Telling tails: a closer look into the hanging tail posture of pigs in relation to their emotional state expressed during a novelty test in isolation

The tail of a pig can be positioned in various postures such as curled, straight out, hanging down or tucked against the body. The tail hanging down is suggested to be indicative of a negative emotional state in several situations. The objective of this study was to explore whether the hanging posture could also be used as an indicator of a negative emotional state in a novel situation. Hereto, the tail posture of 476 pigs was studied and related to the behaviour of the pigs at the start of a 10-min combined novel environment and novel object test, and the latency to get back from a hanging tail, if present, into a curly tail was studied during the novel environment and novel object phase of the test, respectively. In addition, the effect of housing, indirect genetic effects (IGE) for growth, victim of tail biting, coping style and sex on (the change in) tail posture was studied. About 30% of the pigs had their tail hanging at the start of the novel environment phase. This hanging tail posture was more likely to be present in pigs with a relatively negative IGE for growth, in barren housed pigs, and in victims of tail biting. Moreover, it took barren housed pigs longer to have their tail (curled) up during the novel environment phase. Coping style and sex did not affect (the duration of) the hanging tail posture. Besides, pigs that had their tail in a hanging posture at the start of the novel environment phase produced more vocalisations and spent more time at the entrance of the novel environment in the first 10 s of the test. To conclude, these results suggest that the hanging tail posture may be indicative of a negative emotional state in novel situations. However, caution and more research are warranted as not all pigs displayed a hanging tail, and furthermore, tail posture might be influenced by other factors than emotional state.