Behavioural uniformity or diversity? Effects on behaviour and performance following regrouping in pigs

Behavioural differences among pigs can be reliably determined using the human approach test. This study investigated the theory of ‘pre-forming’ a stable hierarchy based on behavioural composition. While in the nursery with littermates, 222 pigs were individually tested in two human approach tests and classified as either being slow, medium, or fast to approach a novel person. Pigs were then regrouped at eight weeks of age. Groups of 12 pigs, consisting of either all slow (uniform slow), all fast (uniform fast), or slow, medium, and fast (diverse) were formed. Intact litters were used as a control treatment. Subsequent aggression, average daily gain, and approach times were assessed. Regrouped pigs took longer to approach than pigs in intact litters (28.6 versus 9.25 ± 3.03 s; P = 0.02) during the final human approach test. There was less fighting per pig in the intact litters (0.45) compared with regrouped pigs (greater than 3.61 ± 0.45 s; P < 0.01). Pigs in the intact litters tended to grow faster than pigs in the uniform fast treatment (863 versus 805 ± 21.6 g/day; P = 0.09). The slow pigs in the intact litters, diverse and uniform-slow treatments all became faster to approach over time (P < 0.05), and did not differ in average daily gain. Whereas, fast pigs in the intact litters became slower to approach than fast pigs in the diverse treatment (49.4 ± 8.61 versus 13.4 ± 7.04 s; P = 0.02) and tended to take longer than the uniform-fast pigs (49.4 ± 8.61 versus 26.7 ± 7.04 s; P = 0.09). There were no differences in average daily gain among fast pigs in any of the treatments (P > 0.1). The human approach test is useful for distinguishing among individuals. Regrouping affects the ability to habituate over time. Furthermore, the type of regrouping strategy influences the ease of habituation. Pigs that are initially classified as slow, medium, or fast may habituate differently to the test. The pigs also have different growth patterns depending on whether they have experienced regrouping. Behavioural uniformity tended to result in more aggression and less weight gain, when compared to intact litters.