Sexual Selection.

 Sexual selection is a special case of natural selection that acts on an organism's ability to mate with a member of the opposite sex.

Males are typically the ones on which sexual selection acts most obviously on because they produce more gametes than females and are not taken out of the operational sex ratio every time they successfully mate. Females, however, are taken out of that ratio because they are developing eggs, carrying embryos, or caring for their young. Therefore, males need to be able to compete with other males because basically there are just too many options.

Think of the peacocks, the females are just kind of a lump of brown feathers but the males have these huge, colorful, flashy tails in order to catch the attention of the available females and successfully mate with them.