Yawning in sync: implications for social cohesion in horses

Area of Science:

• Ethology
• Animal Behavior
• Comparative Psychology

Background:

• Spontaneous yawning (SY) and contagious yawning (CY) are recognized in various species, particularly primates.
• Research on yawning in horses, despite their complex social structures, remains limited.

Purpose of the Study:

• To investigate SY and CY in horses, a species with significant social dynamics.
• To analyze yawning morphologies and their muscular basis in horses.
• To explore the social modulation of CY in equine populations.

Main Methods:

• Video analysis of 48 horses in pasture settings.
• Identification and classification of two yawning morphologies: Covered Teeth (YCT) and Uncovered Teeth (Y UCT).
• Quantitative muscle recruitment analysis using Equi-adapted Facial Action Coding System (EquiFACS).
• Comparison of yawning frequency under chewing-yawn-chewing versus chewing-chewing-chewing conditions to assess CY.

Main Results:

• Two distinct yawning morphologies, YCT and Y UCT, were identified, differing in muscle activation patterns.
• The study provides the first evidence of CY in horses.
• Contagious yawning was more prevalent in mares with positive social relationships.
• Horses responded faster to yawns from kin than non-kin, with faster responses observed between kin who frequently groomed each other.

Conclusions:

• Yawning in horses exhibits distinct morphological and muscular characteristics.
• Contagious yawning in horses is socially modulated, supporting the Social Modulation Hypothesis.
• Enhanced sensitivity to yawns from familiar individuals, especially kin, may promote behavioral synchronization and strengthen social bonds, conferring selective advantages for group cohesion.