MEASUREMENT OF SELECTION USING DISTANCE STATISTICS IN THE PREHISTORIC ORANG-UTAN PONGO PYGMAEUS PALAEOSUMATRENSIS.

Abstract

A large collection of isolated teeth of the prehistoric orang-utan Pongo pygmaeus palaeosumatrensis from two caves in Sumatra has been analyzed using distance statistics to estimate selection intensities. When the characters are considered jointly, some differences are significant between worn and unworn teeth in antero-posterior and transverse diameters. The change in the entire dental complex could not be considered since the teeth were isolated specimens. Van Valen (1965a) has demonstrated significant selection intensity for unit characters using fossil collections, following the earlier work of Kurten (1953, 1957). Marcus (1964) suggested two multivariate procedures for measuring selection. One was through the use of Mahalanobis distance squared statistics (D2), and the other was generalization of Haldane's (1954) formula. Distance is used here, and lower bound for selection intensity is estimated using the graphs given by Van Valen (1965a). The intensities are low in every case (I, < .08). Kurten (1957) and Van Valen (1963a) reported both directional and stabilizing selection components. The mode of selection was directional in this study. No significant stabilizing selection was discovered. At one cave, Sibrambang, the selection for the large fossil subspecies is toward the smaller size and shape of the living form, while at the other cave, Lida Ajer, there was no significant selection. These effects may be explained in part by the difference in sex-ratio between juveniles and adults. MATERIALS Hooijer (1948) studied the large collection of prehistoric orang-utan teeth collected during the latter part of the 19th century by Eugene Dubois from caves in the Padang Highlands of central Sumatra. Measurements for the 3170 teeth are given in Hooijer (1948). On the basis of study of the collection (including considerable biometrical analysis) Hooijer named subspecies Pongo pygmaeus palaeosumatrensis a larger and more simian form than the recent orang. Measurements for 80 specimens of the recent subspecies P. pygmaeus pygmaeus are also given in Hooijer's paper together with detailed analysis and comparisons of the collections. Antero-posterior and transverse diameters, and height-of-crown measurements are given for the unworn teeth and the first two measurements only for the worn teeth. The transverse diameter is unaffected by wear and in cases of appreciable intertooth wear the antero-posterior diameters were omitted (Hooijer, in litt.). All permanent teeth and six of the 10 deciduous teeth are represented. The specimens were apparently brought into the caves by porcupines who chewed up the less resistant jaws, leaving the isolated teeth. The fauna of the caves in addition to Pongo includes, Panthera tigris, Pantkera pardus, Acanthion brachyurus, Helarctos malayensis, Elaphas maximus, Tapirus indicus, and Rhinoceros sondaicus (Hooijer, 1948).