How Do Platypuses Find Food? Electroreception Underwater

A bill packed with sensors

The platypus bill looks like a duck’s beak but works very differently. It is covered in soft, leathery skin that contains about 40,000 electroreceptors, arranged in rows running from front to back, along with tens of thousands of touch-sensitive receptors called push rods.

Every living animal produces small electric signals when its muscles contract, for example when a shrimp flicks its tail or an insect larva moves its gills. Water carries these signals a short distance, and the electroreceptors can pick them up. The touch receptors, meanwhile, sense pressure waves and contact as prey moves or the bill brushes against it.

In the brain, signals from both kinds of sensor arrive in the same area, and some brain cells respond to both. Scientists think the platypus may combine them to judge where prey is and how far away it lies, because the electric signal arrives a fraction of a second before the pressure wave. The bill takes up a huge part of the platypus’s sensory map in the brain, much as the hands and lips do in humans.

How a platypus hunts underwater

A feeding platypus follows a regular rhythm. It dives, swims to the bottom, and works its way along with quick side-to-side sweeps of the head. This scanning movement helps it compare the signal on each side of the bill and turn toward the strongest source.

When it finds something, it digs it out of mud, gravel or leaf litter with its bill and stores it in two cheek pouches. Most dives last around 30–40 seconds. The platypus then rises to the surface, floats for a short time and grinds its catch between horny pads, often together with fine gravel that helps break up hard shells. Then it dives again.

Experiments in the 1980s showed how important electricity is to this process. Captive platypuses swam toward and attacked small batteries placed in their tanks, reacting to the weak electric field around each battery. Wild platypuses also hunt successfully in muddy water and at night, when eyesight would be almost useless.

What platypuses eat

The platypus is a carnivore that eats small animals living on the bottom of rivers, creeks and lakes. Its menu changes with the place and season, but typically includes:

• Insect larvae, the most important food in many streams, especially caddisfly, mayfly, dragonfly and midge larvae.
• Freshwater crustaceans such as shrimps and yabbies (small crayfish).
• Worms, snails and small freshwater mussels.
• Occasional extras, such as tadpoles, fish eggs and small fish.

Because adults have no teeth, they focus on small and fairly soft-bodied prey. The platypus also lacks a true stomach that produces acid and protein-digesting enzymes, so food passes almost straight from the gullet into the intestine, where digestion takes place.

How much and how often platypuses feed

For a small mammal living in cold water, keeping warm takes a lot of energy. Platypuses therefore feed for long periods, often 10 hours or more at a stretch. They are most active at night and around dawn and dusk, although in winter they are often seen feeding during the day.

Food needs rise sharply for females raising young. A nursing mother must catch enough prey for herself and produce rich milk, so she spends longer in the water and returns to the burrow to feed her young between foraging trips. Healthy streams with plenty of insect larvae are therefore essential, and droughts or pollution that reduce invertebrate numbers can quickly hit local platypus populations. Learn more about the rivers they need in Where do platypuses live?.

Electroreception in other animals

Electroreception is common in fish, especially sharks and rays, and in some amphibians, but it is extremely rare in mammals. The monotremes are the main mammals known to have it. The echidnas, the platypus’s closest relatives, also have electroreceptors in the tips of their snouts, but far fewer, because they hunt ants, termites and worms in damp soil rather than in water. You can compare their diet in What do echidnas eat?.

Apart from monotremes, the only mammal confirmed to sense electric fields is the Guiana dolphin of South American coasts, which uses pits on its snout.

Myths about how platypuses feed

Myth: platypuses use venom to catch prey. Only adult males have venom, delivered through spurs on their hind ankles, and it is used against rival males, not prey. See Are platypuses venomous? for details.

Myth: platypuses find food by smell. The nostrils are closed during dives, so smell plays no part underwater. Electroreception and touch do almost all the work.

Myth: platypuses eat fish all the time. Fish make up only a small, occasional part of the diet. The bulk is invertebrates, with insect larvae often making up most of what a platypus eats.