Sensory surprises in platypus, mantis.

Sensory surprises in platypus, mantis

In the realm of the senses, animals continue to amaze scientists. No longer impressed by a dog's ability to hear high-pitched whistles or a cat's ability to see in dim light, researchers have gone on to document far more unexpected animal perceptions in such animals as the platypus and praying mantis.

Take the bill of the duck-billed platypus. It serves as an antenna to pick up weak electrical signals, scientists report in the Jan. 30 NATURE. This is the first report of electroreception in mammals, say Henning Scheich of the Technical University of Darmstadt, West Germany, Anna Guppy of the Australian National University in Canberra City and their colleagues.

In the recent experiments, hungry platypuses were observed to explore underwater with their eyes, ears and nostrils closed. By swinging their bills rapidly back and forth, they detected direct and alternating electrical currents, which they used both to locate and avoid objects. For example, the platypuses would swim down to and try to bite active batteries, but not inactive ones. The scientists propose that the platypus, which feeds on live crustaceans, frogs and small fishes, naturally uses its bill to detect the electric field generated by the muscles of moving prey.

Some fish and amphibians are known to sense electrical currents, but the platypus appears to use a distinctly different, independently evolved type of electroreceptor. These receptors may be located in the ducts of mucous skin glands, which would prevent them from drying out when the platypus is out of the water, the scientists suggest.

The praying mantis provides another sensory surprise--a single "ear" that is a groove in the underside of its thorax. Long thought to be deaf, the insect possesses a "sensitive and specialized acoustic sense," David D. Yager and Ronald R. Hoy of Cornell University report in the Feb. 14 SCIENCE.

What the mantis hears is ultrasonic frequencies, perhaps wings rubbing abdomen during courtship or the sonar signals of insect-eating bats. The sensitivity to ultrasound is shared by some other insects that detect sound with more conventional organs. But all other insects that hear have two "ears," widely separated on the body. Their auditory organs are on the forelegs, on the back of the thorax or on the abdomen.

The first evidence for hearing in the mantis was nerve signals recorded after stimulation with sound. To locate the "ear" responsible, the scientists used a process of elimination. They removed a mantis's legs and coated various parts of its body with a heavy layer of petroleum jelly or melted wax. Nerve recordings show that the mantis does respond to sound when it has no legs, and also when most of its thorax is covered. Only a drop of wax in the deep thoracic groove renders the nerve insensitive to sound.

The "ear" of the mantis consists of a thinned region of cuticle folded into a groove shaped like an elongated teardrop. Under each wall of the groove is a large air sac, connected to the insect's respiratory system. A small neural structure, thought to carry auditory information from the vibrating cuticle to the central nervous system, is located near the top of the sac.

Because animals obtain information about the location of a sound by comparing the input of their two ears, the scientists reasoned that the "cyclopean ear" of the mantis would not be useful in localizing sound. Their data occur: The location of a sound appeared to make no difference in the activity recorded from the nerve.

Although Yager and Hoy do not yet know what role detection of ultrasonic signals plays in the natural lives of mantises, some preliminary experiments do provide a hint. The scientists are interested in learning "who hears" among the 1,700 mantis species of the world, Yager says. When the researchers provided "batlike" ultrasound pulses to an Asian hymenopodid mantis in flight, it responded by extending its forelegs and flexing its abdomen. This response caused an abrupt and dramatic deviation in the flight path. Yager and Hoy conclude, "The mantis thus has independently evolved not only a novel ear but possibly a complex nocturnal predator avoidance system."