Cognitive bias in pigs: Individual classification and consistency over time

Abstract

The aim of the study was to ascertain if the cognitive bias (CB) test can be used to assess pigs' emotional state and classify them individually. Moreover, the test was repeated over time to assess its consistency. Thirty-six male pigs were individually trained during 14 training sessions to discriminate between a bucket with (A) or without (NA) access to chopped apples depending on its position (left or right) in a test pen. Once pigs were able to discriminate between both positions, each animal was subjected to 2 A and 2 NA reminder sessions before performing the CB test session, where the bucket was placed on a central position with access to 2 pieces of apple. The trial was repeated after 5 weeks, reducing the number of training sessions to 4. Time to contact the bucket, time to eat (or try to eat in the case of NA sessions), number of vocalizations, number of times pigs were reluctant to move, number of escape attempts, and number of urination and defecation events were recorded. In the first trial, time to contact the bucket and time to eat was significantly lower in A than in NA from session 10 (P < 0.0012), indicating that pigs were able to discriminate between both positions. In the second trial, both variables were significantly lower in A compared to NA from session 2 (P < 0.005) onward, confirming the pigs' capacity to remember the task. Pigs were individually classified as having positive, negative, or neutral CB, according to the time to contact the bucket during the CB test session in comparison with the time taken during the remainder sessions. A large percentage of pigs were classified as positive CB in both trials (84.85% and 94.29%, respectively). However, there was no consistency between the results of both trials, suggesting that during the second CB test session animals were able to remember the content of the bucket of the first CB test session. Alternatively, other factors such as the time of the day that pigs were tested, the age of the animals, or their hunger state could have an effect on the results.