 In 1999, Whiten et al.examined data from 151 years of chimpanzee observation in an attempt to discover how much cultural variation existed between populations of the species.The synthesis of their studies consisted of two phases, in which they (1) created a comprehensive list of cultural variant behavior specific to certain populations of chimpanzees and (2) rated the behavior as either customary – occurring in all individuals within that population; habitual – not present in all individuals, but repeated in several individuals; present – neither customary or habitual but clearly identified; absent – instance of behavior not recorded and has no ecological explanation; ecological – absence of behavior can be attributed to ecological features or lack thereof in the environment, or of unknown origin.