Okano, Tsuneya 1965. Preliminary survey of the orang-utan in North Borneo (Sabah). Primates, Vol. 6, Issue. 1, p. 123.

Heymann, E. W. and Ganzhorn, J. U. 2015. History of primate behavioural and ecological field research at the German Primate Center. Primate Biology, Vol. 2, Issue. 1, p. 73.

Kretzschmar, P. Kramer-Schadt, S. Ambu, L. Bender, J. Bohm, T. Ernsing, M. Göritz, F. Hermes, R. Payne, J. Schaffer, N. Thayaparan, S.T. Zainal, Z.Z. Hildebrandt, T.B. and Hofer, H. 2016. The catastrophic decline of the Sumatran rhino (Dicerorhinus sumatrensis harrissoni) in Sabah: Historic exploitation, reduced female reproductive performance and population viability. Global Ecology and Conservation, Vol. 6, Issue. , p. 257.