Why do some animals have black sclera?

Question

Why the difference in color? Does it do anything special? Zebras and some horses, lizards have this.

Answer 1

Melanin is the dark pigment in the skin and is thought to provide protection for cells in areas that receive uv radiation. It protects the DNA in the cells' nucleus from mutating and forming cancer cells. Melanin is transported to the skin when the animal is  an embryo is a series of advanced signaling pathways, resulting in random coat patterns in animals. Horses with white face markings will often have light sclera or even blue eyes because the signal pathways were not recruiting the melanin to those areas.The black layer in the back of the horse eye is the tapetum lucidum and is a reflective layer that allows the horse to have superior night vision.