Why are most animals externally symmetrical

Why are almost all animals built axially symmetrically?

Symmetry especially externally

The symmetry in the physique of animals is primarily an external one. The internal organs - including those of humans - are not arranged symmetrically. The heart is more to the left, the liver more to the right, and the intestine meanders through the abdomen, devoid of any geometric order. The exterior, on the other hand - the “packaging” - is not perfect, but in principle is built symmetrically.

From an evolutionary point of view, there are three reasons for this.

  1. Stability of locomotion: An asymmetrical body could not move as elegantly and would be easily unbalanced. It is therefore an advantage if the legs on the right and left are equally long and the weight is evenly distributed in the right and left halves of the body. Birds with wings of different sizes could not fly well either. Smooth motor skills are therefore a good reason for a symmetrical physique.
  2. Sensor technology: We have two symmetrically arranged specimens of the most important sense organs: two eyes, two ears - also two nostrils. This paired arrangement provides us with important information: With the help of the two eyes, we see three-dimensionally: Both eyes receive optical stimuli - and the brain calculates the depth information from the comparison of the two pieces of information. This works best when both eyes are a mirror image of the body axis and are of the same size. The same thing with the ears - we have two ears so that we can better locate sounds. This also works best when the information is processed in the same way in both ears.
  3. Partner choice: In most animals, the females also choose partners based on their physique; Males with a strongly symmetrical build are preferred. Symmetry is an indicator of health or “fitness”. This can be understood from an evolutionary psychological perspective. If a body functions motor-wise, the better the more symmetrical it is, and if the more symmetrically arranged the sensory organs, the better the perceptual performance - then symmetrically built animals have better chances of survival. This means that not only does the characteristic symmetry prevail in evolution, but also the preference of females for symmetrically built males. Because a symmetrical physique signals to the females: This male obviously has good genes, with which I can produce healthy offspring with a high probability. - Over a long period of time, evolution has led to the fact that symmetrically built males were selected more often as sexual partners, thereby reproducing more and therefore the characteristic symmetry has also established itself.

But there are exceptions: in many crabs and crabs, for example, the claws on the left and right are different sizes. The shells of snails are also asymmetrical - they spiral upwards.

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