Sucrose Bobs and Quinine Gapes: horse (Equus caballus) responses to taste support phylogenetic similarity in taste reactivity.

The research article discusses a study aiming to explore how horses (Equus caballus) respond to sweet and bitter tastes indicative of potential perceptual similarities to human, nonhuman primates, and rodents. The findings exhibit certain similarities in the reactions observed in horses with those of rodents and primates.

Primary Objectives of the Study

• The study aimed to investigate the presence of the sweet receptor gene in horses, an aspect that is still debatable in the scientific community.
• It sought to widen the scope of research on facial reactions to taste in diverse animal groups beyond the existing focus on closely related lineages of rodents and primates.
• The research intended to test if certain social signals in horses are derived from movements caused by taste reactions.

Methodology

• The horses were provided with oral infusions of either sucrose, symbolizing sweetness, or quinine, representing bitterness.
• Behavioral reactions towards these infusions were closely monitored through frame-by-frame video analysis to capture subtle changes.
• To ensure the validity of the experiment, control groups were also considered, which received water or were subjected to syringe insertion only.

Key Findings and Observations

• The study found that the horses displayed distinctive responses to sweet and bitter infusions.
• The characteristic response to sucrose involved a nodding motion, termed as a “bob,” slight protrusion of the tongue, and forward movement of the ears.
• In contrast, the characteristic response to quinine consisted of stretching the neck and mouth open wide, which is referred to as a “gape,” coupled with a large protrusion of the tongue and backward movement of the ears.

Implications of Findings

• The study’s findings provide insights into the possible presence of sweet and bitter taste receptors in horses, evident from their facial responses to the respective tastes.
• There are similarities in the taste expression features in horses to those documented in rodents and primates, suggesting potential phylogenetic likeness.
• However, the research also found a dissimilarity between facial reactions to taste and other social signals exhibited by horses, provoking further investigations on this aspect.