Abnormal behaviours shown by zoo-living chimpanzees (n = 40).

Prevalence = number of individuals performing the behaviour/total number of individuals; duration = minutes/30 hrs observation; frequency = number of performances per 30 hrs observation. Abnormal behaviours were not mutually exclusive categories, and in some cases two or more occurred at the same time; duration is specific to each individual behaviour. Range only stated where min. and max. values differ.