Creatures of the Deep: Shark Trackers
Of all the creatures of the deep, sharks inspire the greatest horror, but their reputation may be largely undeserved. In this story, Frontiers travels to Hawaii to join scientists learning more about two species, the tiger shark and the scalloped hammerhead. Host Alan Alda ventures out to sea with a research crew to attach an acoustic transmitter to a tiger shark for a tracking study, then finds out what makes the hammerhead shark such an efficient swimmer.
Activity 1: Measuring Sound Energy
Activity 2: Detecting Electric Flow
ACTIVITY 1: MEASURING SOUND ENERGY
As you see on Frontiers, a signal can be used to track an unseen target. By analyzing the signal strength and frequency, accurate estimations of distance and depth can be made. In this activity, you'll assemble an audio meter that will display relative signal strengths. These values can then be used to determine the relative distance to the signal transmitter.
Learn more about the science of acoustic tracking by observing and charting energy changes in sound volume and frequency.
- 2 connecting wires (with alligator clip terminals)
- digital multimeter
- 2 paper cups
- 2 paper clips
- kite string
- meter stick
- Optional: portable keyboard, electric razor or similar battery-driven device that produces a steady audible signal
- Use the connecting wires to join the multimeter leads to the exposed connecting surfaces of the microphone plug.
- Switch the multimeter to AC volts (if not auto-adjusting, the range should be set for millivolts).
- Speak into the microphone and observe the multimeter readout. Then, sing! Holding the same note, vary its volume and observe how the voltage changes.
- Use paper cups, 3 meters of string and paper clips to make a paper-cup "telephone."
- Sing or use any of the devices mentioned in the optional materials list to produce a steady tone of fixed volume and direct it into one cup. Place the microphone in the other cup. Record the mV (millivolts) generated by this tone.
- Reconstruct the string telephone using various lengths of string: one, six and nine meters. Record all observed voltages.
- What happens to the generated voltage when the volume increases?
- What happens to the generated voltage when the distance to the source increases?
- Why was the string needed for the paper-cup telephone?
- Beginning with vocalization, describe the sequence of energy changes and transfers that take place when you sing or direct a sound into the microphone.
- Voltage increases.
- Voltage decreases.
- The string helped maintain the strength of the audio signal. Without the string, the microphone could not detect the distant sounds.
- Sequence of energy changes: vocal cords vibrate; KE (kinetic energy) transfers to air molecules; KE transfers to cup; KE transfers along string; KE transfers to cup; KE transfers to microphone; microphone changes KE to electricity; electric energy transfers through the wire within the multimeter; the electricity produces a field that influences the alignment of the liquid crystals.
- Explain how the acoustic signals help researchers locate and track the tiger sharks.
- Compare other species, such as the nurse shark, the megamouth, the great white. How do they differ from each other and the two on Frontiers? Why are sharks such efficient swimmers?
ACTIVITY 2: DETECTING ELECTRIC FLOW
Like all sharks, the hammerhead has super senses that help it navigate and detect prey. A shark's electromagnetic sense is an interesting adaptation -- sensory pores detect weak electric fields equivalent to one AA battery. Sensing the bioelectric activity produced by muscle contractions in their prey helps sharks find their dinner. The sense is well developed in hammerheads, which are said to use their heads like metal detectors.
In this activity, you'll construct an instrument called an electroscope. You may have used an electroscope in physics labs, but this homemade version works just as well to detect electric charges.
- aluminum foil
- paper clip
- clear plastic cup
- wool fabric
Note: Drill a small hole into the center of the bottom of the cup before beginning the activity.
Develop a basic understanding of electric fields by making and using a simple electroscope.
- Cut two strips of aluminum foil 4 cm long and 0.5 cm wide for the "leaves" of the electroscope.
- Use the tip of a straightened paper clip to punch a hole in each of the leaves. Holding the leaf horizontally, make the hole along the horizontal midline about 0.5 cm from the left-hand edge.
- Partially straighten a paper clip into the shape of a "J." Hang the two aluminum strips on the curve of the clip.
- Working from the inside of the cup, push the straight end of the clip through the hole in the cup, so that it hangs with the leaves inside the cup. Use a small piece of clay to secure the clip at the hole opening.
- Crumple a small piece of aluminum foil into a ball. Add this ball to the top of the clip that sticks up through the cup.
- Rub a scrap of wool fabric across an inflated balloon. Now pass the balloon near the crumpled aluminum. Observe the behavior of the aluminum strips.
- Why did you rub the balloon?
- What happens when the balloon approaches the electroscope?
- How does the distance to the balloon affect the behavior of the aluminum leaves?
- Do you think that the balloon's electric field can penetrate paper, aluminum foil or plastic?
- So it would become electrically charged.
- The leaves fly apart because the foil and the leaves each gain the same kind of charge, so they repel each other.
- The closer the balloon, the more active the strips.
- Answers will vary.
- How might individual body shapes and sizes give advantages to human swimmers?
- How might certain prey have evolved protective devices to counteract the shark's detecting ability?
- Define electric charge, electric field, electromagnetic field, bioelectricity.
- How does the wing-shaped head of the hammerhead create lift? Compare its head to airplane wings to learn more about fluid mechanics of air and water.
- Find out more about how electric phenomena is measured in living things (EKG, EEG, for example).
- Examine boats to see how their design contributes to better hydrodynamics.
Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.