 This approach turned out not to be particularly fruitful and 20 years later, Heyes, reviewing all the extant data, observed that there had been "no substantial progress" in the subject area.A 2000 paper approached the issue differently by examining competitive foraging behaviour between primates of the same species (conspecifics).This led to the rather limited conclusion that "chimpanzees know what conspecifics do and do not see".