The effect of serotonin and oxytocin on equine docility and friendliness to humans

Neurotransmitters play important roles in animal behaviors. Serotonin (5-HT) and oxytocin (OXT) are well-known neurotransmitters affecting animals' social behavior. Recently, we demonstrated the effect of OXT on horse docility and friendliness to humans. However, the correlation of 5-HT and OXT with horse docility and friendliness to humans remains to be elucidated. This study aims to (1) evaluate the effect of breed, age, and sex on 5-HT and OXT concentrations in horses, (2) examine the correlation between 5-HT and OXT, and (3) to test the effect of 5-HT on the docility and friendliness toward humans. Blood samples were collected from 29 horses at the Horse Industry Complex Center of Jeonju Kijeon College. The horse breeds comprised Thoroughbreds, ponies, Warmbloods, and Haflingers. There were 20 mares and 9 geldings. Based on age, the horses were grouped into postpubertal, adult, and aged horses. The plasma concentration of 5-HT or OXT was measured by ELISA. The temperament of each horse was evaluated by 3 professors who were familiar with all the horses. The mixed model through least significant difference posthoc analysis was conducted to analyze the effect of fixed factors (breed, age, and sex) and interaction factor (breed × age × sex) on random effect (neurotransmitter levels). The Student T-test was performed to analyze the effect of sex (geldings and mares) on 5-HT or OXT concentrations. The linear regression analysis was performed to test the correlation between (1) the plasma concentration of 5-HT and OXT and (2) the plasma concentration of 5-HT or OXT with the score of equine docility and friendliness toward humans. The 5-HT concentration of Warmbloods and OXT concentration of Warmbloods and Thoroughbreds were significantly higher than those of ponies. The concentration of OXT in the aged horse was considerably higher than that of postpubertal horses. Interestingly, there is a positive correlation between the concentrations of plasma 5-HT and OXT. Also, 5-HT showed a positive correlation with horses’ docility and friendliness toward humans. In conclusion, the plasma concentration of 5-HT in combination with OXT can be used as a parameter to identify a horse's docility and friendliness to humans.