Reason we have two eyes and two ears
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Reason we have two eyes and two ears


This time, I will consider the placement of animals' eyes. Before that, I first write about why the eyes of living things are often two, then think about the ideal placement of eyes.



-Contents-
・Reason we have two eyes
・Eco-location
・Carnivore eyes and herbivorous animals eyes
・Reason we have two ears
・Why does spider have eight eyes?



Reason we have two eyes


Why is often the living thing's eyes two? There are two reasons. The first reason is to stereoscopically view the object. Second reason, it is important, to grasp the distance from the object. At least two eyes are necessary to accurately grasp the position of the object. But even though one eye is closed, we can pick up the cup at hand. 

This is only by an empirical judgment from the continuous part of the object such as a desk. We should think from the standpoint of wild animals. In order to measure the distance from the prey under the environment without other objects, such as the sea and grassy plain, you still need at least two eyes. Animals judge the distance from objects by viewing angle as shown below. 

The bigger the viewing angle, the closer the object is recognized. There are some primitive creatures which have only one eye, for example daphnia. Daphnia's eye feels the light and darkness of the light only and is not able to identify distance from the prey or predator.


Reason we have two eyes and two ears 1

Eco-location


Some animals grasp the position of objects using sound waves without using light. This method is called eco-location. For example, dolphin, bat. This time, I will and talk about animals that understand perspective by light, not consider animals which using eco-location.

(Zaku that appears in Japanese animation kidousenshi Gundam have only one eye called mono-eye. I think that Zaku shoots image data for pilot with a mono-eye, and at the same time it works sonar to recognize the accurate position of the enemy aircraft.)


Carnivore eyes and herbivorous animals eyes


The carnivore has eyes on front of the face, and herbivorous animals have their eyes on the side of the face. 

The former is an arrangement of eyes suitable for measuring distance from the prey, and the latter is an arrangement of eyes suitable for finding predators extensively. In short, in order to measure the distance of objects by the viewing angle, it is important that animals face does not block the line connecting the two eyes.

When I was in elementary school, I learned that herbivorous animal's eyes are away each other, and carnivore’s eyes are close in each other. This is not an accurate explanation. 

The eyes of carnivores should be on the front of the face. There is no need for the two eyes to be close. On the contrary, the more distance the eyes are each other, the more advantageous the distance measurement of the prey becomes. The more the eyes are away from each other as shown in the figure below, the more the change in the viewing angle becomes clear, which makes it possible to grasp the distance more accurately. 

A typical example of this is a hammerhead shark. Although the positions of the two eyes are far each other, it can be said to be an ideal eye placement for capturing the prey. Because there are no other objects in the ocean, position capture in ocean becomes more important than on ground. 

If predators can measure the distance to the prey precisely, they will not use extra physical strength, so it will work advantageously in survival competition. (There are various opinions on the reason of the structure of the face of the hammerhead shark)

Reason we have two eyes and two ears 2

Reason we have two ears


Next, I will write about why we have two ears. It is to grasp the direction of the sound source. Sound travels slower than light. Therefore, slight misalignment will occur at the moment the sound wave arrives in the left and right ears. 

If the sound source is on your right side, the sound will be transmitted to the right ear early and enter the left ear with a slight delay. Although we are not conscious of the misalignment of sound usually, in the brain, the direction of the sound source is grasped from such information. Even when making a humanoid robot, there are occasions when designing multiple ears sensors, which are called microphone arrays.



Why does spider have eight eyes?


This time I wrote about animals which have the two eyes. However, animals do not necessarily have two eyes. There are creatures which only have one eye like Daphnia, and there are animals which have eight eyes like spiders. 

We can’t explain exactly why a spider has eight eyes. Originally, we only need two eyes. But spiders can’t accurately measure the distance of prey with eight eyes. Too many eyes disturb the focusing, so spiders can’t clearly see the prey shape. Evolution occurs randomly.

But having the eight eyes should have some merit in survival competition. There is still needs for research on the eyes of living things. Also, this time I did not consider the compound eye of insects. I would like to describe any opportunity.