Tail-Length Evolution in Fascicularis-Group Macaques (Cercopithecidae: Macaca)

In the four species of macaques that constitute the fascicularis-group, relative tail length generally decreases with increasing latitude, in accord with Allen's Rule. Although this generalization applies to Macaca mulatta in the northern part of its range—north of ca. 26°N, it does not apply south of ca. 26°N, where the tail is anomalously short in Macaca mulatta. This suggests that the anomalously short-tailed population of Macaca mulatta did not originate within its present latitudinal range, but instead dispersed there from farther north. The anomalously short-tailed population apparently replaced a now-extinct longer-tailed population, from which founders of insular Macaca cyclopis previously had been derived. Southward dispersal of the anomalously short-tailed population of Macaca mulatta, and correlated extinction of the longer-tailed population that it apparently replaced, may have been induced by a major glacial advance.