Swetha Ravichandran, B.S. Optometry

M.Optom Student, Manipal College of Health Professions, Manipal University, India

Introduction: 

Most often we would have wondered why there is a wide diversity in the size and shape of pupils among various animals compared to simple circular ones that we tend to have. There is an associated significance and purpose for such varying dimensions which seem to be miraculous when we dive into thoughts about it. Recently, scientists have also found a striking correlation between the shape of the pupil and the ecological niche ⁽¹⁾ [Refer Table 1]. Goats, sheep, horses, domestic cats, and many other organisms are capable of varying their pupil shape from being circular in dim illumination to narrow vertical or horizontal slits in bright light ⁽²⁾. This review summarizes the significance behind such diverse pupil types.

Vertical Pupils:

It has been proved that nocturnal and predators’ species mostly tend to have a verticalslit pupil [Figure 1] that happens to be a befitting adaptation to their lifestyle ⁽³⁾. These pupils create an astigmatic depth of field which in turn helps in stereopsis, enabling accurate estimation of the prey location ⁽¹⁾. They also rely on a defocus blur for the purpose of evaluating horizontal contour distances. Owl being a nocturnal bird shows a round pupil that deviates from this established theory ⁽⁴⁾.

Figure 1: Animals with Vertical Pupils. Left: Cat; Right: Beddome’s cat snake (Picture courtesy: Mr. Shakthi Mageshwaran R Optometrist at Sankara Nethralaya and  Wildlife Photographer])

Table 1: Some common animals and their pupil types

Horizontal Pupils:

Figure 2: Animals with Horizontal Pupils. Top Left: Rhacophorus frog; Top Right: NilgiriTahr; Bottom Left: Vine snake; Bottom Right: Cross back bush frog; (Picture courtesy: Mr. Shakthi Mageshwaran R [Optometrist at Sankara Nethralaya and Wildlife Photographer])

Horizontal pupils [Figure 2] and laterally placed eyes are very common among terrestrial preys but there are also theories that it is an adaptation to facilitate the presence of their multifocal lenses ⁽⁵⁾. This is pertaining to the fact that they require a wide and horizontal panoramic field of view to assist the recognition of predators and to permit rapid locomotion during the chase ⁽¹⁾. This pupil type is capable of reducing the horizontal blur contour and it further increases the depth of field at that orientation. It’s also fascinating to know that they are even capable of exerting a large amount of cyclovergence, so as to maintain the pupil horizontal and parallel to the horizon for maintaining their wide visual field even while grazing ⁽¹⁾.

Circular Pupils:

Birds, wolf, mouse, rabbits, dogs, tigers, etc. have a circular pupil [Figure 3] similar to humans ⁽⁴⁾. They may also be accompanied by a multifocal lens system and pinhole constriction principle which helps them overcome the deficiency in the depth of focus ⁽⁶⁾.

Figure 3: Animals with Circular Pupils. Top Left: Owl; Top Right: Elephant; Bottom Left: Tiger; Bottom Right: Monkey; (Picture courtesy: Mr. Shakthi Mageshwaran R [Optometrist at Sankara Nethralaya and Wildlife Photographer])

Other varieties:

Geckos have constricted multiple aperture-like pupils [Figure 4] in bright illumination. The multiple images formed using these apertures can in turn create a greater depth of field which can be used to guide accommodation for detecting distances ⁽⁷⁾.

Skates, rays, fish, and squid have “U” shaped pupils which decrease the effects of lenticular spherical aberration and increase the field of view ^{(8, 9)}. It further helps in camouflage ⁽¹⁰⁾.

Narrow “W” shaped pupils in Cuttlefish are designed specifically to balance the uneven vertical light field. This also cuts down scattered light and improves image contrast ⁽¹¹⁾.

Figure 4: Animals with other different types of Pupils. Top Left: Gecko (Multiple-pinhole pupil), PC: Reddit; Top Right: Cuttlefish (W-shaped pupil), PC: Pinterest; Bottom Left: Skates (U-shaped pupil), PC: Edward Farrell, UCD School of Biology and Environmental Science; Bottom Right: Oriental fire-bellied toad (Heart-shaped pupil), PC: Reddit

Conclusion:

Despite the theories put forth regarding the dimensions of the pupil and its adaptation, there is still some unexplained and unexplored nature regarding pupils. Understanding the significance of the types of the pupil will indeed be an entrancing and a thought-provoking challenge for future researchers.

References:

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