Pupil shapes and lens optics in the eyes of terrestrial vertebrates

Abstract

SUMMARY Animal eyes that are primarily used under low-light conditions usually have optical systems of short depth of focus, such that chromatic defocus may lead to considerable blurring of the images. In some vertebrates, the problem is solved by multifocal lenses having concentric zones of different focal lengths, each of which focuses a different relevant spectral range onto the retina. A partially constricted circular pupil would shade the peripheral zones of the lens, leading to the loss of well-focused images at relevant wavelengths. The slit pupil, however, allows for use of the full diameter of the lens even in bright light. We studied species of terrestrial vertebrates from a variety of phylogenetic groups to establish how widespread multifocal lenses are and how pupil shapes are adapted to the optical systems. We found that multifocal lenses are common from amphibians to mammals, including primates. Slit pupils were only present in animals having multifocal optical systems. Among the felids, small species have multifocal lenses and slit pupils, while large species have monofocal lenses and round pupils. The Eurasian lynx, a cat of intermediate size, has an intermediate eye design. The functional significance of the absence of multifocal optical systems in large felids remains mysterious, because such systems are present in other large-eyed terrestrial vertebrates. Multifocal optical systems in nocturnal prosimians suggest that those animals have colour vision despite being described as cone monochromats.

Multifocal optical systems and pupil dynamics in birds

“…Various vertebrates compensate for LCA with multifocal optical systems (Kröger et al, 1999;Malkki and Kröger, 2005;Malmström and Kröger, 2006;Karpestam et al, 2007;Gustafsson et al, 2008). In multifocal systems, the crystalline lens has concentric zones of different refractive powers created by a complex gradient of refractive index.…”

“…Indeed, multifocal optical systems are primarily present in eyes of aquatic, crepuscular and nocturnal vertebrates that often have low minimum f-numbers (the f-number depends on the pupil size, and the minimum f-numbers refer to the eye with a maximally opened pupil). Diurnal terrestrial animals with high minimum f-numbers commonly have monofocal systems (Kröger et al, 1999;Malmström and Kröger, 2006), i.e. no distinct zones in the lens and one single focal point for monochromatic light of a certain wavelength (Fig.…”

“…2). In terrestrial vertebrates, multifocal optical systems are therefore usually correlated with slit pupils (Malmström and Kröger, 2006).…”

Adaptation in the optical properties of the crystalline lens in the eyes of the Lessepsian migrantSiganus rivulatus

“…A multifocal lens creates a well-focused color image on a background of defocused light that has passed through 'wrong' zones in the lens (Kröger et al, 1999b). Compensation for chromatic defocus by the multifocal principle is common in fishes (Kröger et al, 1999b;Malkki et al, 2003;Karpestam et al, 2007;Gustafsson et al, 2008;Kröger et al, 2009;Schartau et al, 2009) and tetrapods (Malmström and Kröger, 2006;Hanke et al, 2008;Lind et al, 2008).…”

Design explanation: determining the constraints on what can be alive

“…I argue that this insight can be accounted for if design explanations are seen as reasonings that are concerned with the constraints on what forms of organization 2 See Wouters (2003) for a half dozen examples from textbooks and research papers. A quick look into any zoology textbook or journal would reveal many more, including explanations of why bats are almost exclusively nocturnal rather than diurnal (Thomson and Speakman 1999); of why emperor penguins produce calls by using two independent sound generators, rather than one (see Sturdy and Mooney 2000); of why tiger moths inform predatory bats of their unpalatability by using clicks, rather than other kinds of cues (Ratcliffe and Fullard 2005); of why nocturnal, terrestrial vertebrates have multifocal, rather than monofocal, lenses; of why multifocal eyes have split pupils rather then round ones (Malmström and Kröger 2006); and of why the mammalian inner ear has a spiral shape (rather than being stretched out) (Manoussaki et al 2006). …”