Identifying blood-based biomarkers associated with aggression in weaned pigs after mixing

Animal behavior, including agonistic behavior, is regulated by various neurotransmitters and hormones in the blood. To Identify blood-based biomarkers associated with aggression in weaned pigs after mixing, a total of 500 weaned pigs from 65 litters were selected and mixed into 51 pens with littermates separately placed into different pens, and their behavior was recorded for 36 h. Blood samples were taken at 24 h before mixing and 24 h after mixing. An aggressive behavior index composite aggressive score was used to assess the aggressive behavior of weaned pigs after mixing. The most aggressive (n = 51) and the least aggressive pigs (n = 51) were selected in each pen. The concentrations of plasma triiodothyronine (T-3) in the most aggressive pigs were lower (P < 0.01) than those of the least aggressive pigs at 24 h before (3.42 vs. 5.56 ng/mL) and 24 h after regrouping (3.88 vs. 5.51 ng/mL). In addition, skin lesion scores on the front body in the most aggressive pigs was greater (Pearson Chi-Square = 14.053; P = 0.001) than those of the least aggressive pigs 2 h after mixing. The concentrations of salivary cortisol from pen at 2, 24, and 48 h after mixing were higher than that at 24 h before mixing (2.2, 1.8, 2.1 vs. 0.9 ng/mL; P < 0.05). Plasma triiodothyronine levels at - 24 (3.42 vs. 5.56 ng/mL; P < 0.05) and 24 h (3.88 vs. 5.51 ng/mL; P < 0.05) of mixing were higher in the most aggressive pigs than in the least aggressive pigs. The most aggressive pigs had less whole blood 5-hydroxytryptamine (293 vs. 330 ug/mL; P < 0.05) and more platelet number (1926 vs. 1571 x 10(9) ; P < 0.05) than the least aggressive pigs at 24 h after mixing. Number of defeat was positively related (r = 0.241, P < 0.05) to plasma tryptophan at 24 h after mixing. A significant canonical correlation (r = 0.7734, P = 0.006) was found between blood-based biomarkers and aggressive behavior indicators at 24 h before mixing. In conclusion, in this research blood-based biomarkers were partly associated with social stress-induced aggression in pigs, but the relationships are complicated.