Not only are the claws bulky and heavy structures to build, they are energetically costly structures to maintain. Crab claws are not mere hollow threats; they are packed with powerful muscles that can crush the skeletal shells of rival males. Muscle tissue is incredibly energy demanding, because muscle cells are loaded with dense concentrations of mitochondria, the microscopic organelles responsible for converting stored nutrients and oxygen into usable energy. Mitochondria are often called the “cellular power plants,” and inside muscle cells they provide energy needed to contract the muscle and close the claw. … Because of their many mitochondria, muscle cells are expensive to maintain even when they are resting, and males with big claws have the most muscle. Male fiddlers burn energy like crazy to keep their muscle cells alive. Resting metabolic rates of males with big claws are almost 20 percent higher than those of females (who lack enlarged claws) simply because of the costly muscles inside the claw.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 302