The lion's mane is the most recognizable feature of the species. It starts growing when lions are about a year old. Mane colour varies and darkens with age; research shows its colour and size are influenced by environmental factors such as average ambient temperature. Mane length apparently signals fighting success in male-male relationships; darker-maned individuals may have longer reproductive lives and higher offspring survival, although they suffer in the hottest months of the year. The presence, absence, colour and size of the mane are associated with genetic precondition, sexual maturity, climate and testosterone production; the rule of thumb is that a darker, fuller mane indicates a healthier animal. The main purpose of the mane is thought to be the protection of the neck and throat in territorial fights with rivals.
Info source: en.wikipedia.org/wiki/Lion