Gaping for Relief? Rein Tension at Onset and End of Oral Behaviors and Head Movements in Unridden Horses

Pressure from the bit in the horse's mouth, rein tension, likely feels unpleasant to the horse due to sensitive oral tissues. Through trial and error, the horse may learn how to adjust their behavior in order to avoid, diminish or cease uncomfortable sensations from the bit. We hypothesized that oral behaviors and head movements in response to rein tension have the function to avoid or escape the rein tension. The study objective was to assess in what way oral behaviors and head movements affect rein tension and determine the magnitude of rein tension at the onset and end of these behaviors. Twenty Warmblood horses were fitted with a bitted bridle and subjected to 8 trials of backing up in response to a rein tension signal with the handler standing next to the horse's withers. The rein tension signal was gradually increased and then immediately released when the horse stepped back. A rein tension meter and video recordings were used for data collection. Linear mixed models were used for the statistical analysis. There was a decrease in mean rein tension (sum of left and right rein) from onset to end for open mouth (P < 0.001, from 19 to 11 Newtons (N), biting on the bit (P = 0.004, from 11 to 5 N), and head upward (P = 0.024, from 16 to 12 N), while there was an increase in rein tension associated with head forward (P = 0.015, from 27 to 37 N) and head downward (P < 0.001, from 17 to 46 N). Our results suggest that horses will open their mouth, or bite on the bit, to alleviate the oral tissues from pressure; move the head upward to avoid rein tension and move the head forward or downward to increase rein tension, likely in a presumed attempt to break free from the pressure applied. The horse's oral behaviors and head movements during training can be used to gain a greater understanding of how the horse perceives the magnitude of rein tension.