Why are male animals often more attractive than females?

Charles Darwin's theory of evolution introduced the idea of natural selection. According to the principle of "survival of the fittest," animals that are strong, healthy and well adapted to their environment are more likely to pass on their genes to the next generation. 
But Darwin also noticed something curious: natural selection alone could not explain why males in many species often display bold, colorful, and conspicuous traits. Features such as massive antlers, dazzling plumage and striking patterns don't appear to offer a survival advantage. So why and how did they evolve?
To solve this puzzle, Darwin proposed a second evolutionary mechanism: sexual selection. These distinctive traits, he argued, help males compete for mates – by outshining rivals and catching the attention of choosy females.