Assessing walking posture with geometric morphometrics: effects of rearing environment in pigs

Rearing social animals like pigs in isolation from conspecifics can have consequences on behaviour and physiology. The aim of this experiment was to determine whether rearing conditions affect body postures. We adapted a method for quantitative evaluation of postures based on geometric morphometrics, developed in horses, for pigs and applied it in different conditions. Forty eight 75-day old females were reared either alone in 2.25 m2 pens (IH, N = 24 animals and 4 groups) or in groups of four in 4.64 m2 pens (GH, N = 24) for two weeks. They were habituated to human handling (stroking, speaking) and marking on their backs every day, and tested individually once a day for 10 min in a corridor outside the home pen during the two subsequent weeks. We observed their behaviour and posture during the first exposure to the test (novelty), and the fourth and fifth (after habituation). On the sixth and seventh tests, a familiar stockperson was present in the corridor (human presence). Before each test, the animals were marked with seven landmarks along their length, corresponding to anatomical points and easily located. An experimenter took pictures of the animals walking along the corridor, and these pictures were transferred to tps software for analysis. GH animals were more often active in the rearing pen than IH (median (IQ) 15% of observations [12-20%] versus 2% [0-13%]; P < 0.05). All animals except one IH initiated contact with the handler during the last sessions of handling (Fisher's exact test, ns). Principal Component Analyses revealed significant effects of rearing and testing conditions on pigs’ behaviour and posture. Novelty led to fewer vocalisations and more exploration for IH than GH animals (P < 0.05), but there were no differences between treatments after habituation to the testing situation. The backs of IH animals were more rounded than those of GH (P < 0.05; dimension 1 of PCA), independently of the test condition. Human presence had no effect on posture. In conclusion, the method based on geometric morphometrics that we developed to study pig posture detected variations in walking posture in pigs associated with rearing conditions. Postures might reflect affective states in pigs, as shown in other species, but further studies are needed to verify this