Male and female lions can be distinguished from one another easily. This is known as sexual dimorphism. A different example of this is the peacock and the hen--where the brilliant feathered bird is the male. In most animals in nature, males are more showy than females. In lions, the presence of a mane allows the male to stand out, appear larger and more intimidating. It is part of a display and its absence may mean an immature male lion or a female. Females prefer males with denser and darker manes to mate with although in the hotter months, the mane causes discomfort to the male. And, because females lack the mane, it allows them to be able to stalk and kill prey effectively. Males would overheat too quickly to do so.
Male lions are believed to have manes for a couple of reasons. First, it helps to make the lion look larger and more intimidating. This comes in handy in confrontations with their biggest competitor, the spotted hyena, as well as in territorial disputes with other male lions. The mane is seen as a cue to the animals fighting ability so if one lion's mane is larger than his opponent's, many times the smaller maned animal will back down. It is often thought that the mane also protects the lion's throat in fights with other lions but since they usually attack from the back and hips, this isn't really the case. Secondly, because of the physical toll that all the heat from the extra hair takes on the animal, the ones with the larger manes are seen by lionesses as healthier and therefore more fit for siring offspring.