Associations between resilience to weaning stress and startle response in nursery age gilts

Resilience is the capacity of animals to return quickly to their pre-stress status following a disturbance, including social, physical, and/or disease challenges. In modern farming, where there are many stressors, resilience is crucial for pigs because it affects their welfare and production. 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 at weaning (27 +/- 2d of age) as stress-resilient (SR) or stress-vulnerable (SV). Blood samples were collected from female piglets (n = 170) from 26 litters surrounding weaning at multiple time points. Using serum cortisol levels from these samples, two female pigs from each litter (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 7wk-of-age while they were housed in the nursery room and recordings were captured utilizing cameras positioned on the ceiling. We assessed startle magnitude score (SM) and time to resume home pen behavior as indicators of the pigs' behavioral responses. In addition to stress resilience designation, pigs' relaxed-tense score (RT), orientation to stimulus (OS), and pen position were also observed which may affect startle response. Our data suggest no relationship between behavioral response to an auditory startle test and resilience or vulnerability to weaning stress. The difference in the type of stressors needs to be considered as an acute, simple auditory stimulus was used for the startle test and a longer-lasting, multi-modal weaning stress was used for initial resilience designation. However, the startle test could be a relatively easy way to assess the fearfulness of pigs on farms, as it requires no training of pigs and can be conducted in the home pen, but further methodological improvement is required. Though our results did not support a clear connection between a behavioral fear response and physiological stress resilience, examining the resilience of pigs during their early developmental phases using behavioral markers is worthwhile. Early detection of non-resilient animals could offer the opportunity to implement more effective management strategies beginning earlier in life when pigs might be more vulnerable to stressors or the effects more long-lasting, thereby improving the welfare of pigs. Furthermore, it facilitates the selection of robust animals for future breeding. Further research is required to identify behavioral traits clearly indicative of stress resilience.