Brief communication: Weights of wild orang utans

Abstract

Exact records of the weight of wild primates are of great value, even more so nowadays than formerly because of the extensive use of such data in allometric studies (see, for example, Kay, 1981; Pilbeam and Gould, 1974). Too few accurate records exist, and, in the case of a large ape like the orang utan (Pongo pygmaeus), it is difficult to obtain data on living examples and would be inappropriate, for both conservation and ethical reasons, to obtain any new data on further wild-killed specimens. It is, therefore, imperative to use what data we already have in the most accurate possible way. The only accurate weights for wild orang utans appear to be those of Abbott, as reported by Lyon (1907, 1908, 1911). These were analysed by Eckhardt (19751, who pointed out that a failure to transform avoirdupois to metric in the case of the Bornean specimens had resulted in a spurious disparity in subsequent literature between reported weights of Bornean and Sumatran subspecies ofP. pygmaeus. According to Eckhardt’s re-analysis, adult male orang utans (both subspecies together) range in weight from 32 to 91 kg and females from 32 to 46 kg (in rounded figures). His conclusions have been widely cited and have been utilised by such authors as Fooden and Izor (1983) in important growth studies. What is missing from Eckhardt’s study, and subsequent utilisation of it, is an appreciation of the degree to which orang utans, especially males, grow after achievement of formal maturity. According to Fooden and Izor (19831, the third molars erupt at about age 9 to 10 years; this being the traditional criterion of maturity, both Schultz (1941) and Eckhardt (1975) include as “adult” all specimens in which third molars are reported by Lyon (1908,1911) to have the third molars at least visible in their alveoli. From Fooden and Izor’s data (19831, however, it is clear that males continue to grow well into their teen years and females perhaps somewhat beyond 10 years of age. This we can confirm in a growth study in preparation. The delay in physical maturity beyond the apparently correlated achievement of sexual maturity has important behavioural consequences in this species (Galdikas, 1985). The question, then, is which of Abbott’s orangs, “adult” though they may formally have been, were actually fully grown? Lyon (especially in his 1908 paper) gives quite detailed information on the maturity of the skull, as well as the stage of dental eruption. One of us (C.P.G.) has examined the specimens in question, in the Smithsonian Institution (Natural History), Washington D.C., and independently verified these assessments. It is absolutely clear that Lyon’s sample included many that are far from mature and ought to be excluded from any sample purporting to represent the sizes of fully grown animals: