Why do male lions have manes?

Since lionesses roam in groups and allow only limited males to live with them, competition between males is fierce. Rival males often fight to death — with their huge teeth and claws — to gain control over the pride.

This behaviour led many biologists, including the great Charles Darwin, to believe that the purpose of thick manes is to make it difficult for attackers to reach the vulnerable throat area. But over the years this theory has been questioned, as the area covered by manes was rarely attacked. 

Another idea attached with the manes is that they make lions look larger and stronger than they really are. 

Although, it turns out that the shaggy manes are actually used for attracting females. Research published in 2002 revealed that male lions who had darker, and longer manes were mature, better fed, in good health, and overall better fighters. Since females rely on males for the protection of their cubs, it makes sense that they would prefer males with large manes.