Conservation of absolute foveal area in New World monkeys. A constraint on eye size and conformation.

The foveal specializations of five New World monkeys, the marmoset, Callithrix jacchus; the golden-handed tamarin, Saguinus midas niger; the squirrel monkey, Saimiri ustius; the capuchin monkey, Cebus apella; and the howler monkey, Alouatta caraya were compared. Although retinal area varies by over a factor of two in these monkeys, the area of the fovea does not covary with retinal area and remains approximately the same absolute size, as measured by the dimensions of the high density region of cones, or the rod-free region. This constancy in foveal size also holds for rhesus monkeys and humans, bringing the variation in retinal area to a factor of five. Alouatta caraya is unusual, distinguished by a very high central cone density and a small rod-free zone. Physiological constraints that might limit foveal area over a wide range of eye sizes are considered.