Courtship · Sexual Selection
A peacock's tail is a paradox: the showiest males win the most mates, but those same tails are heavy, costly, and easy for predators to spot. You play the peahen. Pick the fathers of the next generation — and watch the tail evolve, pulled bigger by your choices and smaller by the predators.
Two forces pull on a peacock's tail at the same time. Natural selection favours small, cheap tails — they are easier to carry and harder for predators to spot, so plain males live longer. Sexual selection favours big, dazzling tails — because peahens prefer them, showy males win more matings even if they die younger.
When peahens strongly prefer big tails and predators are scarce, the two traits can spiral together in a Fisherian runaway: the preference and the ornament reinforce each other, driving tails to absurd sizes. Predation is the brake. Crank up the predators and you'll see the tail held small — but watch the male survival rate collapse, the hidden price of all that beauty.
There's a deeper idea too: the handicap principle. Precisely because a huge tail is costly, only a genuinely fit male can grow one and still survive — so the tail becomes an honest advertisement of quality. You are the peahen in this story; every mate you pick tips the balance.