Platypuses have ~40,000 electroreceptors, but short-beaked echidnas have ~400. What best explains the drop?

Answer: Dry habitats reward it less

Snout length sets count — Snout length sounds anatomical, but it does not match the pattern well. A long-billed echidna has more electroreceptors than a short-billed echidna, yet still far fewer than the aquatic platypus. The larger contrast is not just shape; it is how often the animal's world lets weak electric fields help with food.

Dry habitats reward it less ✓ — Correct. Platypuses hunt underwater, while short-beaked echidnas are much more terrestrial and often live in dry habitats. JEB's review treats the short-beaked echidna's few hundred receptors as a remnant system, because dry life gives fewer chances to use electroreception for prey. Evolution often trims senses when the environment stops paying for them.

Bigger prey need fewer — Bigger prey would not automatically require fewer sensors; if anything, a scarce or hidden target could reward more sensory resolution. The receptor counts line up better with medium and habitat: water gives electric fields a useful path, while dry ground usually does not. That is why the aquatic platypus keeps the dense array.