Recent molecular research has provided a consistent estimate of phylogeny f
or the living papionin monkeys (Cercocebus, Lophocebus, Macaca, Mandrillus,
Papio, and Theropithecus). This phylogeny differs from morphological phylo
genies regarding the relationships of the mangabeys (Cercocebus and Lophoce
bus) and baboons (Mandrillus, Papio, and Theropithecus). Under the likely a
ssumption that the molecular estimate is correct, the incongruence between
the molecular and morphological data sets indicates that the latter include
numerous homoplasies. Knowledge of how these homoplasies emerge through de
velopment is important for understanding the morphological evolution of the
living papionins, and also for reconstructing the phylogenetic relationshi
ps and adaptations of their fossil relatives. Accordingly, we have used geo
metric morphometric techniques and the molecular phylogeny to investigate t
he ontogeny of a key area of morphological homoplasy in papionins, the face
. Two analyses were carried out. The first compared allometric vectors of C
ercocebus, Lophocebus, Macaca, Mandrillus, and Papio to determine which of
the facial resemblances among the genera are homoplasic and which are plesi
omorphic. The second analysis focused on early post-natal facial form in or
der to establish whether the facial homoplasies exhibited by the adult papi
onins are to some degree present early in the post-natal period or whether
they develop only later in ontogeny. The results of our analyses go some wa
y to resolving the debate over which papionin genera display homoplasic fac
ial similarities. They strongly suggest that the homoplasic facial similari
ties are exhibited by Mandrillus and Papio and not by Cercocebus and Lophoc
ebus, which share the putative primitive state with Macaca. Our results als
o indicate that Mandrillus and Papio achieve their homoplasic similarities
in facial form not through simple extension of the ancestral allometric tra
jectory but through a combination of an extension of allometry into larger
size ranges and a change in direction of allometry away from the ancestral
trajectory. Thus, the face of Mandrillus is not simply a hypermorphic versi
on of the face of its sister taxon, Cercocebus, and the face of Papio is no
t merely a scaled-up version of the face of its sister taxon, Lophocebus, L
astly, our results show that facial homoplasy is not restricted to adult pa
pionins; it is also manifest in infant and juvenile papionins. This suggest
s that the homoplasic facial similarities between Mandrillus and Papio are
unlikely to be a result of sexual selection.