We present a compilation of endocranial volumes (ECV) for 176 non-human primate species, based on individual data collected from 3813 museum specimens, at least 88% being wild-caught. In combination with body mass data from wild individuals, strong correlations between endocranial volume and body mass within taxonomic groups were found. Errors attributable to different techniques for measuring cranial capacity were negligible and unbiased. The overall slopes for regressions of log ECV on log body mass in primates are 0.773 for least-squares regression and 0.793 for reduced major axis regression. The leastsquares slope is reduced to 0.565 when independent contrasts are substituted for species means (branch lengths from molecular studies). A common slope of 0.646 is obtained with logged species means when grade shifts between major groups are taken into account using ANCOVA. In addition to providing a comprehensive and reliable database for comparative analyses of primate brain size, we show that the scaling relationship between brain mass and ECV does not differ significantly from isometry in primates. We also demonstrate that ECV does not differ substantially between captive and wild samples of the same species. ECV may be a more reliable indicator of brain size than brain mass, because considerably larger samples can be collected to better represent the full range of intraspecific variation. We also provide support for the maternal energy hypothesis by showing that BMR and gestation period are both positively correlated with brain size in primates, after controlling for the influence of body mass and potential effects of phylogenetic relatedness.

Examination of orbit size and optic foramen size in living primates reveals two adaptive phenomena. First, as noted by many authors, orbit size is strongly correlated with activity pattern. Comparisons of large samples of extant primates consistently reveal that nocturnal species exhibit proportionately larger orbits than diurnal species. Furthermore, nocturnal haplorhines (Tarsius and Aotus) have considerably larger orbits than similar-sized nocturnal strepsirrhines. Orbital hypertrophy in Tarsius and Aotus accommodates the enormously enlarged eyes of these taxa. This extreme ocular hypertrophy seen in extant nocturnal haplorhines is an adaptation for both enhanced visual acuity and sensitivity in conditions of low light intensity. Second, the relative size of the optic foramen is highly correlated with the degree of retinal summation and inferred visual acuity. Diurnal haplorhines exhibit proportionately larger optic foramina, less central retinal summation, and much higher visual acuity than do all other primates. Diurnal strepsirrhines exhibit a more subtle but significant parallel enlargement of the optic foramen and a decrease in retinal summation relative to the condition seen in nocturnal primates. These twin osteological variables of orbit size and optic foramen size may be used to draw inferences regarding the activity pattern, retinal anatomy, and visual acuity of fossil primates. Our measurements demonstrate that the omomyiforms Microchoerus, Necrolemur, Shoshonius, and Tetonius, adapiform Pronycticebus, and the possible lorisiform Plesiopithecus were likely nocturnal on the basis of orbit diameter. The adapiforms Leptadapis, Adapis, and Notharctus, the phylogenetically enigmatic Rooneyia, the early anthropoids Proteopithecus, Catopithecus, and Aegyptopithecus, and early platyrrhine Dolichocebus were likely diurnal. The activity pattern of the platyrrhine Tremacebus is obscure. Plesiopithecus, Pronycticebus, Microchoerus, and Necrolemur probably had eyes that were very similar to those of extant nocturnal primates, with a high degree of retinal summation and rod-dominated retinae. Leptadapis and Rooneyia likely had eyes similar to those of extant diurnal strepsirrhines, with moderate degrees of retinal summation, a larger cone:rod ratio than in nocturnal primates, and, more speculatively, well-developed areae centrales similar to those of diurnal strepsirrhines. Adapis exhibited uncharacteristically high degrees of retinal summation for a small-eyed (likely diurnal) primate. None of the adapiform or omomyiform taxa for which we were able to obtain optic foramen dimensions exhibited the extremely high visual acuity characteristic of extant diurnal haplorhines.

Comparative studies of mammalian eye morphology have shown that relative cornea size is an important correlate of visual ecology. Nocturnal species tend to have large corneas relative to eye size as an adaptation for increasing visual sensitivity. By contrast, diurnal species tend to have smaller corneas relative to eye size because their eye morphology maximizes visual acuity. Although qualitative analyses suggest that activity pattern may have a similar influence on eye morphology in primates, various current hypotheses have proposed that either diurnal anthropoids or diurnal lemurs have visual systems that diverge from those of other diurnal mammals. The goal of this analysis is to quantify the relationship between eye morphology and activity pattern in primates and to determine whether primates exhibit variation in eye morphology comparable to that of other mammals. Data on eye size and cornea size were collected for 147 specimens of 55 primate species. These data reveal that, within primate suborders, diurnal species have significantly smaller relative cornea sizes than nocturnal or cathemeral species. Both haplorhines and strepsirrhines thus exhibit variation in eye morphology that is consistent with functional expectations. However, comparisons between the two primate suborders demonstrate that haplorhines and strepsirrhines differ significantly in eye morphology. Whereas strepsirrhines have relative cornea sizes that are similar to nonprimate mammals of comparable activity pattern, diurnal anthropoids have smaller relative cornea sizes than most nonprimate mammals. This derived eye morphology in anthropoids probably evolved in the anthropoid stem lineage as a result of selection for highly acute diurnal vision.