Grandin, T., Curtis, S. E., Greenough, W. T. 1983. Effects of rearing environment on the behavior of young pigs. Journal of Animal Science 57(Supplement 1), 137 (Abstract).

Pigs raised in an enriched environment with weekly petting were less excitable compared to pigs raised with no extra environmental stimulation. Pigs exposed to an enriched environment approached novel objects more quickly than animals raised in a barren environment. Observation indicated that the pigs would stop playing with an object [plastic milk crate, garbage can, chains, cloth strips, dirt, stones, newspapers, cardboard boxes, ropes and twine] unless it was changed often.