1. Agusti, J. et al. J. hum. Evol. (in the press).
2. Erikson, G. E. Symp. zool. Soc. Land. 10, 136–164 (1963).
3. Andrews, P. & Groves, C. in Gibbon and Siamang 167–218 (Karger, Basel, 1976).
4. Ward, C. V. Am. J. phys. Anthrop. 92, 291–328 (1993).
5. Rose, M. D. J. hum. Evol. 26, 387–412 (1994).