 Sexual dimorphism exists in some species . Individuals of one sex , usually males develop secondary sexual characteristics that increase their chances of reproductive success . In dolphinfish , males have larger and blunter heads than females . In several minnow species , males develop swollen heads and small bumps known as breeding tubercles during the breeding season . The male green <unk> parrotfish has a more well @-@ developed forehead with an " ossified ridge " which plays a role in ritualised headbutting . Dimorphism can also take the form of differences in coloration . Again , it is usually the males that are brightly coloured ; in <unk> , <unk> and wrasses the colours are permanent while in species like minnows , sticklebacks , darters and sunfishes , the colour changes with seasons . Such coloration can be very conspicuous to predators , showing that the drive to reproduce can be stronger than that to avoid predation . 
