Why do platypuses have around 40,000 electroreceptors while short-beaked echidnas have only around 400?

Answer: Dry habitats give weak electric fields less use, so echidnas kept only a reduced sense.

A longer snout creates more room, so receptor count follows snout length. — Snout length sounds anatomical, and shape does affect how receptors are packed. But long-billed echidnas have more electroreceptors than short-billed echidnas yet still far fewer than the aquatic platypus. The bigger contrast tracks habitat: water lets weak electric fields travel and help locate prey, while dry ground usually does not.

Dry habitats give weak electric fields less use, so echidnas kept only a reduced sense. ✓ — Correct. Platypuses hunt underwater, where weak electric fields can travel and reveal prey. Short-beaked echidnas are much more terrestrial and often live in dry habitats, so electroreception gives fewer chances to find food. Their few hundred receptors are best seen as a remnant system, and evolution often trims senses when the environment stops paying for them.

Larger prey are easier to detect, so echidnas need fewer sensors to find them. — Bigger prey can be easier to detect, so it is tempting to think fewer sensors would do. But a scarce or hidden target could reward more sensory resolution, not less. 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.