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Did you know that vision is different between predator and prey animals? Most predators have front-facing eyes, which give them a large binocular visual field and superior depth perception for accurate spatial localisation when zooming in on their prey. Herbivores, on the other hand, often have lateral orbits and an almost 360 field of vision to help detect approaching predators. Both have evolved for optimum survival, but come with a visual price; predators have a large blind spot behind them and prey animals have a small frontal binocular visual field. ‘Binocular vision’ means using both eyes together, and is important for depth perception.
There is a striking correlation between terrestrial animals’ pupil shapes and their ecological niche: animals with horizontal pupils are much more likely to be prey animals, and those with vertical pupils predators (think goat eye vs cat eye). But why? In 2015, researchers decided old theories about the function of shape (e.g. controlling retinal illumination in different light environments) failed to explain why pupil slits are elongated vertically in some species and horizontally in others. Their new research shows there is a strong relationship between activity time and foraging mode, and pupil shape – this suggests there are some functional advantages of certain species having these shapes in their specific ecological niche. The study concludes vertical pupils facilitate ‘stereopsis’ and aid the use of defocus blur for estimating distances of horizontal contours along the ground. Basically, vertically shaped pupils provide more effective means for estimating distances along the ground – important for ambush predators who can’t use motion parallex as a depth cue, lest they give away their position to their prey. For horizontal shaped pupils, the researchers concluded this shape improves image quality for horizontal contours in front and behind, helping guide rapid locomotion in a forward direction despite lateral eye placement. They also think horizontal eyes present a horizontally panoramic view for detecting predators approaching from the ground. For some herbivores like sheep that graze, when they face down their eyeballs roll up: the horizontal bar shaped pupil remains parallel to the horizon.
It seems as though independent selective pressure has determined pupil shape in different species. Or, it could be evolution from a handful of common ancestors. The study concluded that transitions in pupil shape happened multiple times within and between lineages and are generally associated with ecological niches: round pupils with daytime activity and active foraging, vertical pupils with nighttime activity and ambush foraging, and horizontal pupils with prey. So, pupil shape is likely to have evolved as a response to the species’ environments, and not due to emergence from a few common ancestors. Interesting!
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