Sexual dimorphism is the phenomenon of markedly different physical appearance between the genders of the same species. It does go a little deeper than that, but we'll speak to dimorphism at its most obvious. Dimorphism can be demonstrated in simple ways like size. A male tiger is typically 50% heavier than its female counterpart. Of course this is the least interesting manifestation of dimorphism and generally not a highly reliable indicator of gender. Other species have much more pronounced dimorphic traits across the genders. The mane on a lion immediately distinguishes it from a lioness. Peacocks have their gorgeous tail feathers whereas the peahen could not be more unprepossessing. Biologists have posited that species where sexual dimorphism is more pronounced are indicators of polygamous mating behaviors. Makes sense to me. I'm sure you're thinking, umm, okay, so what? Here's the payoff.
Readers, I give you the triplewart seadevil anglerfish. The female of the species is approximately about a foot long and between you and me is an unattractive looking sea denizen. The male of the species is approximately 1.25 inches long, i.e. one tenth the length of the female. The male seadevil upon maturation attaches itself to female and enters into a parasitic relationship where its sole role is as an on-demand sperm supplier. When it's not supplying sperm its living off the female. Upon attachment, the male's organs begin to deteriorate into uselessness. In addition from attachment onwards the male relies entirely on the female for blood flow and supply.
Fantastic. A model that some, no doubt, strive for socially has a precedent in nature.