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William Sprague, Zachary Helft, Jared Parnell, Jurgen Schmoll, Gordon Love, Martin Banks; Pupil shape is adaptive for many species.. Journal of Vision 2013;13(9):607. doi: 10.1167/13.9.607.
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Pupil shape varies considerably across species, though most are circular or elliptical.
Elliptical pupils are always elongated vertically or horizontally relative to the
head. We examined the usefulness of elongated pupils: specifically, why horizontal
elongation is useful to some species and vertical elongation to others. Walls (1942)
proposed that such pupils are adaptive for nocturnality, providing more control
over retinal illumination. Malmström and Kröger (2006) proposed that they
preserve image quality in eyes with multifocal lenses. However, these proposals do
not explain why pupil orientation varies across species. We propose a new theory
based on the visual requirements of different species in their environments. We
examined pupil shape in over 200 animals and related it to activity time, foraging
mode, and height. Clear correlations emerged. Round pupils occur in tall or diurnal
predators. Vertical elongation occurs in short, nocturnal predators; these animals
usually have forward-facing eyes and stereovision. Horizontally elongated pupils
occur in prey animals; they tend to have lateral eyes. We argue that vertical pupils
are well suited for using stereopsis to estimate distances of vertical contours and
using depth-of-field blur for distances of horizontal contours. In an analysis of image
formation, we found that the potential usefulness of blur is inversely related to
height. This may explain why vertical-slit pupils are more common in short than
tall predators. For horizontally elongated pupils, our optical analyses show that the
elongation expands field of view horizontally allowing these terrestrial prey animals
to see objects near the ground plane both in front of and behind them. Our analyses
also show that horizontal elongation allows sharper imaging of horizontal contours
on the ground in different directions, particularly directions well off the optic axis.
We conclude that elongated pupils evolved to optimize visual information near the
ground plane in predators and prey.
Meeting abstract presented at VSS 2013
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