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Sexual dimorphism and allometry between males has developed evolutionarily in some species to see that the males are actually bigger then the females, but also there is competition between the males size so they can fight each other to get a female.

Some dimorphism is reversed, where the female is much larger than the male, but competition can still take place between the males to secure a mating.

You then enter a world of resource holding power, game theory, evolutionary stable strategies, etc etc. At the centre is very often first mate sperm priority (sperm competition at work).

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