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How the Platypus Sees With Its Eyes Shut

This month a most unusual animal had its genome sequenced by molecular biologists -- the platypus. Some of its genes match those of humans, like a cluster of casein genes involved in milk production. This was not unexpected, as both of us are mammals and possess mammary glands. Other genes were very different from ours, more like those found in birds and reptiles. Again, this was not unexpected -- after all, the platypus is a very primitive mammal, not far removed from reptiles and birds on the evolutionary ladder.

Still other genes were unlike any science has ever seen before, genes intimitely involved in making a platypus a platypus. As a way of exploring what is so unusual about the platypus, I want to examine one of these platypusian features, an ability derived from genes not found in any other creature

-- the platypus hunts with its eyes shut!

To understand this, we should first go back to 1799, when the skin of a most unusual animal was sent to England by Captain John Hunter, governor of the British penal colony in New South Wales (Australia). Covered in soft fur, it was less than two feet long. As it had mammary glands with which to make milk for its young, it was clearly a mammal, but in other ways it seemed very reptilian. Males have internal testes, and females have a shared urinary and reproductive tract opening called a cloaca, lay eggs as reptiles do, and like reptilian eggs, the yolk of the fertilized egg does not divide. It thus seemed a confusing mixture of mammalian and reptilian traits.

Adding to this impression was its appearance: It has a tail not unlike that of a beaver, a bill not unlike that of a duck, and webbed feet! It was as if a child had mixed together body parts at random—a most unusual animal.

Individuals like the one pictured here are abundant in freshwater streams of eastern Australia today. What does one call such a beast? In its original 1799 description it was named Platypus anatinus (flatfooted ducklike animal), which was later changed to Ornithorhynchus anatinus (ducklike animal with a bird’s snout). To everyone but scientists, it immediately became known as the duckbill platypus.

Until recently, few scientists had studied the platypus in its natural habitat—it is elusive, spending its days in borrows it constructs on the banks of waterways. A platypus is active mostly at night, diving in streams and lagoons to capture bottom-dwelling invertebrates such as shrimps and insect larvae. Unlike whales and other marine mammals, a platypus cannot stay under water long. Its dives typically last a minute and a half (Try holding your breath that long!).

When scientists began to study the platypus’ diving behavior, they soon observed a curious fact: the eyes and ears of a platypus are located within a muscular groove, and when a platypus dives the sides of these grooves close over tightly. Imagine pulling your eyebrows down to your cheeks—effectively blindfolded, you wouldn’t be able to see a thing! To complete its isolation, the nostrils at the end of the snout also close. So how in the world does the animal find its prey?

For over a century biologists have known that the soft surface of the platypus bill is pierced by hundreds of tiny openings. In recent years Australian neuroscientists (scientists that study the brain and nervous system) have learned that these pores contain sensitive nerve endings. Nestled in an interior cavity, they are protected from damage by the bill, but linked to the outside streamwater via the pore. These nerve endings act as sensory receptors, communicating to the brain information about the animal’s surroundings. These pores in the platypus bill are its diving “eyes.”

They have two types of sensory cells. Clustered in the front are so-called mechanoreceptors, which act like tiny pushrods. Anything pushing against them triggers a signal. Your ears work the same way, sound waves pushing against tiny mechanoreceptors within your ears. These pushrods evoke a response over a much larger area of the platypus brain than stimulation from the eyes and ears—for the diving platypus the bill is the primary sense organ. What responses do the pushrod receptors evoke? Touching the bill with a fine glass probe reveals the answer—a lightning-fast snapping movement of its jaws. When the platypus contacts its prey, the pushrod receptors ensure its capture, seizing it with a rapid snap of the jaws.

But how does the platypus locate its prey at a distance, in murky water with its eyes shut? That is where the other sort of sensory receptor comes in. When a platypus feeds, it swims along steadily wagging its bill from side to side, two or three sweeps per second, until it detects and homes in on prey. How does the platypus detect the prey individual and orient itself to it? The platypus does not emit sounds like a bat, which rules out the possibility of sonar as an explanation. Instead, electroreceptors in its bill sense the tiny electrical currents generated by the muscle movements of its prey as the shrimp or insect larva moves to evade to approaching platypus!

It is easy to demonstrate this, once you know what is going on. Just drop a small 1.5 volt battery into the stream. A platypus will immediately orient to it and attack it, from as far away as 30 centimeters. Some sharks and fishes have the same sort of sensory system. In muddy murky waters, sensing the muscle movements of a prey individual is far superior to trying to see its body or hear it move—which is why the platypus that you see in the photo above is hunting with its eyes shut.

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