Arellano, P. E., Pijoan, C., Jacobson, L. D. et al. 1992. Stereotyped behaviour, social interactions and suckling pattern of pigs housed in groups or in single crates. Applied Animal Behaviour Science 35, 157-166.

Our observations confirm that providing straw and space for exercise reduced the frequency of stereotypies (14.9 per sow-hour), but did not completely suppress them (0.3 per sow-hour). ... Sows in crates demonstrated more stereotypies, whereas sows in pens had more peaceful interactions.