Not Just Little Black Holes
English is a language of ‘homonyms’ as our English, French and Russian teacher Bruno Mannewitz used to say. Many English words sound the same but mean totally different things. Mean and mean, current and current, bat and bat, bark and bark and, of course pupil and pupil come to mind. (On the other hand words that sound the same but are spelt differently like soar and sore, you and ewe, cell and sell, etc. are homophones and equally numerous). This essay, however, is about ‘pupils’, i.e., gaps in the eyes’ iris through which light passes on its way to the retina. That the size of our pupil changes and turns into a tiny circular “black hole” when the ciliary muscle (an involuntary, smooth and not striated muscle) contracts, yes ‘contracts’ is known! But in human eyes only the pupil’s diameter changes upon an exposure to light; in some animals, the shape of the pupil changes as well.
Among animals one can find pupils of a variety of sizes and shapes, and it has repeatedly been tried to correlate the way a pupil looks with the way an animal lives and behaves. A pupil like that of the domestic cat, which appears as a vertical slit under illumination, has been linked with small ambush predators that require good distance estimations. It can also be encountered in many birds, reptiles and sharks; not all of them, however, being small if we think of crocodiles and, for example, the lemon shark. Large predators such as tigers and lions, which like the domestic cat may hunt during the day as well as during the night, possess circular pupils like humans. Such pupils will dilate, i.e. increase in diameter at night and contract during the day, giving the predator a superior sensitivity to the dim light available at night and a better acuity, i.e. resolving power during daylight hours.
Pupils that look like horizontal slits are easily observable in sheep and goats, animals in other words that are grazers and in constant danger of being attacked by a predator. The horizontal pupil provides an excellent field of view of the horizontal surroundings, the area in front and around the animal, but not above it (attacks are not likely to come from there). That, however, is different for creatures of the ocean that do face dangers from above and this could explain the weird often ‘W-shaped’ pupils seen in many squid or the horse-shoe or crescent-shaped pupils of sting-rays and related rhinobatid guitar fish. Given sunny flickers of light from above and dimmer more stable light conditions from below, such unusually shaped pupils are thought to even out the effects of light distortions, excluding unwanted light and shadows and providing the animal with a large visual field.
Perhaps some of the weirdest pupils can be found among some geckos that possess beaded pupils with vertically arranged wider and narrower regions in between. How to relate that to their lifestyle and behaviour is hard to understand and something else, too, is: which has been puzzling me since childhood when I kept Hyla arborea tree frogs and common toads. The latter had beautifully golden horizontal pupils, often considered to be typical of prey animals (but toads are not preyed upon by many species), while the former, when sitting on a horizontal surface, had vertical pupils, commonly seen as a sign of a small ambush predator. However, tree frogs do not always sit horizontally on a leaf but frequently cling vertically to a surface (like they did when resting on the glass walls of my terrarium). Will their vertical pupils then not be oriented horizontally? And grazing animals like goats with normally horizontal pupils, aren’t the latter vertical when the animal grazes with its head down? Martin Banks et al. in a 2015 article in “Science Advances” have shown that when goats and other grazing animals put their heads down, their eyes rotate to maintain the pupils’ horizontal alignment! But did the eyes of my tree frogs also rotate? Alas, it’s too long ago; I can’t remember.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.