Weapons shrink when predator pressure falls and armor gets costly.

Evidence

Today, sticklebacks in many lakes lack defensive weapons. Dolph Schluter and his students at the University of British Columbia found that lake habitats have far fewer predators than marine habitats, and this appears to relax the pattern of natural selection for larger plates and longer spines. With fewer predators, lake fish benefit less from large weapons than marine fish. Armor also costs more in lakes than it does in the ocean. Low freshwater concentrations of the ions necessary for bone growth mean that fish pay a higher price for mineralizing bony plates in lakes. Unarmed sticklebacks are larger as juveniles and begin breeding sooner than their armed counterparts. In freshwater, it appears, the costs of long spines and large plates are steeper than the benefits they provide. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 56

Categorization

• XREF: Directly parallels the military UCP trade-off at the chapter’s opening — cost of a defensive trait vs. its benefit. — connects to: Arms races inevitably stall as weapon costs neutralize reproductive benefits — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 432; Rising predator pressure quickly enlarged lake stickleback armor; Predation risk is a near-universal cost of elaborate weapons — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 307 (+6 more)

Related

• Rising predator pressure quickly enlarged lake stickleback armor (similarity 0.92)
• Natural selection stabilizes at local optima, as armor-plate shifts in sticklebacks show (similarity 0.88)
• Weapons evolve only when reproductive benefits exceed costs (similarity 0.86)
• Armor evolution tracks a benefit-cost trade-off identical to that shaping animal weaponry (similarity 0.86)
• Early human weapon points shrank rather than grew over time (similarity 0.86)
• Predation risk is a near-universal cost of elaborate weapons (similarity 0.85)
• Weapon size in animal species is under stabilizing selection, not open-ended directional selection (similarity 0.85)
• Natural selection usually favors modest weapon size because cost offsets benefit (similarity 0.85)
• Bigger weapons usually fail because their costs outweigh their benefits (similarity 0.85)
• Arms races inevitably stall as weapon costs neutralize reproductive benefits (similarity 0.85)
• Weapons grow late because development prioritizes mandatory survival expenses first (similarity 0.85)
• Predator weapons stay small because larger ones hinder hunting (similarity 0.84)
• Ambush predators evolve large weapons because they strike rather than chase prey (similarity 0.84)
• Weapon growth widens the gap between the best- and worst-armed males. (similarity 0.84)
• Big weapons only benefit males with the reserves to wield them effectively (similarity 0.84)

Broader Connections

• Armor evolution tracks a benefit-cost trade-off identical to that shaping animal weaponry #corroborates (0.80) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 62
• Weapons evolve only when reproductive benefits exceed costs #corroborates (0.70) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 199
• Human spear points shrank as prey species successively dwindled in size #exemplifies (0.65) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 103
• Weapon escalation ends when costs outweigh benefits and cheaters invade #expands (0.65) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 457
• Balancing selection normally curbs weapon size until some lineages escape into runaway arms races #expands (0.60) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 22