Weapon size honestly signals fighting ability because growth tracks health and nutrition

Evidence

When fiddler crabs size up rival males, they look at their claws. For one thing, they’re huge and, in many species, brightly colored, making them supereasy to see. They’re also the most variable part of a crab, ranging from tiny to extreme and including everything in between. Finally, like all big weapons, claw growth is sensitive to parasites, disease, and nutrition, so the male with the bigger claw really is the most likely to win. This makes claws worth paying attention to, and males use them to evaluate one another before battle. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 371

Categorization

• XREF: connects to: Arms-race weapon growth makes resource trade-offs drastically impair body functions — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 296; Weapons evolved as signals so males could predict fight outcomes before risking battle — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 370; Each weapon-size increase resets the competitive baseline, ratcheting populations upward — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 183

Related

• Fiddler crabs use their oversized claws far more as signals than as weapons (similarity 0.87)
• Weapons are better signals than body size because they vary from zero to enormous (similarity 0.86)
• Fiddler crabs assess rival claw size and only approach equally-matched opponents. (similarity 0.85)
• Fiddler crabs have the largest relative weapons yet rarely fight (similarity 0.85)
• Weapon size should track resource availability more than any mandatory body part (similarity 0.84)
• Infections shrink weapon growth because immunity consumes the same resource pool (similarity 0.84)
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection (similarity 0.83)
• Growing large weapons diverts resources and stunts other body parts (similarity 0.83)
• Costly claw muscle raises male fiddler crab resting metabolism nearly twenty percent (similarity 0.82)
• Weapons work primarily as deterrents that prevent fights rather than win them (similarity 0.82)
• Weapon growth is far more resource-sensitive than other body parts (similarity 0.82)
• Weapons evolved as signals so males could predict fight outcomes before risking battle (similarity 0.82)
• Arms-race weapon growth makes resource trade-offs drastically impair body functions (similarity 0.82)
• Each weapon-size increase resets the competitive baseline, ratcheting populations upward (similarity 0.81)
• Weapon growth widens the gap between the best- and worst-armed males. (similarity 0.81)