Fiddler crabs assess rival claw size and only approach equally-matched opponents.

Evidence

Imagine the beachscape from the vantage of a wandering crab, eyes perched on stalks a mere inch above the sand. Everywhere he looks are claws jerking up and down, bursting above the horizon and back again. Over and over they rise and fall, surrounding him with an incessant barrage of motion. The wandering male doesn’t simply march up to the first burrow he encounters. Nor does he approach at random. As he weaves between waving males, he sizes them up—bigger claws reach higher than smaller claws—advancing only toward burrows guarded by males whose claws are equal in size or just a bit smaller than his own. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 375

Categorization

• XREF: connects to: Fiddler crabs have the largest relative weapons yet rarely fight; Fiddler crabs use their oversized claws far more as signals than as weapons — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 366

Related

• Fiddler crabs have the largest relative weapons yet rarely fight (similarity 0.93)
• Fiddler crabs use their oversized claws far more as signals than as weapons (similarity 0.91)
• Weapon size honestly signals fighting ability because growth tracks health and nutrition (similarity 0.85)
• Costly claw muscle raises male fiddler crab resting metabolism nearly twenty percent (similarity 0.84)
• Even resting male crabs pay high energy costs for big claw muscles (similarity 0.81)
• Predators evolve counter-tactics that exploit weapon-costly prey (similarity 0.80)
• Waving a fiddler crab claw raises blood lactate measurably. (similarity 0.79)
• Waving a fiddler crab claw incurs measurable energetic costs (similarity 0.79)