Emotion and cognition in low birth weight pigs

Abstract

In commercial pig farming, increasing numbers of low birth weight (LBW) piglets are born, due to selection for larger litter sizes. These piglets are known to have an increased risk of poor animal welfare during the neonatal period, as they are more likely to suffer from injury and disease than normal birth weight piglets. While this results in LBW piglets having a higher risk of pre-weaning mortality, a considerable number of these piglets survive to slaughter age. Whether the welfare of these surviving pigs is affected by their birth weight has received far less scientific attention. In humans, LBW is a risk factor for long-term emotional and cognitive impairments. As humans and pigs share many similarities in brain development, it is possible that the adverse effects of LBW are comparable as well. Impaired emotional and cognitive functioning could negatively affect the welfare of pigs. A negative emotional state reflects an animal’s negative subjective response to its environment, while impaired cognition may hinder an animal’s interaction with its environment. Therefore, the aim of this thesis was to evaluate if LBW influences emotion and cognition in pigs. To assess emotion, pigs were tested in a judgment bias task. Judgment bias is a cognitive measure of emotional state which reflects the influence of emotion on an animal’s interpretation of ambiguous stimuli. In the judgment bias task, pigs first received discrimination training. Pigs were trained to perform a specific behavioral response to two auditory stimuli, predicting either a positive or negative outcome. Once pigs successfully discriminated between these stimuli, they were presented with intermediate, ambiguous stimuli. The pigs’ responses to ambiguous stimuli were scored as optimistic (performance of ‘positive’ response) or pessimistic (performance of ‘negative’ response). Optimistic interpretation of an ambiguous stimulus is indicative of a positive emotional state, whereas a pessimistic interpretation is indicative of a negative emotional state. In addition to judgment bias, pigs’ cortisol concentrations in hair and saliva were measured as a physiological marker of stress. Cognitive performance was evaluated using discrimination training in the judgment bias task, as well as performance in the spatial holeboard task. In this task, pigs had to learn and remember the locations of hidden food rewards in an open arena. After a pig had successfully acquired the task, it was presented with a reversal phase during which it was presented with a new configuration of reward locations. The holeboard allows for simultaneous assessment of working and reference memory, as well as measures of motivation, exploration, and behavioral flexibility. We found that LBW does not have to result in a more negative emotional state in pigs, if they are housed in an enriched environment. However, such housing conditions were not sufficient to counter LBW pigs’ impaired cognitive development based on decreased performance in both behavioral tasks. Although the cognitive impairments we found were mild, it is likely they were ameliorated by the presence of (cognitive) enrichment. LBW pigs on commercial farms may suffer from exaggerated deficits compared to the pigs tested in this thesis.