Associating behavioral and physiological biomarkers of affective states with judgment bias task outcomes in weaned pigs

Abstract

Measuring positive affective states in animals remains challenging. One common method is the judgment bias task (JBT), where animals interpret ambiguous cues as optimistic or pessimistic. While many studies have examined the impact of enrichment on JBT responses, links between JBT outcomes and animal-based affective indicators—such as behavioral and physiological biomarkers—remain unclear. Additionally, personality traits may influence JBT responses. To explore these associations, we observed 36 pigs (aged 37–75 days) in their home pens using continuous focal sampling, collected blood and hair samples, and conducted a go/no-go JBT. Animals learned to associate a reward with the positive location (left/right) and a punishment with the negative location (right/left) and were tested at three test locations: near positive (nS+), middle (M), and near negative (nS-). We hypothesized that pigs exhibiting more positive behaviors would show greater optimism and explored the relationship between JBT performance and physiological biomarkers. Coping strategies and personality indicators were evaluated via a back-test (5d) and voluntary human approach test (72d). Results revealed limited associations between physiological markers and JBT responses. Elevated immunoglobulin (Ig)M levels were linked to greater optimism at the M (est.: 0.44, 95 %CI: 0.21–0.93) and nS- (est.: 0.37, 95 %CI: 0.18–0.77) locations. Conversely, higher IgG levels at the M location were associated with pessimism (est.: 2.95, 95 %CI: 1.50–5.82). Unexpectedly, higher hair cortisol correlated with optimism (nS+: est.: 0.97, 95 %CI: 0.95–0.99; M: est.: 0.95, 95 %CI: 0.92–0.98). Other physiological biomarkers, including serotonin, showed no or inconsistent effects. Positive behaviors, including play, enrichment exploration, social nosing, play barks, grunts, and tail wagging, showed no associations with optimistic JBT responses. However, increased levels of certain social behaviors were linked to pessimism across test locations (nS+ – social nosing: est.: 1.07, 95 %CI: 1.02–1.13; M – play barks and grunts: est.: 1.03, 95 %CI: 1.01–1.05; nS- – play fighting: est.: 1.09, 95 %CI: 1.03–1.15), suggesting that social isolation may influence JBT responses. Finally, no effects of personality indicators were detected. In conclusion, most biomarkers showed no or inconsistent associations, contradicting our expectations. Although positive behaviors have previously been linked to affect, our results indicate that they are not associated with optimism in the JBT. These findings suggest that factors such as social isolation and other unaccounted variables may influence JBT responses. To improve the reliability and efficiency of assessing animal affective states, we recommend direct measurement of animal-based affective indicators and further exploration of their interrelations.