1. Animal Tool Behavior: the Use and Manufacture of Tools by Animals;Beck,1980
2. Tool use by wild and captive elephants;Chevalier-Skolnikoff;Animal Behaviour,1993
3. Evolution of longevity in ungulates and carnivores;Cutler;Gerontology,1979
4. The Descent of Man and Selection in Relation to Sex;Darwin,1871
5. Among the Elephants;Douglas-Hamilton,1975