Michiels, N. K.; Kuhl, A.: Altruistic sperm donation in a sperm-dependent parthenogenetic hermaphrodite is stabilized by reciprocal sperm exchange. Journal of Zoology 259 (1), pp. 77 - 82 (2003)
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.