When consumed sugar water, they consistently bobbed their heads—as if they were very pleased. They also tended to move their ears forward and relax them, and they licked their lips with the tongue sticking out just slightly.
Our horses can make some pretty funny expressions sometimes, and the way they appear to react to certain tastes can give us a good laugh. Now Canadian scientists have put the evidence behind the humor: recent research results showed that horses appear to react differently to sweet tastes compared to bitter.
Ian Q. Whishaw, MSc, PhD, a professor in the Canadian Centre for Behavioural Neuroscience (CCBN) at the University of Lethbridge in Alberta, said horses have distinct facial expressions related specifically to taste. In his study, Whishaw and colleague Emilyne S. Jankunis, a PhD candidate at the CCBN, identified clear equine reactions to bitter quinine and sweet sucrose. They’ve aptly named the reactions the “sucrose bob” and the “quinine gape.”
Whishaw and Jankunis observed 44 stabled riding horses as they received an oral infusion of either sucrose (“sugar water”) or quinine. Fourteen of the horses received one of each infusion at separate times so the researchers could compare individual reactions. A control group of horses received water infusions. The team video-recorded and analyzed the horses' reactions on a frame-by-frame basis.
Whishaw and Jankunis found head-bobbing (aka, the sucrose bob) to be the horses' standard reaction to sugar water. All the horses consuming the sweet infusion consistently bobbed their heads—as if they were very pleased—when they got the sugar treat, Whishaw said. They also tended to move their ears forward and relax them, and they licked their lips with the tongue sticking out just slightly.
When horses consumed bitter quinine liquid, they consistently made a gaping mouth reaction (the "quinine gape"), seen here.
Photo: Courtesy Dr. Ian Q. Whishaw
When horses consumed bitter quinine liquid, on the other hand, they consistently made a gaping mouth reaction (the quinine gape), Whishaw said. The standard “bitter” expression also included back-pointing ears and a significant extension of the head forward. Horses would pull their lips up away from their teeth, open their mouths, and stick out their tongues.
“It looks as if most animals studied (now and previously), including humans, have the same reactions,” Whishaw said. “The sweet receptors are on the tip of the tongue, and the sweet reflex response is to make licking movements to bring the sweet substance into the mouth. The bitter receptors are largely at the back of the mouth, and the bitter response is to push the bitter substance out.”
Taste's purpose is probably an evolutionary one, he said, as it helps a species survive by preferring high-calorie foods (sweet tastes) over toxic plants and poisons (bitter ones). Because the taste reactions appear so similar among species, taste probably originated with a distant common ancestor—“perhaps a fish or very early vertebrate,” Whishaw said.
From a neuroscience point of view, Whishaw said observing taste reactions in horses is exciting research, as it suggests a connection between taste and pleasure/distaste centers in the brain.
The researchers’ identification of the "sucrose bob"—which has never been described in science before—also provides evidence that horses do indeed recognize sweet tastes, despite modern arguments to the contrary. “Some people had proposed that horses have lost sweet sensitivity, but our work shows that is not so,” he said.
From a practical point of view, this also means owners can help horses that object to unpleasant tastes, like deworming medication, Whishaw said. “My colleague (Jankunis) took a horse that refused to take its oral dewormer and treated it with sugar,” he said. “It did not take many trials before it became very accepting of having things squirted into its mouth, and then it readily accepted the dewormer.”
The study, "Sucrose Bobs and Quinine Gapes: horse (Equus caballus) responses to taste support phylogenetic similarity in taste reactivity," was published in Behavioural Brain Research.