Behavioral responses of horses (Equus ferus caballus) vary in response to potentially threatening odor condition and aversive social odor stimuli

Abstract

Horses (Equus ferus caballus) engage their sense of smell in numerous behaviors. However, only few studies so far evaluated responses of horses to olfactory stimuli presumed to be either attractive or aversive and rather focused on presenting horses with only one type of odor. Therefore, we assessed behavioral responses of horses towards seven natural odors that are either potentially threatening, or socially attractive, or socially aversive. To this end, we presented a total of 10 horses with the following odors: same-sex and opposite-sex horse feces; same-sex and opposite-sex horse urine; tiger (Panthera tigris) feces and urine; and horse blood attached to one of two simultaneously presented food buckets and recorded their behavior. We found that the horses displayed clear behavioral reactions towards potentially threatening odors (tiger feces and urine; horse blood) compared to socially attractive (opposite-sex feces and urine) and, in some cases, socially aversive (same-sex feces and urine) odors. More precisely, the horses put their ears back significantly more frequently and tended to do it for longer in the potentially threatening odor condition compared to the socially attractive one. In the potentially threatening odor condition, the horses ate from the control bucket significantly more frequently compared to the other two conditions. Detailed analyses showed that the horses’ behavioral responses were particularly pronounced following exposure to tiger feces and urine. Furthermore, we found that, in some situations, socially aversive odor stimuli caused higher vigilance and more cautious approach, as reflected in increased time of sniffing the odor-bearing bucket and decreased frequency of eating from it in the socially aversive compared to the potentially threatening odor condition. These findings suggest that horses, which are known as prey and social animals, are sensitive to social odors and even more so to odors from predators.