HALL, M.I.: Another look at Leuckart’s Law

Birds have particularly large eyes relative to their body mass when compared to other vertebrates. This has been hypothesized to be an adaptation for flight because increases in axial length of the eye enhance spatial resolution, possibly improving an animal’s ability to judge distances, of obvious importance for flight. This hypothesis has been tested in birds using estimates of eye size derived from Plasticine modeling from dry skulls. Here the hypothesized relationship between increased relative eye size and flight is evaluated using actual measures of eye size in both bats and birds: Megachiroptera, Psittaciformes, Columbiformes, Procelliformes, Falconiformes, and Strigiformes. Eyeball axial diameters and the density of water are used to estimate eye mass, and are then compared to body mass figures either estimated from wing length or culled from the literature. This study confirms that Plasticine modeling from dry skulls is a reliable method of estimating eye mass. It also confirms previous work demonstrating that eye mass scales with negative allometry relative to body mass. Although eye and body mass scale with negative allometry in all groups studied, the scaling relationships differ between groups. This suggests that taxon-specific factors other than size influence eye mass. Leuckart’s Law, that swifter moving animals have larger eyes, receives scant support from these data: Nocturnality is also an important determinant of large eyes among birds. Megachiropteran bats have relatively smaller eyes than birds and other nonvolant mammals, such as primates, lagomorphs, many treeshrews, and elephant shrews. Flight per se does not necessitate relatively larger eyes more than do other factors, although the relatively large eyes of birds remain unexplained. Extension of this study to fossil relatives of birds promises to be interesting in this regard.