Variations in salivary oxytocin and eye caruncle temperature indicate response to environmental enrichment material in fattening pigs

Conventional pigs are typically raised under intensive conditions, which often do not meet pigs’ basic needs regarding behavioural expression and cognitive challenge, likely threatening their welfare. Thus, enabling the experience of positive affective states and, consequently, positive welfare should be encouraged. We investigated the potential of salivary oxytocin (sOXT) and eye (i.e., lachrymal caruncle region) temperature measured by infrared thermography as indicators of environmental enrichment material valuing in 32 conventional Yorkshire × Landrace pigs (110-day-old, mean ± SD body weight: 80.9±7.6kg/pig) housed in 8 identical pens (3 female and 1 castrated male/pen; 3.3 m2/pig). Pigs were subjected to two contexts/days: fulfilment of expectation for fresh straw (i.e., delivery of 200g of fresh straw/pig) and violation of expectation (i.e., no straw delivered). Additionally, based on footage of individual pigs from -30 min to +30min relative to the moment when an experimenter left the pen after straw had been delivered or not (reference moment), we labelled behaviours more extensively investigated as measures of affective states (body posture; locomotor-rotational play, LOC; social play, SOC; aggression; tail motion; tail posture; tail tip lateralisation). Individual saliva samples were collected at -10 min and +20min for analysis of sOXT and cortisol (sCORT), and individual thermograms were recorded at -10 min, -5 min and +10min relative to the reference moment. Pigs tended to move their tail more often and tended to display more aggressions following straw provision. Furthermore, pigs displayed more playful events after the reference moment than before this event, irrespective of straw availability. However, pigs were mostly inactive (i.e., sitting or lying down) across contexts and no clear behavioural differences between contexts were detected. Similarly, we found no clear differences in sCORT concentrations between contexts. Alternatively, eye caruncle temperature and sOXT concentrations were higher on days when straw was provided. Our results suggest that salivary oxytocin and lachrymal caruncle temperature can be useful non-invasive tools to detect short-term responses to non-social stimuli in fattening pigs. Further research is needed and this study supports continuous development and validation of measures of affective state in domesticated animals.