Female and male pigs' performance in a spatial holeboard and judgment bias task

Abstract

Studies of the cognitive abilities of pigs are increasing in number, due to their relevance for the fields of animal welfare and biomedical research. While both female and male pigs have been used in cognitive tasks, possible sex differences in performance have not yet received extensive attention. This is of interest, as sexual dimorphism in cognitive abilities has been documented for a variety of species. The aim of this study was to assess the effects of sex on pigs' performance in two cognitive tasks. Spatial learning and memory of ten female and ten male pigs was compared in a spatial holeboard task. Working and reference memory, as well as measures of motivation and exploration were assessed. Both females and males acquired the task and no differences were found between sexes for any measures of spatial memory. However, female pigs performed more successfully during reversal trials (shorter latency to first reward, higher number of rewards found), indicating greater response flexibility. This difference between sexes was transient, with males eventually reaching the same level of performance as the females. Judgment bias, a cognitive measure of affective state, was subsequently assessed using an active choice judgment bias task. Pigs were trained to respond differently to a negative and a positive stimulus, signaling either a small or a large reward. During judgment bias testing, pigs were presented with ambiguous cues and their trained 'positive' and 'negative' responses were recorded as optimistic or pessimistic choices, respectively. Both females and males displayed a slightly optimistic judgment bias. Optimistic choosing decreased with repeated testing for both groups. It is likely the pigs learned about the unrewarded outcome of ambiguous cues, rendering them no longer ambiguous. Further improvement of the judgment bias task as a cognitive measure of affective state is deemed necessary. Overall, our results indicate that sex is not a confounding factor when measuring baseline performance of pigs in the spatial holeboard or judgment bias task. Sex effects were only found when subjecting the pigs to a reversal task, warranting further study of sex differences in response flexibility. Such a difference would have implications for pig welfare, as it suggests that males are slower to cope with changes in their environment.