Why do males and females of some species look so different?
Question via Chimera Liana, from Habitica!
The phenomenon in which males and females of the same species look very different is called “sexual dimorphism”. At the most basic level, it develops because there are different demands on the two sexes. For example, in a given species males need to defend territory, while females rely on the males for defence and focus on bearing young. Males are often bigger than females because of their increased need to fight, where being larger is an advantage in helping them to win.
Sometimes display is a big part of this: among peafowl, the peacock is extremely showy because each male is competing with the others. Glossy feathers and an elaborate display help to convince the peahens that a given male is healthy and strong, i.e. going to be the best possible father for her brood. In peafowl, the males compete for female attention, which is why they need to be showy, while the females have their pick of the males, and don’t need to have the same displays.
In other species, females can be the larger sex, and again this would depend on their role. If it’s females that hold territory in a given species, then they’re likely to be bigger, on average, than the males.