A growing number of studies have investigated the evolutionary drivers
of external eye appearance in primates, but conclusive evidence is
lacking. The literature has distinguished between two types of
functions. Communicative functions, such as announcing a tame
temperament via conjunctival depigmentation, and photo-regulatory
functions towards the amount and quality of light in a given species'
environment. Here, we assess the relative contribution of
photo-regulatory and communicative functions to macaques' external eye
appearance. Macaques' relatively well described social structure and
wide distribution make them interesting to explore. We find that their
sclera is more pigmented closer to the equator, suggesting
photoprotective functions. However, this is not the case for the
conjunctiva. We also explore individual variation in pigmentation
adjacent to the iris, suggesting eyeball pigmentation in macaques is
distributed to reduce damage to the corneal limbus. We find no evidence
that communicative functions drive variation in external eye appearance
in macaques.