Fig. 1. A – form pupil: vertical (cat), rounded (lynx), round (men) and horizontal (sheep). B – distribution of 214 species of animals in the form of the pupil and lifestyle (Herbivorous – herbivores, Active – active predators, searching for prey, and Ambush – predators lying in wait to ambush the victim). Drawing from the discussed article in Science Advances
In terrestrial predators attacking from ambush, most common vertical slit pupils, while their victims, herbivores, slit pupil often oriented horizontally. Scientists explain this pattern different purposes of the two groups of animals. Vertical pupil allows predators to see sharper vertically oriented objects, such as their victim, herbivore, on the contrary, it is necessary to survey the wide horizons in time to notice a predator, so their pupils are often adapted to ensure that the horizontal lines are sharply visible.
The pupil regulates the amount of light falling on the retina. In addition to the round pupils, the lumen of which is determined by the circular muscles – sphincter of the pupil, in the nature of frequent slit-shaped pupils are provided with an additional pair of muscles. The range of the slot-like clearance pupil wider than that of a round: the area of the vertical pupil cat can vary 135 times, and the round pupil person – only 15. Therefore, slit pupils useful for animals that are active during the day and at night, a wide range of lighting conditions. After all, the day when the light, the pupil should not miss too much light, and at night it is, on the contrary, should be as wide as possible to the retina fell out, and more so the small number of photons.
But the gap pupil can be oriented in different ways – vertically or horizontally. Scientists from Durham University and California have recently suggested that the orientation of the pupil is not random and depends on the lifestyle of the animal, and the objects in the visual field, which is especially important for him. They compared the shape data 214 pupils terrestrial species to their way of life (Fig. 1). It turned out that the pupils herbivores are mainly oriented horizontally in active predators most common round shape of the pupil, and the predators that lie in wait to ambush the victims, most of the pupils are oriented vertically. The shape of the pupil and depend on the period of daily activity: nocturnal animals often pupils were slotted than round. Regularities found corresponded very stringent levels of significance, so had to understand what the reasons explained by the difference in the shapes of pupils of different groups of animals.
Fig. 2. The depth of field vertical pupil
Fig. 2. Vertical pupil allows the animal to sharply enough to see the vertical contours, located a little closer or farther focal distance. However, horizontal lines, even a little bit far from the focal distance, look much blurred. If the pupil is horizontal, the picture will be the opposite. Drawing from the discussed article in Science Advances
When the eye or the camera focuses on a certain point, the rest of the image becomes blurred. The diameter of the blur circle around a specific point of the visual field depends on the difference between the distance to it and to the point on which the eye is focused, and the diameter of the pupil. If the pupil is not round, the cutting depth of the visible space will be different for different directions. For example, an animal with a vertical pupil will sharply enough to see not only the vertical line on which it is focused, but also vertical lines located farther or nearer the focal length. But to the horizontal contours of a pupil adapted worse and horizontal lines on which the animal itself is not focused will be very blurred (Fig. 2). In animals with a horizontally-oriented slit-shaped pupils is the opposite: they see a sharp horizontal contours, and to perceive vertical objects adapted not so good.
Fig. 2. Vertical pupil allows the animal enough to clearly see the vertical contours, located a little closer or farther focal distance. However, horizontal lines, even a little bit far from the focal distance, look much blurred. If the pupil is horizontal, the picture will be the opposite. Drawing from the discussed article in Science Advances
The benefit of a particular form of the pupil depends on the height at which the animal’s eyes are placed. It is easy to understand, considering the photos taken with different distances from the subject, the closer to the surface of the camera is, the higher the gradient blur that is not in focus (Fig. 3). Therefore, if we compare, for example, a cat and a man, the cat is much more important to correct motion blur, because it is “closer to the ground” than people. For these reasons, scientists have suggested that animals smaller growth slit pupils are more common, and this hypothesis was confirmed when researchers analyzed data on the size of the animals from their sample. Interestingly, birds pupils almost always round, with one exception – at the skimmer vertical slit pupils. This exclusion is suitable for theoretical calculations of researchers because of lifestyle skimmer reminds terrestrial predator of small stature. This bird is flying very low at the surface of the water, preying on fish, so that all concerns about the vagueness of the image on which the face at close range, hold for skimmer.
Fig. 3. Simulated view of a plane for animals of different heights (0.6, 0.2 and 0.1 m). Left greatest height, the right – the smallest. The closer to the ground located the eye of the animal, the more blurred look objects do not fall into focus. Drawing from the discussed article in Science Advances
Why the eyes of predators attacking from ambush, better able to see the vertical line, and their eyes herbivorous victims, on the contrary, are set to horizontal contours? One can only assume that the predator is more important to see the victim (which is more like a vertically oriented object), and the victim, on the contrary, it is necessary to survey the wide horizons, which must be clearly visible to her in time to notice the threat. In addition, the victim need to determine the direction of where to go, and you need only predator chasing the victim, and the surrounding open spaces is not very interested.
Various forms of the pupil several times independently during evolution occurred. It turns out that some form of pupil in animals with different ways of life – this is an example of convergent evolution, when the useful life sign parallel develops in organisms other than the closest groups.