Why might high energy costs of armor be selected against in small or young sticklebacks?

• A. Because energy saved from reduced armor can be allocated to growth and reproduction, increasing fitness when predation risk is moderate or low.
• B. Because armor enhances camouflage.
• C. Because armor directly improves feeding efficiency.
• D. Because smaller fish have more energy to invest in armor.

Answer: (A)

The main idea here is how energy is allocated between defense and growth/reproduction. Armor in sticklebacks costs energy to build and maintain. For small or young fish, energy is especially precious because their access to food and growth opportunities are limited. If predation risk is moderate or low, the benefit of having heavy armor is not as strong, so the energy saved by not investing heavily in armor can be redirected toward growth and earlier reproduction. Growing faster and maturing sooner often boosts lifetime fitness more than carrying heavy armor for a long time, so natural selection can favor lighter armor in juveniles under these conditions.

Armor isn’t primarily about camouflage, nor does it directly boost feeding efficiency, and smaller fish don’t inherently have more energy to invest in armor—their energy budget is typically tighter, so the costs of armor would more likely hinder growth rather than help it.