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First published online December 14, 2005
Journal of Experimental Biology 209, 18-25 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01959

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Pupil shapes and lens optics in the eyes of terrestrial vertebrates

Tim Malmström and Ronald H. H. Kröger^*

Department of Cell and Organism Biology, Lund University, Zoology Building, Helgonavägen 3, 22362 Lund, Sweden

View larger version (32K): [in a new window]   Fig. 1. Schematic illustration of the function of a colour-corrected multifocal fish lens. The spherical lens has a number of discrete zones, three in this example, of different focal lengths for monochromatic light of intermediate wavelength (green). Because of colour dispersion, the lens refracts light of short wavelength (blue) more strongly than green light, such that the zone of the lens having too long a focal length for green light focuses blue light on the retina. Accordingly, the zone of the lens having too short a focal length for green light focuses light of long wavelength (red) on the retina. By this mechanism, a sharp colour image is created by a single lens. That image, however, is contaminated by defocused light having passed through `wrong' zones of the lens (e.g. the peripheral and intermediate zones for green light).

View larger version (17K): [in a new window]   Fig. 2. The functional significance of the slit pupil in combination with a multifocal lens. In the fully dilated state of the pupil (A), all zones of the lens (shown in the colours they are focusing) can be used. A concentrically constricting iris (B) would cover the outer zone of the lens such that a spectral range (blue in this example) could not be focused on the retina. By contrast, all lens zones can be used if the pupil constricts to a slit (C).

View larger version (87K): [in a new window]   Fig. 3. Multifocal optical systems were found in all studied groups of terrestrial vertebrates, ranging from amphibians (A,B) to primates (C-F). Ring-like structures are visible in the photorefractive images (A,C,E). Because of larger eye size, the rings are more prominent in the primates (C,E) than in the toad (A). All three species have slit pupils; horizontal in the toad (B) and vertical in the primates (D,F). Scale bars are 5 mm. Species: Bufo marinus (A,B), Nycticebus coucang (C,D), Galago senegalensis (E,F).

View larger version (124K): [in a new window]   Fig. 4. Multifocal optical systems in combination with circular pupils are present in the beauty snake (Orthriophis taeniurus friesi; A,B) and house mouse (Mus musculus; C,D). The partially constricted pupil of the house mouse was visualized using infrared light because in the visual range the iris was as black as the pupil and the flashlight of the camera did not illuminate the eye at close range. Scale bars are 2 mm.

View larger version (84K): [in a new window]   Fig. 5. The eyes of felids. The domestic cat (Felix sylvestris domestica; A,B) has a multifocal optical system (indicated by the ring-like structure in the photorefractive image) and a slit pupil. The Siberian tiger (Panthera tigris altaica; E,F) has a monofocal optical system (the photorefractive reflex has a smooth intensity gradient) and a circular pupil. The pupil appears slightly oval in both images because the tiger did not look at the camera. The photorefractive reflex is appearing sideways because the animal was lying on its side. The Eurasian lynx (Lynx lynx; C,D) has an eye of intermediate size and design. A faint ring-like structure is visible in the photorefractive image and the pupil is oval to rhomboid in shape. Scale bars are 5 mm.

View larger version (74K): [in a new window]   Fig. 6. The eyes of canines. The red fox (Vulpes vulpes) has vertical slit pupils and multifocal optical systems (A,B). Grey wolves and dogs (Canis lupus lupus and Canis lupus familiaris, respectively) have circular pupils (D). The status of their optical systems is unclear, because some dogs have smooth photorefractive reflexes (dachshund; C), while others have clear ring-like structures (giant schnauzer; E). All members of a group of four closely related tame wolves had a central region of irregularity in photorefractive images (F). Scale bars are 5 mm.

View larger version (133K): [in a new window]   Fig. 7. Multifocal optical systems and slit pupils in large-eyed terrestrial vertebrates. Clearly visible ring-like structures were observed in photorefractive images of the eyes of domestic sheeps (Ovis aries domesticus; A) which have horizontal slit pupils (B). Similar pupil shapes were found in other artiodactyls, such as the reindeer (Rangifer tarandus; C). The Nile crocodile (Crocodylus niloticus; D) has a vertical slit pupil in combination with a multifocal optical system. Scale bars are 10 mm.