Why is the sclera, the white part of the eye, usually not visible in animals eyes?
In: Biology
The leading theory is that humans evolved this due to social reasons. Being able to tell where another individual is looking is useful when interacting with them.
The sclera being less visible in the eyes of animals isn’t due to the evolution of social interaction. The reflective index of the lenses of the eyes of animals gives you a clear indication of where the animal is looking. It’s the same reason the rest of us have evolved as well: Creature comforts.
Animals have a less visible sclera because their iris and pupil are larger proportionally than the eyes of humans. Why? Because of the food chain. Animals are divided into two categories: Predators and prey.
Predators, particularly apex predators have extremely large pupils to allow in more light for improved night vision, this improves the hunting ability of nocturnal predators. For an example, look at the eyes of an owl.
Conversely, prey also has large pupils to allow in more light for improved night vision, whereas prey use this improved night vision not to hunt, but to spot advancing predators and avoid being killed.
Humans used to have this trait. It has evolved out of our genetic structure because of our creature comforts. We used to look significantly different, long body hair for warmth, improved skeletal/muscular/tendon processes for increased strength, larger pupils for night vision, more muscle density for improved endurance, increased bone density for physical durability, etc.
Buildings, central heat and air, technology, etc have served only to increase our lethargy and reduce our physical effectiveness. Likewise, our pupil size decreased because we no longer need to hunt in the dark or dodge apex predators.
Hope this answers your question.
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