The allometric dependence of eye size on body mass in adult mammals must be largely due to the fact that small heads cannot bear large eyes. This obvious but neglected point is explored using brain mass as a measure of the relevant aspect of head size. Data are taken from the literature. Although there are large interspecific variations in the ratio of eye mass to brain mass, brain mass itself varies over a much greater range, making it possible to discern a clear correlation between the two masses. Indeed, much of the data suggest a simple proportionality (isometry). Over a wide range of body sizes, body mass seems to have no separate and consistent effect on eye/brain ratio, although this ratio is low in many small mammals with poor vision. In species with brains weighing between about 220 and 5200 g, the data can be interpreted as suggesting that eye mass is independent of brain mass, as if increases in eye size beyond a certain point produce little advantage. Whether this ‘plateau’ is functionally meaningful is unclear, however, and large whales do have much bigger eyes, even allowing for their unusually thick sclera.
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