1. Abordo, E., 1976. The learning skills of gibbons. In:Gibbon and Siamang, Vol. 4,D. M. Rumbaugh (ed.), Karger, Basel, pp. 106–134.
2. Reck, B., 1980.Animal Tool Behavior. Garland STPM Press, New York.
3. Candland, D. K. &C. N. Johnson, 1978. Object-play: test of a categorized model by the genesis of object-play inMacaca fuscata. In:Social Play in Primates,E. O. Smith (ed.), Academic Press, New York, pp. 259–296.
4. Chevalier-Skolnikoff, S., B. M. F. Galdikas &A. Z. Skolnikoff, 1982. The adaptive significance of higher intelligence in wild orangutan: a preliminary report.J. Human Evol., 11: 639–652.
5. Connolly, K. &J. Elliott, 1972. The evolution and ontogeny of hand function. In:Ethological Studies of Child Behaviour,N. Blurton Jones (ed.), Cambridge Univ. Press, London, pp. 329–383.