Berthold, P.; Fliege, G.; Querner, U.; Winkler, H.: Die Bestandsentwicklung von Kleinvögeln in Mitteleuropa: Analyse von Fangzahlen. Journal für Ornithologie 127 (4), pp. 397 - 437 (1986)
Scientists studying a rare population of tool-using chimpanzees have found that aging can have a profound impact on their abilities—but not all chimps decline equally.