuals tended to have a greater variety of grooming partners in summer.

Introduction

Many authors have reported on the social relationships which occur in groups of feral or free-ranging horses. However, most of these reports focused on the dominance–subordinance relationship, and only a few dealt with companionship between horses (Tyler, 1972; Clutton-Brock et al., 1976; Wells and von Goldschmidt-Rothschild, 1979; Crowell-Davis et al., 1986; Ellard and Crowell-Davis, 1989; Feh and De Mazieres, 1993).

The monitoring of the preferred associate (Ellard and Crowell-Davis, 1989) and mutual grooming behaviour patterns provides some insight into the companionship between individual horses. Although both the preferred associate relationship and the mutual grooming relationship are indicative of companionship, the correlation between these two behaviours, if one exists, has not been determined.

Clutton-Brock et al. (1976)suggested that free-ranging Highland ponies tended to spend more time close to the partner with which they most frequently exchanged grooming. Wells and von Goldschmidt-Rothschild (1979)suggested that inter-mare relationships, in a group of Camargue horses, were characterized by close proximity but with very little `friendly contact' between them. Crowell-Davis et al. (1986)suggested that the sex difference of development in mutual-grooming partnership and preferred associate relationships in foals existed in the usual social milieu of unmanaged adults, in which several mares form a cohesive band with one or more stallions which associate with them. Ellard and Crowell-Davis (1989)demonstrated that the preferred associate relationship among mares in a paddock was between those individuals of similar social rank. The present study was conducted to examine the differences in the structure and function of the preferred associate relationship, and the mutual grooming relationship. The relationship between these two behaviours, taking into consideration the effects of season, was also examined. Therefore, understanding the companionship relationships between individual horses might facilitate their handling, improve their comfort and reduce their stress when kept in stables or paddocks, thus enhancing productivity and performance.

Section snippets

Animals and methods

The uninhabited Yururi Island (43°12′N, 145°35′E), which lies 2.6 km east of the port of Konbumori in Nemuro City, Hokkaido, Japan, is the home of a band of free-ranging horses. Their descendants were used as work horses to haul kelp. In 1971, when the last human inhabitants left the island, the horses, a mixed breed of Hokkaido native horse, Percheron, Trotter, and Breton, were left behind and now live with minimal management and disturbance.

The island has a circumference of 7 km, a surface

Preferred associate relationship

There were three clear clusters (subgroups) in the band throughout the year but some changes occurred in the membership (Fig. 1). The dominance hierarchy in the band appeared linear in winter, summer and fall. In each of the three seasons there was a significant correlation between age and rank (r=0.98, n=11; r=0.94, n=10; r=0.83, n=12; p<0.01), indicating that older individuals were higher in rank. In spring, however, aggressive–submissive behaviour was infrequent and social ranks were not

Discussion

This study indicated that the preferred associate relationship and the formation of subgroups between mares reflected the social rank in the band. The band size of feral horses in North America was found to be between 2 and 21, with 4 being the most common (Waring, 1983). The large numbers of mares in the Yururi Island band, and the social circumstances involving the less frequent arrival or departure of members, may have been factors promoting the formation of subgroups that strongly reflected

Conclusion

In the band of mares, the mutual grooming relationship is strongly influenced by seasonal factors, the relationship is a means of communication based on ties between individual horses, rather than hierarchical. The preferred associate relationships, directly reflecting social rank or status, develop as subgroups consisting of individuals of similar social rank and status.

Acknowledgements

I thank the owners of the horses who kindly welcomed and transported us to the island. I also thank the Nemuro Educational Board and the students in the Tokyo Agricultural University who assisted me in the data collection. I am grateful to Dr. T. Tennessen, Dr. R. Kusunose and Z. Lucas for their comments on earlier drafts of the manuscript. Finally, my thanks go to Dr. H. Klingel for reviewing the manuscript.

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