Ontogeny of Orbit Orientation in Primates.

Orbit orientation in primates has been linked to adaptive factors related to activity pattern and size-related variation in structural influences on orbit position. Although differences in circumorbital form between anthropoids and strepsirrhines appear to be related to interspecific disparities in levels of orbital convergence and orbital frontation, there is considerable overlap in convergence between suborders. Unfortunately, putative links between convergence and frontation across primates, and consequent arguments about primate and anthropoid origins, are likely to be influenced by allometry, the size range of a respective sample, and adaptive influences on encephalization and activity patterns. Such a multifarious system is less amenable to interspecific treatment across higher-level clades. An ontogenetic perspective is one way to evaluate transformations from one character state to another, especially as they pertain to allometric effects on phenotypic variation. We characterized the ontogeny of orbital convergence and frontation in 13 anthropoid and strepsirrhine species. In each suborder, correlation and regression analyses were used to test hypotheses regarding the allometric bases of variation in orbital orientation. Growth trajectories were analyzed intra- and inter-specifically. Frontation decreased postnatally in all taxa due to the negative scaling of brain vs. skull size. Further, interspecific variation in relative levels of frontation was linked to corresponding ontogenetic transpositions in encephalization that differed within both suborders. In strepsirrhines, postnatal increases in convergence were largely due to the negative allometry of orbit vs. skull size. In contrast, convergence in anthropoids varied little during growth, being unrelated to ontogenetic variation in either relative orbit or interorbit size. Anat Rec, 302:2093-2104, 2019. © 2019 American Association for Anatomy.