A male stickleback fish will attack other male sticklebacks that invade its nesting territory, it only attacks males with the red belly. why has natural selection favored this

flatwing males favored by natural selection. behavior: a male stickleback fish attacks other male sticklebacks that invade its nesting territory. proximate cause:red belly of intruding male acts as sign stimulus that releases aggression in male sticklebacks.