In the two-arm experiment, why should both options be kept equally moist?

• A. To prevent slugs from preferring moisture over substrate
• B. Moisture has no effect on slug behavior
• C. To attract more predators
• D. To ensure moisture does not confound substrate preference

Answer: (D)

Keeping both options equally moist targets a key factor that could otherwise bias the result. Slugs rely on humidity to stay hydrated and to move comfortably, so a moister option can attract them for reasons unrelated to the substrate itself. If one arm is damp while the other isn’t, the slug may choose the damp area simply to satisfy its moisture needs, not because it prefers one substrate over another. By standardizing moisture across both arms, you ensure that any preference you observe is due to the substrate itself rather than moisture, preserving the test’s ability to compare substrates fairly. This is why moisture must not confound substrate preference.