Oxytocin as a welfare indicator related to the social environment

Jean-Loup Rault, Purdue University

Abstract

The disruption of normal social behavior is a major welfare concern for farm animals. Swine typically experience several regroupings depending on their production stages. Group-housing of breeding sows is particularly becoming more common in the United States. All these situations involve agonistic interactions to establish a hierarchy in newly formed groups. Therefore, social stress is a critical welfare aspect of animal production systems due to its detrimental effects on well-being, especially for subordinates. In contrast, positive social interactions appear to have evolved in social animals as an effective strategy to reduce the adverse effects of stress. The mechanisms through which benefits arise from positive interactions and how that enhances an animal's stress-coping ability are poorly understood. In addition, the neurophysiological mechanisms of social behavior in pigs remain mostly unknown. Recent research on rodents and primates suggests that oxytocin (OT) acts as one of the main regulators of mammalian social behavior at the central level. Research on the role of OT shows anxiolytic effects and an association with positive affective states. This suggests that OT might be the physiological pathway by which animals benefit from positive relationships. No studies have investigated the role of OT in the social behavior of pigs. The first study, presented in Chapter Two, validated that OT administered intranasally was able to attenuate social separation distress 45 min post-administration in 2 wk-old suckling piglets, as evidenced by the lower activity, escape attempts, and heart rate of OT-treated piglets during a 15 min social isolation test. The response to OT was greater in gilts than barrows possibly due to an estrogen influence. This study also confirmed that intranasal administration represents an appealing, non-invasive method to study the effect of neuropeptides on the behavior and welfare of animals. The second study, presented in Chapter Three, investigated the effects of mixing sows in mid-gestation (prenatally-stressed; PNS), by introducing them into a new group weekly for 3 wk, on the response of their female offspring to social separation. Furthermore, we used intranasal OT administration to elucidate the impact of PNS on the offspring's social behavior. Prenatally-stressed piglets displayed lower levels of behavioral activity and heart rate during social isolation. The administration of OT to PNS piglets reestablished vigilance behaviors such as alertness and standing. In control piglets, OT administration increased the root mean square of successive differences (RMSSD) of the heart rate, a measure of parasympathetic activity; however, PNS piglets did not respond to this effect of OT, but OT lowered their heart rate through a different mechanism. These results suggest that social stress during the second trimester of gestation alters behavior and the development of the autonomic nervous system in the sow's offspring. The third study, presented in Chapter Four, investigated the long-term effects of neonatal OT administration on the pigs' ability to cope with social stress. Repeated OT administration to neonatal pigs at 1, 2, and 3 d of age caused pigs to be involved in more agonistic interactions when mixed after weaning at 19 d of age and to have greater cortisol and adrenocorticotropin hormone concentrations, with a few effects persisting at mixing at 8 wk of age. A dexamethasone-corticotropic releasing hormone test revealed that neonatal OT administration resulted in a long-term dysregulation of the hypothalamo-pituitary-adrenal axis until at least 11 wk of age. We originally hypothesized that neonatal OT administration could provide long-lasting protective effects against social stress. Our results showed the opposite, as a disruption of the OT system in early life brought long-term detrimental consequences. This series of experiments show for the first time that OT is implicated in the social behavior of pigs and appears to play a role in their stress-coping abilities. Hence, OT might represent the first welfare indicator to assess the social environment. A better knowledge of the neurobiological mechanisms of the social behavior of domestic animals will help to clarify how different husbandry practices can lead to various outcomes in animal health and well-being and implement practices that meet their social and psychological needs.

Degree

Ph.D.

Advisors

Lay, Purdue University.

Subject Area

Animal sciences|Behavioral Sciences

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