99 Determining dominance hierarchy of horses in an equine assisted services program

Abstract

An ethogram was constructed as part of an undergraduate research course and used to determine the dominance hierarchy of a small (n = 4) herd of horses in a university-based equine-assisted services program. The herd consisted of one gelding and 3 mares ranging in age from 10 to 27 years. An ethogram of mutually exclusive agonistic behaviors was developed, with behaviors further defined and sub-categorized as either aggressive (approach, arched neck threat, bite, bite threat, boxing, circling, chase, ears pinned, following, grasping, head bowing, head threat, herding, interference, kick, kick threat, mount, neck wrestle, head on body, nip, push, rear stomp, strike, strike threat) or submissive (retreat/avoidance, balk, rump presentation, snapping). Horses had been group housed together during daytime hours for 22 mo before the study in a 23 by 38 m dry lot with ad libitum access to clean water and one round bale of mixed brome, timothy and orchardgrass hay. At approximately 4:30 p.m. daily horses were brought inside where they were maintained overnight in individual 3 by 3 m stalls with pine shavings and fed evening and morning grain meals with access to water and grass and/or alfalfa hay to meet their dietary requirements. When group housed, horses had access to shade in the morning hours, but no covered shelter. Horses were observed in June 2020 where the average temperature was 22°C, and ranged from 10 to 37°C and the average relative humidity was 61%, ranging from 18 to 100%. Horse behaviors were recorded in 1-h intervals between 9 a.m. and 4 p.m. on random days with 4 replicates of each hour for a total of 28 h of observation. All agonistic horse interactions were recorded as event behaviors. At the conclusion of the study, a score of +1 was assigned to all aggressive agonistic behaviors performed by a horse and –1 assigned to all submissive agonistic behaviors performed by a horse. The point value of all behaviors performed by each horse was added together. The hierarchy rank was then determined by the total scores, with more positive scores equaling a higher social ranking in the herd. Scores for horses from most dominant to least dominant were +168, +32, +20 and –52. These scores reflected a clearly dominant mare in the herd, as well as a dominance hierarchy that matched the hypothesized social structure based on informal observations. Awareness of social hierarchy among horses is essential for good herd management as well as human and equine safety, particularly in equine assisted services programs.