Salivary oxytocin co-varies with parturition and nursing behavior in domestic pigs (Sus scrofa domesticus)

Abstract

Domestic pigs produce twice as many offspring as wild boars, but little is known about the effects of selection pressures for increased productivity on pig behavior and welfare. From an evolutionary perspective, producing larger litters is expected to increase parent-offspring conflict, which may help to explain why piglets from larger litters are more likely to die by starvation or infant-crushing. Our goals were to identify behavioral and physiological correlates of variation in maternal care that may be useful as selection criteria in breeding programs to improve pig welfare. We observed eleven sows regularly during their lactational period and recorded their hourly postural changes and the proportion of time that piglets had contact with teats (nursing contact) or any physical contact with the sow (social contact). As a potential physiological indicator of maternal care, we measured oxytocin concentrations in 119 saliva samples from sows following observations. Samples were extracted and measured in an Enzyme Immunoassay. As a biological validation, we measured changes in salivary oxytocin during parturition in an additional eleven sows. Laboratory validations confirmed that the assay is suitable for measuring salivary oxytocin in pigs, and oxytocin increased almost three-fold during parturition. Sows nursing contact was positively correlated with social contact and negatively correlated with postural changes. Sow oxytocin concentrations were predicted by their nursing contact, but not by their overall social contact with piglets. We compare our results with current evidence regarding best practice methods for salivary oxytocin measurements and discuss the potential to use indicators of maternal care as selection criteria to improve pig welfare.