Aschoff, J.; Diehl, I.; Gerecke, U.; Wever, R.: Aktivitätsperiodik von Buchfinken (Fringilla-coelebs L.) unter konstanten Bedingungen. Zeitschrift für Vergleichende Physiologie 45 (6), pp. 605 - 617 (1962)
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.