DeLuca, A. M., Kranda, K. C. 1992. Environmental enrichment in a large animal facility. Lab Animal 21(1), 38-44.

The cats' favorite toys contained catnip. ... Cats in single cages often expressed physiological signs of boredom like psychogenic alopecia and polyphagia with resultant obesity. Although we gave these cats a variety of playthings, the problem continued. ... We want to stress that toys can help alleviate research animal boredom, but are not replacement for human companionship. .. All of the dogs preferred toys that they could chew... Empty plastic bottles were a great favorite of the majority of the dogs; chewed plastic pieces, however, clogged the drains. ... We want to stress that toys can help alleviate research animal boredom, but are not replacement for human companionship. .... All the animal species we observed [including miniature pigs] demonstrated a clear preference for human contact over toys.