Does stocking density modify affective state in pigs as assessed by cognitive bias, behavioural and physiological parameters?

Abstract

Recent studies suggest that emotional state can affect cognitive abilities of humans and non-human animals, determining biases in information processing. Negative mental states, such as anxiety or depression, induce pessimistic judgments of ambiguous stimuli. These assumptions may be used to derive indicators of emotional state in captive animals, providing a novel approach to the assessment of animal welfare. This study used a spatial judgement task, in which farmed pigs were trained to expect food inside a bowl in one location and not in another, to determine whether pigs housed in ways that might be expected to result in relatively positive or negative emotional states respond differently to ambiguous stimuli of intermediate spatial locations. Forty growing pigs were housed in groups of 10 at different density for 8 weeks prior to the start of the test. After training, the pigs successfully discriminated between the rewarded and the unrewarded locations as assessed by increased latency to arrive at the unrewarded location, with no rearing treatment difference. Then, pigs were tested on 3 days in which three ambiguous locations, intermediate between the known rewarded and the unrewarded sites, were introduced and latency recorded. In order to compare the novel cognitive bias task with other welfare indicators, during the 8 weeks of the study four behavioural observations, two measurements of skin lesions, two salivary samples for cortisol and α-amylase, and six individual weights to assess growth were collected.Considering the mean of the three test days, there was no difference between the treatments in the pigs’ judgement of the three ambiguous locations. However, the latency trend during the testing days led to difference between treatments on the third day (P=0.026). Pigs housed a higher density seemed to learn faster that the ambiguous stimulus near the unrewarded location was also not reinforced by a reward and showed a higher latency to approach on day 3 (44 vs 15.6s). These animals were also observed to have a higher frequency of sitting posture (P=0.01), and more total skin lesions (P=0.035) due to aggressiveness, in particular at the ear location (P=0.009), but did not differ in other physiological parameters.Although the results showed no immediate effect of stocking density on cognitive bias, differences in latencies to reach the bowl over the three testing days suggest a different learning process between treatments.