Three experiments studied the behaviour of pigs which were brought together for brief periods of observation on successive days. Two distinct patterns of aggressive behaviour were observed, one involving biting and the other involving butting. Biting was common when the pigs first met, and butting seemed to replace biting in some cases. Pairs of pigs which had no previous contact frequently showed a straight forward type of dominance with one pig performing almost all the biting and butting; among well-acquainted pairs, however, the relationships were frequently more complex.

Of all the experimental manipulations, familiarity between two animals had the most profound effect on the behaviour. Biting, but not butting, was reduced if the pigs were housed in neighbouring pens, or if they had previously shared the same pen. Biting was almost eliminated, and butting was reduced, if the pigs lived together throughout the time of the observations. There were no differences between females and castrated males, or between individually-housed and group-housed animals in either type of behaviour.

Pigs which were subjected to repeated biting generally became less active, refraining from some social and non-social activities. Butting did not appear to have similar effects on the recipient animals.

The experiments illustrated some complex social interactions involving a large number of distinct social activities. In this light, the usefulness of such broad terms as ‘aggression’ and ‘dominance’ is discussed.