Pig emotional states reliably identified by ear posture, locomotor play, and cortisol changes across diverse husbandry

Accurately assessing animal welfare requires robust, non-invasive indicators of emotion, particularly positive states, which are challenging to measure due to their subjective nature. Current methods often lack consistency across diverse environments and individual variations, hindering standardized welfare assessments. Understanding these emotional states is crucial for improving husbandry practices and ensuring ethical treatment, but a reliable, cross-contextual toolkit for emotional evaluation remains an unmet need. This study addresses this gap by seeking broadly applicable markers for emotional reactivity in livestock.

Researchers conducted experiments across four facilities using n=124 female domestic pigs in a within-subject design to induce positive and negative emotional reactions. They developed a common ethogram for behavioral recording and standard operating procedures for collecting saliva. Saliva samples were analyzed in a single lab to measure short-term changes in cortisol (as an indicator of arousal) and oxytocin (as a putative indicator of valence). Data was analyzed using linear and generalized linear mixed models (LMM/GLMM) to test for effects of predicted valence and arousal, and multilevel principal component analysis (PCA) to consolidate indicators.

The study identified several robust indicators of emotional states. Locomotor play significantly increased in positive contexts (GLMM, P < 0.001), while escape attempts increased in negative contexts (GLMM, P < 0.001). Ear posture was also a strong indicator: pigs more often had both ears pointed forward in positive contexts (GLMM, P < 0.001) and both ears pointed backward in negative contexts (GLMM, P < 0.001).