Goymann, W.; Geue, D.; Schwabl, I.; Flinks, H.; Schmidl, D.; Schwabl, H.; Gwinner, E.: Testosterone and corticosterone during the breeding cycle of equatorial and European stonechats (Saxicola torquata axillaris and S. t. rubicola). Hormones and Behavior 50 (5), pp. 779 - 785 (2006)
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.