Can Homoplasy Explain Suspensory Adaptations in Living Apes?

Abstract

All living apes share Suspensory adaptations. Spool-shaped Numeral trochleae in apes have been considered as a specialization related to suspensory positional behaviors. While the humerus of Sivapithecus has a spool-shaped trochlea, its shaft is curved like non-hominoid anthropoids (e.g., cercopithecids). So-called Sivapithecus dilemma involves three kinds of question. Is a Sivapithecus-Pongo clade valid? Did Sivapithecus employ substantial suspensory positional behavior? Is the spool-shaped trochlea really originated from suspensory positional behavior? The author hypothesizes that Sivapithecus and Pongo form a clade and that Sivapithecus did not employ suspensory positional behavior (thus, the spool-shaped trochleae are not originated from suspensory behaviors and suspensory adaptations in living apes are results of parallel evolution). In living anthropoids, suspensory behavior (plus climbing) and pronograde quadrupedalism are the dichotomy of arboreal behavioral adaptation. Apes and some atelines engage in the former. However, it is probable that some Miocene apes including Sivapithecus can not be incorporated to this dichotomy and that orthograde climbing represented their basic positional adaptation.