There has been a lot of research done in humans and primates (and even rats) to determine how they react to sweet and sour tastes. Recently a study from the Department of Neuroscience, Canadian Centre for Behavioural Neuroscience, University of Lethbridge, Canada, showed that horses have very similar facial responses to sweet and sour as do rodents and primates.
In humans, non-human primates, and rodents, “The sweet taste of sucrose elicits facial responses that include rhythmic tongue protrusions whereas the bitter taste of quinine elicits facial responses that include gapes, featuring an opening of the mouth and protrusion of the tongue,” noted the study.
According to the researchers, the study was done in horses because: 1) there has been debate about the presence of a sweet receptor gene in the horse [Editor’s note: Maybe they should have asked some horse owners about that one]; 2) there is a need to expand the examination of facial reactions to taste in lineages other than rodents and primates; and 3) the horse provides an opportunity to test the hypothesis that some social signals derive from movements related to taste reaction.
The researchers gave a syringe of either sucrose or quinine (very bitter) and their reactions were videotaped and those tapes were studied frame-by-frame. Control horses either just had a syringe inserted into their mouths or were given water.
“Amongst the many responses made to the infusions, the distinctive response to sucrose was a (head) bob coupled with a slight tongue protrusion and forward movement of the ears; the distinctive response to quinine was a head extension and mouth gape accompanied by a large tongue protrusion and backward movement of the ears.”
The study noted the “similarity of features of taste expression in horses to those documented in rodents and primates.” They noted that the reactions were also dissimilar from facial reactions to other social signals displayed by horses.
It would be interesting to know how the horses reacted to the handler and the syringe depending on whether they were given the sweet or bitter taste first. It also might give us some insights on why some horses are harder to deworm and give bute than others. One older horse I had hated the taste of even 1 gram of bute, and he wouldn’t eat anything or drink for a few hours afterward. However, he didn’t mind being dewormed.
You can find the abstract of this study at www.sciencedirect.com.