Amazing Senses in the Animal World

SCAMPERING about looking for food, the mouse feels safe in the darkness. But it does not anticipate the pit viper’s ability to “see” the heat radiating from the mouse’s warm body​—a fatal misjudgment. A flounder lies completely concealed under a layer of sand in a shark pool, where a hungry shark is cruising in its general direction. The shark cannot see the flounder; yet, in the blink of an eye, the shark stops, plunges its nose into the sand, and devours its quarry.

Yes, the pit viper and the shark are examples of animals with specialized senses that humans do not have. On the other hand, many creatures have senses that are like our own but are more acute or able to capture a different range of perception. Eyes are a good example of this.

Eyes That See a Different World

The range of colors our eyes capture is but a minute fraction of the electromagnetic spectrum. For instance, our eyes cannot see infrared radiation, which has a longer wavelength than red light. However, pit vipers have two small organs, or pits, between their eyes and nostrils that detect infrared radiation. * Hence, even in the dark they can accurately strike at warm-blooded prey.

Beyond the violet end of the visible spectrum is ultraviolet (UV) light. Although unseen to our eyes, UV light is visible to many creatures, including birds and insects. Bees, for instance, orient themselves in relation to the sun​—even on a partly cloudy day when it is hidden—​by locating some blue sky and seeing the pattern formed by polarized UV light. Many flowering plants present patterns visible only in the UV range, and some flowers even have a “nectar marker”​—a section with a contrasting UV reflectance—​to point insects to the nectar. Certain fruits and seeds advertise themselves to birds in a similar way.

Because birds see in the UV range and because this light gives their plumage extra radiance, birds probably look more colorful to one another than to us. They have a visual “depth of richness that we can’t begin to imagine,” said one ornithologist. The ability to see UV light may even help certain hawks and kestrels to locate voles, or field mice. How so? Male voles, says the journal BioScience, “produce urine and feces containing chemicals that absorb UV, and mark their trails with urine.” Thus, birds can “identify areas of high vole density” and focus their efforts there.

 Why Do Birds See So Well?

Bird vision is a marvel. “The chief reason,” says the book All the Birds of the Bible, “is that the image-forming tissue lining the eye’s interior is richer in visual cells than the eye of other creatures. The number of visual cells determines the ability of the eye to see small objects at a distance. While the retina of a man’s eye contains some 200,000 visual cells per square millimeter, most birds have three times that number, and hawks, vultures, and eagles have a million or more per square millimeter.” Additionally, some birds have the extra asset of two foveae​—areas of maximum optical resolution—​per eye, giving them a superior perception of distance and speed. Birds that catch flying insects are similarly endowed.

Birds also have an unusually soft lens that enables rapid focus. Imagine how dangerous life on the wing would be​—especially in forests and thickets—​if everything were a blur. Yes, what wisdom is manifest in the design of the avian eye! *

The Electric Sense

The scenario mentioned earlier involving the hidden flounder and the shark actually occurred during a scientific study of sharks. The researchers wanted to know if sharks and rays sensed the minute electric fields that emanate from living fish. * To find out, they hid electrodes in the sandy floor of the shark pool and applied the appropriate voltage. The result? As soon as the shark neared the electrodes, it viciously attacked them.

Sharks possess what is called passive electroreception; they sense electric fields just as the ear passively hears sound. But electric fish have active electroreception. Like a bat that emits an acoustic signal and reads the echo, these fish emit electric waves or pulses, depending on the species, and then, with special receptors, detect any disturbances made to these fields. * Thus electric fish can identify obstacles, potential prey, or even a mate.

A Built-in Compass

Think what life would be like if your body were equipped with a built-in compass. Getting lost would surely not be a problem! Within the body of a number of creatures, including honeybees and trout, scientists have found microscopic crystals of magnetite, or lodestone, a natural magnetic substance. The cells containing these crystals are connected to the nervous system. Hence, bees and trout have demonstrated the ability to detect magnetic fields. In fact, bees use the earth’s magnetic field for comb building and navigation.

Investigators have also discovered magnetite in a species of bacteria that live in seafloor sediment. When the sediment is stirred up, the earth’s magnetic field acts on the magnetite to align the bacteria in such a way that they propel themselves safely back into their seafloor home. Otherwise, they would die.

Many migratory animals​—including birds, turtles, salmon, and whales—​may also have a magnetic sense. However, they do not seem to rely on this sense alone but, rather, appear to navigate by a variety of senses. Salmon, for instance, probably use their strong sense of smell to find the stream of their birth. European starlings navigate by the sun; and some other birds, the stars. But as professor of psychology Howard C. Hughes observed in his book Sensory Exotica​—A World Beyond Human Experience, “we are obviously a long way from understanding these and other mysteries of nature.”

Ears to Envy

Compared with humans, many creatures possess amazing hearing. Whereas we can hear sounds ranging from 20 to 20,000 hertz  (cycles per second), dogs can hear in the range of 40 to 46,000 hertz, and horses, between 31 and 40,000 hertz. Elephants and cattle can even hear in the infrasonic range (just below human hearing) to as low as 16 hertz. Because low frequencies travel farther, elephants may be able to communicate over distances of two or more miles [4 km]. In fact, some researchers say that we could employ such animals to give us an early warning of earthquakes and severe weather disturbances​—both of which emit infrasonic sound.

Insects also have a wide range of hearing, some in the ultrasonic range over two octaves above the human ear and others in the infrasonic range. A few insects hear by means of thin, flat, eardrumlike membranes, which are found on almost every part of the body except the head. Others hear with the aid of delicate hairs that respond not just to sound but also to the most gentle movements in the air, such as those caused by a human hand. This sensitivity explains why flies are so hard to swat!

Imagine being able to hear an insect’s footsteps! Such amazing hearing belongs to the world’s only flying mammal​—the bat. Of course, bats require specialized hearing to navigate in the dark and to catch insects by means of echolocation, or sonar. * Says Professor Hughes: “Imagine a sonar system more sophisticated than that found in our most advanced submarines. Now imagine that system is used by a small bat that easily fits in the palm of your hand. All the computations that permit the bat to identify the distance, the speed, and even the particular species of insect target are performed by a brain that is smaller than your thumbnail!”

Because precise echolocation also depends on the quality of the sound signal emitted, bats have the “ability to control the pitch of their voice in ways that would be the envy of any opera singer,” says one reference. * Apparently by means of the flaps of skin on the noses of some species, bats can also focus sound into a beam. All these assets contribute to a sonar so sophisticated that it can produce an “acoustic image” of objects as fine as a human hair!

Besides bats, at least two kinds of birds​—swiftlets of Asia and Australia and oilbirds of tropical America—​also employ echolocation. However, it seems that they use this ability  simply to navigate in the dark caves where they roost.

Sonar at Sea

Toothed whales also employ sonar, although scientists have yet to discover exactly how this works. Dolphin sonar begins with distinct clicks, which are believed to originate, not in the larynx, but in the nasal system. The melon​—the bulb of fatty tissue on a dolphin’s forehead—​focuses the sound into a beam that “illuminates” a zone in front of the animal. How do dolphins hear their echoes? Not with their ears, it seems, but with their lower jaw and associated organs, which connect to the middle ear. Significantly, this region contains the same kind of fat as that found in the dolphin’s melon.

Dolphin sonar clicks are strikingly similar to a mathematical waveform called a Gabor function. This function, says Hughes, proves that dolphin clicks “approach a mathematically idealized sonar signal.”

Dolphins can adjust the power of their sonar clicks from a mere whisper to a  cracking 220 decibels. How powerful is that? Well, loud rock music can produce 120 decibels, and artillery fire 130 decibels. Armed with sonar that is much more powerful, dolphins can detect things as small as a three-inch [8 cm] ball 400 feet [120 m] away and possibly even farther in quiet waters.

When you reflect on the amazing senses manifest in the living world, does it not fill you with awe and wonder? Humble, informed people usually feel that way​—which brings us back to the question of how we are made. True, our senses often pale beside those of certain animals and insects. Nevertheless, we alone are moved by what we observe in nature. Why do we have such feelings? And why do we seek not just to understand living things but to comprehend their purpose and to learn our own place among them?

[Footnotes]

[Box/Pictures on page 9]

Insects Beware!

“Each day, just around dusk, a truly astonishing event takes place under the rolling hills near San Antonio, Texas [U.S.A.],” says the book Sensory Exotica​—A World Beyond Human Experience. “At a distance, you might think you saw an enormous black cloud billowing from the depths of the earth. However, it’s not a cloud of smoke that darkens the early evening sky, but the mass exodus of 20 million Mexican free-tailed bats from the depths of Bracken Cave.”

A more recent estimate places the number of bats exiting Bracken Cave at 60 million. Climbing up to 10,000 feet [3,000 m] into the night sky, they pursue their favorite meal, insects. Although the night sky must contain an overabundance of ultrasonic bat calls, there is no confusion, for each of these unique mammals is equipped with a highly sophisticated system for detecting its own echoes.

[Picture]

Bracken Cave

[Credit Line]

Courtesy Lise Hogan

[Picture]

Mexican free-tailed bat​—sonar

[Credit Line]

© Merlin D. Tuttle, Bat Conservation International, Inc.

 [Picture on page 7]

Honeybees​—sight and magnetic sense

[Picture on page 7]

Golden eagle​—sight

[Picture on page 7]

Ray​—electric sense

[Picture on page 7]

Shark​—electric sense

[Picture on page 7]

Starlings​—sight

[Picture on page 7]

Salmon​—smell

[Credit Line]

U.S. Fish & Wildlife Service, Washington, D.C.

[Picture on page 7]

Turtle​—possibly magnetic sense

[Picture on page 8]

Elephant​—low-frequency hearing

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Dog​—high-frequency hearing

[Picture on page 9]

Dolphins​—sonar