Influence of Novel Environments and Group Dynamics
on Equine Dominance Hierarchies

Introduction:
     Extensive studies have been conducted to investigate dominance hierarchies, with a moderate amount lying in the study of young horse dominance behavior.  Many studies have also been conducted concerning individual reactions to novel environments, as well as individual temperament testing.  However, to this point, little to no data has been obtained on the effect of temperament changes that influence an entire herd or the effect of a novel environment on an entire herd.  We investigated whether the dominance hierarchy established in the group’s home environment will remain stable when the group is moved to a new environment.

Methods:
     This study used eight Quarter Horse weanlings; all horses were born at Penn State and had never left the farm.  The initial observation for baseline data was done in a pasture where the weanlings had lived together for a month and a half prior to the study.  For one novel environment the weanlings were taken to a shed lot also within the Penn State horse facilities, about a quarter mile from their home pasture. Important to note is that one horse had previous exposure to the shed lot, while all others were exposed for the first time during the study. After observation, the weanlings were returned to their original pasture.  On a separate occasion the weanlings were taken to the Agricultural Arena, a novel environment for all the horses, across the street from the horse facilities.

     The same behavior observation procedures were used in each location.  In each setting, the horses were observed for one hour with no interaction from the observers.  In both of the novel environments, all horses were released simultaneously, with observation beginning directly after the release.  In the hour of observation, one observer used a scan sampling method, with observations recorded every minute.  Behaviors for the scan sample included grazing/eating, resting, locomotion, and sniffing.  Concurrently, the other observer recorded all occurrences of rearing, biting, kicking, chasing, vocalization, and other behaviors deemed relevant that took place within the hour.  In the all occurrence data, the time each behavior took place, as well as the initiating and receiving horses, were recorded.  After the hour of observation, a bucket test was used to determine the dominance hierarchy.  This procedure involved putting a small amount of grain in one bucket and determining which horse appeared most dominant using factors such as who was eating, who was displaced from eating, and agonistic behavior between horses.  The horse that gained access to the grain was haltered and removed from the group and the testing continued with the rest of the group.

Results:

State Behavior Results (in horse minutes):

Event Behavior Results (in number of events):

Rank Order (as determined by bucket tests):

Discussion:
     This study has shown that some dominance hierarchy variation does occur within horse herds placed in novel environments, but overall, rank orders do seem to remain within a range, regardless of location.  The scan sampling seemed to reveal that behaviors are indicative of the environment.  In the baseline pasture filled with grass, the horses spent most of their time grazing.  In the Ag Arena, which had no available food and was completely enclosed, the horses spent a great deal of time sniffing and in motion, perhaps showing investigatory behavior. 

     The all occurrence data made linking agonistic behaviors to rank order a difficult thing to do.  While the general trend seemed to indicate that the horses initiating a great deal of the agonistic behaviors were dominant in the rank order, a few of the horses that were often on the receiving end of agonistic behaviors were actually dominant as well.  It is important to note that the recorded agonistic behaviors could be interpreted a play behaviors, especially in the novel environment where the group was more active.  When comparing the dominance hierarchies, only the rank orders between the baseline and the shed lot were significantly unchanged (Spearman Rank Correlations).

     In this study, the horse that was most dominant in both of the first environments dropped down to nearly the bottom of the herd in the last location.  This horse seemed to be standoffish and uninterested in the third environment, raising questions about the possibility of illness. Had that horse remained near the top of the rank order, the correlation between dominance hierarchies would have been much greater.

     Also, bucket tests cannot be viewed as a definitive way to determine dominance hierarchies.  First of all, different motivations are not represented—some horses may have grain preferences, one horse might be hungrier than another, and some horses may never gain access to grain at all.  In this particular horse herd, the weanlings were used to sharing grain out of a creep feeder, which might have encouraged them to share grain out of a bucket as well. Ultimately, while bucket testing is helpful in determining dominance hierarchies, the procedure still is imperfect and may not represent an accurate rank order.