Stag beetle .Cyclommatus metallifer . SE Asia : Males with larger mandibles outcompete males with smaller mandibles , and have greater access to females . This has driven the evolution of increasingly larger, more elaborate mandibles . Male beetles commonly develop extravagant weaponry but it has been shown that there is a price to be paid. Growth of these oversize horns or mandibles appears to trade-off with reduced growth of other structures such as eyes, antennae, wings and genitalia.