Short Immobilization in a Sling Does Not Lead to Increased Salivary Cortisol Levels in Pigs

The goal of the present study was to evaluate the potential stress developed in farm hybrid pigs and miniature laboratory pigs briefly restrained in a sling, by measuring salivary cortisol levels. The study was performed in 20 healthy pigs grouped into three groups: group HYB-F: hybrid female pigs (n = 12), housed at the CREBA facility (Lleida, Spain); group MIN-F: Specipig^® miniature female pigs (n = 4), housed at the CREBA facility; group MIN-M: Specipig^® miniature male pigs (n = 4), housed at the Almirall facility (Barcelona, Spain). Upon arrival, the animals were enrolled in a social habituation and training program, which included habituation to a restraint sling. The sling was a stainless steel structure with a canvas hammock which had four openings for placing the animal's feet. The assessment of stress levels in the sling was carried out by measuring cortisol levels in saliva samples. Five saliva samples were collected from each animal over 4 days: Sample 1 (basal sample): taken after animals perceived the presence of the technicians in the pen; Sample 2: taken after animals saw the sling in the pen; Sample 3: taken when animals were in the sling; Sample 4: taken 1 min after the previous one; Sample 5: taken after animals were released back on the floor. In group HYB-F, five animals (5/12) showed strong resistance and could not be restrained in the sling on at least one day. All animals in the groups of miniature pigs could be restrained on all the days. Within each group, the manipulation phase did not affect salivary cortisol levels. Likewise, salivary cortisol levels did not change significantly across days in either group. In conclusion, salivary cortisol levels did not increase when pigs were lifted and briefly restrained in the sling, even though some of them (in particular, the hybrid pigs) showed apparent signs of stress. The lack of correlation between such apparent stress and salivary cortisol levels might be because the vocalizations and movements were not really signs of stress, but simply a way of releasing discomfort, learned in the process of socialization and habituation. In light of this unexpected conclusion, further studies are needed to collect other physiological and behavioral data to clarify what actually happens when pigs are restrained in a sling.