Research ArticleLIFE SCIENCES

Why do animal eyes have pupils of different shapes?

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Science Advances  07 Aug 2015:
Vol. 1, no. 7, e1500391
DOI: 10.1126/sciadv.1500391

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  • RE: iguana iguana

    Professor Banks, thank you for your helpful response. The study’s pupil characterization of iguana iguana is correct. Upon closer observance of an iguana’s eye, I see that the pupil is indeed sub-circular and not round as I stated. A slight vertical slit can be observed in an iguana eye receiving daytime light.

    Reclassifying iguana iguana as diurnal in the Supplemental Figure 1 makes the data more intriguing – iguana iguana would then be the only diurnal species represented in Subcircular, making it fit even less neatly into the general trend of the data. This data appears to further indicate this animal has transitioned from predator to prey, as is also suggested by the unusually high intelligence of iguana iguana (higher intelligence more commonly being associated with predators and not prey animals).

    Thank you again for this interesting study, and for including iguana iguana as one of the species studied.

    Competing Interests: None declared.
  • RE: iguana iguana

    Thank you for the comment.

    We included iguana iguana in our database, but characterized that species somewhat differently than you. First, we categorized the pupil as sub-circular which means in-between circular and vertically elongated. We seem to be right on this because we have seen a number of images of the constricted pupil and it constricts somewhat more horizontally than vertically. At the same time, it’s clearly not a vertical-slit pupil because its aspect ratio does not deviate significantly from 1. Second, we categorized the diel activity as nocturnal as shown in the supplementary material. This was a mistake on our part. As you point out, iguana iguana should have been classified as diurnal.

    Interestingly, iguana iguana does not fit neatly into the general trend of the data, at least as we have the animal currently categorized. You can see this by referring to Supplemental Figure 1 and the link to http://pupils.bankslab.org/ . Iguana iguana is the first blue data point from the left along the sub-circular pupil row. There are two points about the classification of the animal. First of all, as you point out, the animal seems to have moved from predator to prey. If they were predators, our hypothesis would predict vertical-slit pupils. Their behavior has, however, transitioned so it is possible that pupil shape has also transitioned toward circular. Second, iguana iguana is mostly arboreal. Our hypothesis c...

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    Competing Interests: None declared.
  • RE: Why do animal eyes have pupils of different shapes?

    I work with green iguanas (Iguana iguana), large vegetarian diurual lizards, that are common prey when young and small. The larger adults (4-6 feet) are preyed upon less frequently by humans, dogs and larger animals. Iguanas have round irises which I believed represented their strict diurual nature (they are sound sleepers at night). They have excellent eyesight and are cognitive of airplanes flying overhead and tiny ants crawling on the ground. They see color, presumably so they can spot the flowers and fruits that they eat. Would all these attributes, as well as being a prey animal, result in their round pupils? There is some thought that the green iguana may have at one time been a predator, but over time moved to the trees and became vegetarian, resulting in their stomachs chambering off like bovines so that they could digest vegetation. They have very sharp serrated teeth like a predator, but which are believed to be used to rip and tear tough leaves from plants. So my questions are: 1) Are there other reasons I haven't stated as to why a green iguana's pupils are round? 2) Were green iguanas one of the species eyes studied? 3) If a species evolved from a predator to a prey animal, how would that affect eye shape?
    Thank you for your consideration. It is a very interesting study.

    Competing Interests: None declared.

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