Why such long faces? Response to Collard and O'Higgins

In a recent issue of Evolution & Development , Collard and O'Higgins (2001) presented an analysis of the development of the faces of Old World monkeys belonging to the Tribe Papionini. We can applaud this study for at least two general reasons. First, the authors bypass the jaded " morphology versus molecules " debate by using a well-supported molecular phylogeny as a template to analyze morphological evolution. Second, they have investigated a key process—the on-togeny of cranial morphology—that promises to yield fresh insights into parallel evolution within the papionins. Here, I discuss two aspects of the study. First, I discuss limitations in taxon sampling that, if overcome, could enhance the au-thor's analyses. Second, I examine the author's conclusions regarding the relationship between facial lengthening and papionin social systems, as well as concerning the roles that sexual selection and other evolutionary forces have played in papionin evolution. The author's first analysis examined the polarity of facial evolution within the papionin group. Such an analysis requires a sufficient sampling of ingroup and outgroup taxa that does not overlook the diverse facial morphologies of species within genera. However, there appears to be a limitation in the species sampling, which detracts from the article's general conclusions. For all but the genus Lophocebus , the authors analyzed a single species even though most papionin genera have two or more recognized species. Furthermore, some species show considerably different expressions of the crucial features ex-amined—facial length and shape. The problem exists for the ingroup taxa, Papio and Mandrillus , and for the outgroup taxon, Macaca. Admittedly, with respect to Papio and Mandrillus , little has been published describing shape differences among species or subspecies. For example, little is documented about differences between the crania and muzzles of mandrills and drills (Mandrillus). However, at least for Papio, researchers have long noted distinct morphologies among the crania and muzzles of the five subspecies (Hill 1967; Jolly 1967). Perhaps the most significant shortcoming involves overlooking facial diversity within Macaca. For this genus, the authors analyzed the single species M. mulatta (the rhesus macaque), which possesses a relatively short face. However, some macaque species are described (Groves 1980) as having " baboon-like " muzzles (although I should point out that genetic analyses indicate that macaques form a monophy-letic group; Deinard and Smith 2001; Tosi et al. 2000). For example, the Sulawesi Island group of macaques is known to have relatively long …