Why does a hunting platypus sweep its bill side to side instead of just pointing it forward?

Answer: It helps the platypus figure out the direction and distance of prey.

It helps the platypus figure out the direction and distance of prey. ✓ — Correct. The bill is not just asking whether something electric is nearby; it is sampling changing signals across space. Experiments and models describe directionality in the platypus electrosensory system, and neural maps combine electric and mechanical input. A side-to-side scan can therefore help turn a faint twitch in mud into a rough fix on where prey sits.

It lets the platypus map the water temperature around it. — Temperature can matter to aquatic animals, but it does not explain the distinctive bill-sweeping hunt. The key timing clue is not warm versus cold water; it is how pressure waves and electrical pulses arrive across the bill. A fish might use temperature gradients for habitat choice, while the platypus is solving a centimeter-scale target-location problem.

It works like sonar, picking up long-range echoes from prey. — Long-range echoes belong more to dolphins and bats than to platypus bill hunting. The platypus has no known sonar-like system for mapping prey at a distance. Its trick is shorter range and tactile-electric: the bill samples local pressure and electric signals while the head swings over the river bottom.