large species such as the snowy owl as those of humans. The snowy owl's eye, at about in diameter, is slightly smaller than those of great horned and Eurasian eagle-owls but is slightly larger than those of some other large owls. Snowy owls must be able to see from great distances and in highly variable conditions but probably possess less acute night vision than many other owls. Based on the study of dioptres in different owl species, the snowy owl was determined to have eyesight better suited to long-range perception than to close discrimination, while some related species such as great horned owls could probably more successful perceive closer objects. Despite their visual limits, snowy owls may have up to 1.5 times more visual acuity than humans. Like other owls, snowy owls can probably perceive all colors but cannot perceive ultraviolet visual pigments. Owls have the largest brains of any bird (increasing in sync with the size of the owl species), with the size of the brain and eye related less to intelligence than perhaps to increased nocturnality and predatory behavior. Size The snowy owl is a very large owl. They are the largest avian predator of the High Arctic and one of the largest owls in the world. Snowy owls are about the sixth or seventh heaviest living owl on average, around the fifth longest and perhaps the third longest winged. This species is the heaviest and longest winged owl (as well as the second longest) in North America, the second heaviest and longest winged owl in Europe (and third longest) but is outsized in bulk by about 3 to 4 other species in Asia. Despite being sometimes described as of similar size, the snowy owl is somewhat larger in all aspects of average size than the great horned owl while the similarly specialized taiga-dwelling great grey owl (Strix nebulosa), is longer in total length and of similar dimensions in standard measurements, but is shorter winged and much less heavy than the snowy owl. In Eurasia, the Eurasian eagle-owl is larger in all standards of measurements than the snowy owl not to mention two additional species each from Africa and Asia that are slightly to considerably heavier on average than the snowy owl. Like most birds of prey, the snowy owl shows reverse sexual dimorphism relative to most non-raptorial birds in that females are larger than males. Sexual dimorphism