The use of olfactory and other cues for social recognition by juvenile pigs

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Social recognition is essential for the maintenance of a stable group structure. Failure to recognise familiar conspecifics in social groups of juvenile pigs may initiate agonistic encounters that can compromise welfare and productivity. Current housing systems may allow build up of atmospheric ammonia that might, in turn, interfere with the olfactory system and compromise olfactory perception. In the present study, 16 juvenile pigs were housed in fresh air while another 16 pigs were kept in an ammoniated atmosphere (approximately 36 ppm) for 1 week prior to test and another week during testing. We then assessed the role of olfaction in social recognition and determined whether chronic exposure to ammonia compromised discrimination based on olfactory perception by comparing the pigs’ responses to selected cues from a familiar and an unfamiliar pig presented simultaneously in separate chambers of a modified Y-maze in each of two test situations (near, remote). Visual, auditory, olfactory, and tactile cues were all provided in the “near” test situation; here, the stimulus pigs were presented in two separate chambers behind clear perspex walls containing an aperture that allowed nose-to-nose contact between the test and stimulus pigs. On the other hand, the “remote” test provided only olfactory cues via air passed from the chambers containing the stimulus pigs into the test chamber. Each test lasted 5 min and the pigs’ behaviour was recorded via overhead video cameras; we then measured the accumulated times spent near and the numbers of visits made to the familiar and the unfamiliar stimulus pigs as well as the transitions between them. Overall, pigs made more visits to and spent significantly longer near both the stimulus pigs in the near test than in the remote one (ANOVA, P<0.001). They also made more transitions between the stimuli in the former test than the latter ( P<0.001). Pigs from both the ammonia and the fresh-air treatment groups showed social discrimination. However, pigs that had received chronic exposure to ammonia visited the familiar pig more often and spent longer near it than the unfamiliar one regardless of the test situation ( P<0.05) whereas those reared in fresh air spent longer near the unfamiliar animal ( P<0.05). The present results suggest that pigs from both treatment groups employed olfactory cues in social recognition, but that chronic exposure to ammonia did not interfere with this ability. However, ammonia treatment seemingly affected social preferences, thus indicating an unknown and more fundamental effect of living in ammoniated atmospheres.