Unit Six. Animal Life


29. The Senses


29.6. Other Vertebrate Senses


Vision is the primary sense used by all vertebrates that live in a light-filled environment, but visible light is by no means the only part of the electromagnetic spectrum that vertebrates use to sense their environment.



Electromagnetic radiation with wavelengths longer than those of visible light is too low in energy to be detected by photoreceptors. Radiation from this infrared (“below red”) portion of the spectrum is what we normally think of as radiant heat. Heat is an extremely poor environmental stimulus in water because water has a high thermal capacity and readily absorbs heat. Air, in contrast, has a low thermal capacity, so heat in air is a potentially useful stimulus. However, the only vertebrates known to have the ability to sense infrared radiation are the snakes known as pit vipers.

The pit vipers possess a pair of heat-detecting pit organs located on either side of the head between the eye and the nostril (figure 29.19). The pit organs permit a blindfolded rattlesnake to accurately strike at a warm, dead rat. Each pit organ is composed of two chambers separated by a membrane. The infrared radiation falls on the membrane and warms it. Thermal receptors on the membrane are stimulated. The nature of the pit organ’s thermal receptor is not known; it probably consists of temperature-sensitive neurons innervating the two chambers. The two pit organs appear to provide stereoscopic information, in much the same way that two eyes do. Indeed, in snakes the information transmitted from the pit organs is processed in the brain by the homologous structure to the visual center in other vertebrates.



Figure 29.19. "Seeing" heat.

The depression between the nostril and the eye of this rattlesnake opens into the pit organ. In the cutaway portion of the diagram, you can see that the organ is composed of two chambers separated by a membrane. Snakes known as pit vipers have these unique organs which allow them to sense infrared radiation (heat).



While air does not readily conduct an electrical current, water is a good conductor. All aquatic animals generate electrical currents from contractions of their muscles. A number of different groups of fishes can detect these electrical currents. The electrical fish even have the ability to produce electrical discharges from specialized electrical organs. Electrical fish use these weak discharges to locate their prey and mates and to construct a three-dimensional image of their environment even in murky water.

The elasmobranchs (sharks, rays, and skates) have electroreceptors called the ampullae of Lorenzini. The receptor cells are located in sacs that open through jelly-filled canals to pores on the body surface. The jelly is a very good conductor, so a negative charge in the opening of the canal can depolarize the receptor at the base, causing the release of neurotransmitter and increased activity of sensory neurons. This allows sharks, for example, to detect the electrical fields generated by the muscle contractions of their prey. Although the ampullae of Lorenzini were lost in the evolution of teleost fish (most of the bony fish), electroreception reappeared in some groups of teleost fish that use sensory structures analogous to the ampullae of Lorenzini. Electroreceptors evolved yet another time, independently, in the duck-billed platypus, an egg-laying mammal. The receptors in its bill can detect the electrical currents created by the contracting muscles of shrimp and fish, enabling the mammal to detect its prey at night and in muddy water.



Eels, sharks, bees, and many birds appear to navigate along the magnetic field lines of the earth. Even some bacteria use such forces to orient themselves. Birds kept in blind cages, with no visual cues to guide them, will peck and attempt to move in the direction in which they would normally migrate at the appropriate time of the year. They will not do so, however, if the cage is shielded from magnetic fields by steel. Indeed, if the magnetic field of a blind cage is deflected 120° clockwise by an artificial magnet, a bird that normally orients to the north will orient toward the east-southeast. There has been much speculation about the nature of the magnetic receptors in these vertebrates, but the mechanism is still very poorly understood.


Key Learning Outcome 29.6. Pit vipers can locate warm prey by infrared radiation (heat), and many aquatic vertebrates can locate prey and ascertain the contours of their environment by means of electroreceptors. Magnetic receptors may aid in bird migration.


A Closer Look

How the Platypus Sees With Its Eyes Shut

The duck-billed platypus (Ornithorhynchus anatinus) is abundant in freshwater streams of eastern Australia. These mammals have a unique mixture of traits— in 1799, British scientists were convinced that the platypus skin they received from Australia was a hoax. The platypus is covered in soft fur and has mammary glands, but in other ways it seems very reptilian. Females lay eggs as reptiles do, and like reptilian eggs, the yolk of the fertilized egg does not divide. In addition, the platypus has a tail not unlike that of a beaver, a bill not unlike that of a duck, and webbed feet!

It turns out that platypuses also have some very unique behaviors. Until recently, few scientists had studied the platypus in its natural habitat— it is elusive, spending its days in burrows it constructs on the banks of waterways. Also, a platypus is active mostly at night, diving in streams and lagoons to capture bottom-dwelling invertebrates such as shrimps and insect larvae. Interestingly, 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, the nerves are protected from damage by the bill but are linked to the outside streamwater via the pore. The 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.”

Platypuses have two types of sensory cells in these pores. 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 does 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 are stimulated, and the jaws rapidly snap and seize the prey.

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 the 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.



Inquiry & Analysis

Do Birds Use Magnetic Particles as Compass Needles?

Some migrating birds use infrasound to orient themselves. Others may use visual cues, like the angle of polarizing light or the direction of sunset. Many birds that migrate long distances use the earth's geomagnetic field as a source of compass information. If the magnetic field of a blind "orientation cage” (see photo below) is deflected by 120 degrees clockwise by an artificial magnet, a bird that normally orients to the north will orient toward the southeast.

The sensory system underlying the magnetic compass of these birds is one of the great mysteries of sensory biology. There are two competing hypotheses.

The Magnetite Hypothesis. One hypothesis is that crystals of the magnetic mineral magnetite within brain cells of migrating birds act as miniature compass needles. While trace amounts of magnetite are indeed present in some brain cells, intensive research has failed to confirm that information about the orientation of magnetite particles within these cells is transmitted to any other cells of the brain.

The Photoreceptor Hypothesis. An alternative hypothesis is that the primary process underlying the compass is instead a magnetically sensitive chemical reaction within the photoreceptors of the bird's eyes. The alignment of photopigment molecules with the earth's magnetic field might alter the visual pattern in a way that could be used to obtain directional information.

Which hypothesis is correct? Experiments have shown that the magnetic detector used by birds in blind cages is light sensitive, as a photoreceptor compass should be—but this would also be true of a light-activated magnetite compass.

In 2004, University of California Irvine researchers devised a clever experiment to distinguish between the two hypotheses. They studied the way in which migrating European robins held in orientation cages use the magnetic field as a source of compass information to hop in the appropriate migratory direction. They found that the robins oriented 16 degrees north during the spring migration, the appropriate direction. To distinguish between the magnetite and photoreceptor hypotheses, the robins in the cages were exposed to oscillating, low-level radio frequencies (7 MHz) that would disrupt the energy state of any light-absorbing photoreceptor molecules involved in sensing the magnetic field, but would not affect the alignment of magnetite particles.

The chart above presents the results of this study. Each data entry is the mean of three recordings. For each recording, the bird was placed in a 35-inch conical orientation cage lined with coated paper, and the vector (the directional position of first contact with the paper) recorded relative to magnetic north (north = 360 degrees; east = 90 degrees; south = 180 degrees; west = 270 degrees).



1. Applying Concepts. In the chart, is there a dependent variable? If so, what is it? Discuss.

2. Interpreting Data. Plot each column on a circle. For birds orienting to the geomagnetic field without radio interference, what is the greatest difference (expressed in degrees) between recorded vectors and the mean vector of 16 degrees? For birds orienting with 7 MHz radio interference?

3. Making Inferences

a. For birds orienting to the geomagnetic field without radio interference, how many of the 12 birds oriented with an accuracy of +/- 30 degrees relative to the mean vector of 16 degrees? For birds orienting with 7 MHz radio interference, how many?

b. If you were to select a bird at random, what is the probability that it would orient within +/- 30 degrees of the appropriate migration direction (16 degrees north) without radio interference? With radio interference?

4. Drawing Conclusions Is the ability of European robins to orient correctly with respect to geomagnetic fields disrupted by 7 MHz radio frequencies? Is it fair to conclude that the birds' compass sense involves a molecule sensitive to radio disruption, such as a photoreceptor? That it does not involve particles not sensitive to radio disruption?



Test Your Understanding

1. What is the correct order for the stages of sensory perception?

a. transduction, stimulation, transmission

b. transmission, transduction, stimulation

c. stimulation, transmission, transduction

d. stimulation, transduction, transmission

2. Which of the following is an example of an exteroceptor?

a. stretch receptors

b. rods and cones

c. baroreceptors

d. temperature receptors

3. When arm muscles hurt after heavy exercise, the pain is detected by

a. neurotransmitters.

b. interoceptors.

c. associative neurons.

d. exteroceptors.

4. Which of the following structures of the ear is associated with sensing motion and gravity?

a. cochlea

b. ear bones (the ossicles)

c. semicircular canals

d. eardrum

5. Which of the following could not provide an animal information about its food?

a. photoreceptors

b. taste receptors

c. pain receptors

d. smell receptors

6. The ear detects sound by the movement of

a. hair cells within the cupula.

b. a membrane within the cochlea.

c. otoliths within a gelatinous matrix.

d. the eustachian tube.

7. The lateral line system in fish detects

a. pressure waves.

b. light.

c. taste.

d. magnetism.

8. Which of the following statements is incorrect?

a. Vertebrates focus the eye by changing the shape of the lens.

b. The eyes of arthropods and vertebrates use the same lightcapturing molecule.

c. Rod cells detect different colors, and cone cells detect different shades of gray, allowing vision in dim light.

d. Light changes ds-retinal into trans-retinal.

9. Binocular vision

a. is more common in prey animals.

b. enlarges the overall receptive field.

c. allows for a better depth of perception.

d. is only possible when the fields of vision do not overlap.

10. Which of the following can some types of animals detect in their environment?

a. infrared heat

b. magnetic fields

c. electric fields

d. All of the above.