Associations Between Resilience to Weaning Stress and Startle Response in Nursery-Age Gilts

Bora Lee, Andrea M Luttman, Catherine W Ernst, Nancy E Raney, Juan P Steibel, Janice M Siegford

Piglet Resilience is the capacity of animals to return quickly to their pre-stress status following a disturbance, including social, physical, and/or disease challenges. Understanding impact of individual resilience on behavior is key to improving the performance and welfare of farm animals. The goal of the study was to assess whether behavioral response to an auditory stimulus during a startle test (acute stress) differed between pigs designated as stress-resilient (SR) or stress-vulnerable (SV). Blood samples were collected from female piglets (n = 170) from 26 litters on the day before weaning, the day of weaning, and four days after weaning. Using serum cortisol concentrations from these samples, female pigs (n = 52) were classified as either SR (n = 26) or SV (n = 26) and used for the startle test. The startle test was conducted when pigs were 6 weeks-of age while they were housed in the nursery room. Video recordings were made with ceiling-mounted cameras from 1.5h before the test to 1.5h after the test. Videos were decoded to determine the relaxed-tense score of the pigs (RT; 1: relaxed, 3: tense; scored 10 min before testing began to describe the underlying mood of the pig), orientation to stimulus (OS; direction pigs were facing when air horn sounded), and startle magnitude score (SM; 0: no response, 4: remained frozen >1min). A mixed-effects ordinal logistic regression model was fit for SM including fixed effects of stress group (resilience designation (SR/SV), OS, RT, and pen position (distance from the air horn) and random effect of pen composition (social group). SM was affected by RT (P = 0.02), with pigs that had the greatest levels of underlying tension expressing less response to the stimulus. Pen position also impacted the SM (P < 0.01), with pigs in pens closer to the air horn displaying a stronger startle response than those housed further away. However, SR/SV designation and OS had no influence on SM. Pigs that are already tense may not be responsive to changes in their surroundings, in this case showing little response to a startling noise. Pigs closer to a startling stimulus are likely to perceive it as more threatening than pigs further away from the potential danger. Our data suggest no relationship between intensity of behavioral response to an auditory startle test and resilience or vulnerability to weaning stress. Behavioral and physiological responses to acute stress are influenced by a number of confounding factors inherent in individual animal variability, including the personalities and experiences of the pigs. However, the startle test could be a relatively easy way to assess fearfulness of pigs on farm, as it requires no training of pigs and can be conducted in the home pen, but further methodological improvement is required to enable instantaneous data collection.