Ontogeny of Orbit Orientation in Primates

Mammalian retinal specializations for high acuity vision evolve in response to both foraging strategies and morphological constraints

Vision is a complex sensory system that requires coordination among cellular and morphological traits, and it remains unclear how functional relationships among traits interact with ecological selective pressures to shape the evolution of vision. Many species have specialized high visual acuity regions in the retina defined by patterns of ganglion cell density, which may evolve in response to ecological traits. For example, ganglion cell density can increase radially towards the center of the retina to form an area centralis, which is thought to improve acuity towards the center of the visual field in predators. Another example is the horizontal streak, where ganglion cells are dense in a horizontal pattern across the retina, which is thought to be beneficial in horizon-dominated habitats. At the morphological level, many have proposed that predation selects for high orbit convergence angles, or forward-facing eyes. We tested these hypotheses in a phylogenetic framework across eutherian mammals and found support for the association between the horizontal streak and horizon-dominated habitats. However, we did not find a significant association between orbit convergence and predation. We also tested if retinal specializations evolve in response to orbit convergence angles. We found that horizontal streaks were associated with side-facing eyes, potentially facilitating panoramic vision. Previous studies observed that some species with side-facing eyes have an area centralis shifted towards the temporal side of the retina, such that the high acuity region would project forward, but this relationship had not been tested quantitatively. We found that the temporal distance of the area centralis from the center of the retina was inversely correlated with orbit convergence, as predicted. Our work shows a strong relationship between orbit convergence and retinal specializations. We find support that both visual ecology and functional interactions among traits play important roles in the evolution of ocular traits across mammals.

Postnatal growth and spatial conformity of the cranium, brain, eyeballs and masseter muscles in the macaque (Macaca mulatta)

Spatial growth constraints in the head region can lead to coordinated patterns of morphological variation that pleiotropically modify genetically defined phenotypes as the tissues compete for space. Here we test for such architectural modifications during rhesus macaque (Macaca mulatta) postnatal ontogeny. We captured cranium and brain shape from 153 MRI datasets spanning 13 to 1090 postnatal days and tested for patterns of covariation with measurements of relative brain, eyeball, and masseter muscle size as well as callosal tract length. We find that the shape of the infant (<365 days) macaque cranium was most closely aligned to masseter muscle and brain size measured relative to face size. Infant brain and juvenile (365-1090 days) cranium shape were more closely linked with brain size relative to basicranium and face size. Meanwhile, the juvenile macaque brain shape was dominated by the size of the brain relative to that of the basicranium. Associations with relative eyeball size and commissural tract lengths were weaker. Our results are consistent with a spatial-packing regime operating during postnatal macaque ontogeny, in which relative growth of the masseter, face and basicranium have a greater influence than brain growth on the overall shape of the cranium and brain.

Patterns and magnitudes of craniofacial covariation in extant cercopithecids

The cranium contains almost all of the vertebrate sensory organs and plays an essential role in vertebrate evolution. Research on the primate cranium has revealed that it is both highly integrated and modular, but studies have historically focused on covariance between the neurocranium and facial skeleton rather than on bones specific to special senses such as vision. The goal of this work is to investigate patterns and magnitudes of craniofacial covariation in extant cercopithecids with particular attention to the orbits. This study takes a quantitative approach using data collected from 38 homologous cranial landmarks across 11 genera of cercopithecid monkeys (Cercopithecidae, N = 291). These data demonstrate that both patterns and magnitudes of craniofacial covariation differ across Cercopithecidae at subfamily, tribe, and genus levels, with the strongest integration in the papionins (and specifically Papio) and significantly weaker covariation in the colobines, particularly Presbytis. Orbital height does not covary with other measurements of the cranium to the same degree as other cranial traits in Cercopithecidae and is highly constrained across the family. This study has important implications for our understanding of the evolution and development of morphological diversity in the cercopithecid cranium and evolution of the primate eye. This study also highlights the potential error of broad assumptions about generalizing patterns and magnitudes of modularity and integration across primates. Additionally, these findings reiterate the importance of trait selection for interpreting fossil taxonomy, as craniofacial covariation may impact phenotypes commonly used to differentiate fossil primate species.