Storhas, M. G.; Weinzierl, R. P.; Michiels, N. K.: Paternal sex in parthenogenetic planarians: A tool to investigate the accumulation of deleterious mutations. Journal of Evolutionary Biology 13 (1), pp. 1 - 8 (1999)
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.