Sea Turtle Hatchling Strategies for Navigation

Sea turtles’ eggs are laid at night to minimize the likelihood of their discovery by predators, and the offspring, when ready to emerge from their eggshells and dig their way out of the sand, hatch at night for the same reason. Since the offspring are especially vulnerable immediately after hatching, it is vital for them to get to the sea as soon as possible. Turtle hatchlings use a number of cues to tell them where the sea is.

 

 

The most important cue seems to be light. The night sky is usually brightest over the sea. Cover a turtle hatchling’s eyes, and it cannot find the sea even if there is other information available, such as a downward slope of the sand toward the water’s

edge. The hatchlings respond to light cues covering a vertical range of only about 30˚ above the horizon or, depending on the species, even less. Responding only to lights that are close to the horizon decreases the risk that hatchlings will become confused. They seem less attracted to yellow light than to other colors — loggerhead turtles show an aversion to yellow light — and this preference may keep them from becoming disoriented by the rising Sun.

 

 

It is usually safest to have more than one internal compass, and hatchlings seem to be guided by more than light alone. They steer away from sand dunes and vegetation. Possibly these objects merely block light behind them that might mislead turtle

hatchlings about where the sea is, but it is also possible that turtles are sensitive to the shape of such objects and process these shapes as signals that the sea is located in some other direction. Such reinforcing cues, however, are not enough to guide

hatchlings away from the artificial lights that now burn on many a beach environment. Artificial lighting is often strong enough to completely overcome the signals a hatchling sea turtle is programmed to recognize. Artificial light, if it is bright enough, becomes a stimulus so powerful that the hatchlings respond to nothing else, crawling toward it from hundreds of meters away.

 

 

If all goes well and the hatchlings scramble over the sand in the right direction, avoid their enemies, and reach the surf, a new set of orienting mechanisms takes over. As soon as they are afloat, the hatchlings begin to swim at something over 1.5 kilometers per hour. They dive into the path of the wave undertow, where the receding waters sweep them outward, away from the beach. When they surface again, the head for open sea. This time, they are guided not by sight but apparently exclusively by the direction of the incoming waves. Experiments with loggerheads, greens, and leatherbacks have shown that hatchlings swim toward approaching waves; but if the sea is calm, they swim randomly or in circles. Under experimental conditions, hatchlings will swim into the waves even if doing so sends them back to the beach again.

 

 

The farther a hatchling gets from shore, the less reliable wave direction becomes as a pointer to the open sea. Researchers have shown that hatchling green sea turtles released from a hatchery in Borneo, East Malaysia, are able to navigate around small islands and keep swimming offshore, even when there are few waves to guide them. They may be relying on yet another internal compass this time oriented to Earth’s magnetic field. Recent experiments suggest that leatherback and olive ridley hatchlings “switch on” their geomagnetic compass almost as soon as they are out of the nest. Though the hatchlings position themselves geomagnetically as soon as they leave the nest and appear to be able to use that position as a reference point, they will not follow it automatically if other cues, such as light and sound, are available. Hatchlings find their geomagnetic compass useful only after they have already been able to determine the direction they should swim. A simple directional compass –one that always sent the turtles westward, for instance—would be useless if the open sea lay in some other direction. Therefore, a magnetic compass does not so much tell a hatchling turtle which way to go as keep it on course once it has determined the direction it should swim from some other cue.

 

 

【Paragraph 1】

Sea turtles’ eggs are laid at night to minimize the likelihood of their discovery by predators, and the offspring, when ready to emerge from their eggshells and dig their way out of the sand, hatch at night for the same reason. Since the offspring are especially vulnerable immediately after hatching, it is vital for them to get to the sea as soon as possible. Turtle hatchlings use a number of cues to tell them where the sea is.

 

 

1. Which of the sentences below best express the essential information in the highlighted sentence in the passage? Incorrect 

choices change the meaning in important ways or leave out essential information.

A. Sea turtle eggs are laid at night and hatch at night for the same reason.

B. To minimize the likelihood of their discovery by predators, sea turtle hatchlings dig their way out of the sand at night.

C. To minimize the likelihood of discovery by predators, sea turtle eggs are laid and hatch at night.

D. Eggs that laid at night and offspring that emerge from the eggs at night are less likely to be discovered by predators.

 

 

【Paragraph 2】

The most important cue seems to be light. The night sky is usually brightest over the sea. Cover a turtle hatchling’s eyes, and it cannot find the sea even if there is other information available, such as a downward slope of the sand toward the water’s edge. The hatchlings respond to light cues covering a vertical range of only about 30˚ above the horizon or, depending on the species, even less. Responding only to lights that are close to the horizon decreases the risk that hatchlings will become confused. They seem less attracted to yellow light than to other colors — loggerhead turtles show an aversion to yellow light—and this preference may keep them from becoming disoriented by the rising Sun.

 

 

2. In paragraph 2, why does the author provide the information that sea turtle hatchlings cannot find the sea when their 

eyes are covered?

A. To identify a major reason that such turtles might not reach the sea

B. To support the claim that light is an important directional cue for such turtles

C. To point out the effect of yellow light on the attempts of such turtles to reach the sea

D. To provide evidence showing that such turtles easily lose their sense of direction

 

 

3. According to paragraph 2, which of the following is true of turtle hatchlings that are trying to make their way to the sea?

A. They are unable to distinguish the light of the rising Sun from other types of light.

B. They require the presence of yellow light in order to reach the water’s edge.

C. They have trouble crossing downward slopes near the water’s edge.

D. They respond to light only when it is close to the horizon.

 

 

【Paragraph 3】

It is usually safest to have more than one internal compass, and hatchlings seem to be guided by more than light alone. They steer away from sand dunes and vegetation. Possibly these objects merely block light behind them that might mislead turtle hatchlings about where the sea is, but it is also possible that turtles are sensitive to the shape of 

such objects and process these shapes as signals that the sea is located in some other direction. Such reinforcing cues, however, are not enough to guide hatchlings away from the artificial lights that now burn on many a beach environment. Artificial lighting is often strong enough to completely overcome the signals a hatchling sea turtle is programmed to recognize. Artificial light, if it is bright enough, becomes a stimulus so powerful that the hatchlings respond to nothing else, crawling toward it from hundreds of meters away.

 

 

4. Which of the following is offered in paragraph 3 as a possible explanation for sea turtle hatchlings’ behavior of avoiding sand dunes and vegetation?

A. Sand dunes and vegetation may provide hiding places for animals that attack hatchlings.

B. Sand dunes and vegetation may block the pathway between hatchlings and the water’s edge.

C. The shapes of sand dunes and vegetation may indicate the wrong direction to travel in.

D. The shapes of sand dunes and vegetation may resemble the shapes of some hatchling predators.

 

5. Paragraph 3 supports which of the following statements about the artificial lights now found on many beaches?

A. The lights have sometimes helped sea turtle hatchlings find their way to the sea.

B. The lights can be bright enough to affect sea turtle hatchlings within hundreds of meters of them.

C. The lights are commonly turned off during periods when sea turtle offspring are hatching.

D. The lights tend to affect the behavior of sea turtle hatchlingsless than other programming signals do.

 

【Paragraph 5】

The farther a hatchling gets from shore, the less reliable wave direction becomes as a pointer to the open sea. Researchers have shown that hatchling green sea turtles released from a hatchery in Borneo, East Malaysia, are able to navigate around small islands and keep swimming offshore, even when there are few waves to guide them. They may be relying on yet another internal compass this time oriented to Earth’s magnetic field. Recent experiments suggest that leatherback and olive ridley hatchlings “switch on” their geomagnetic compass almost as soon as they are out of the nest. Though the hatchlings position themselves geomagnetically as soon as they leave the nest and appear to be able to use that position as a reference point, they will not follow it automatically if other cues, such as light and sound, are available. Hatchlings find their geomagnetic compass useful only after they have already been able to determine the direction they should swim. A simple directional compass—one that always sent the turtles westward, for instance—would be useless if the open sea lay in some other direction. Therefore, a magnetic compass does not so much tell a hatchling turtle which way to go as keep it on course once it has determined the direction it should swim from some other cue.

 

 

6. In paragraph 5, why does the author provide the information that hatchling green sea turtles can navigate around 

small islands and keep swimming offshore even when few waves are present?

A. To point out a benefit of the fact that hatchlings use their geomagnetic compasses almost as soon as they leave the nest

B. To provide evidence that green sea turtles use different navigational techniques than leatherbacks and olive ridley turtles do

C. To provide a reason for considering the possibility that sea turtles navigate by detecting Earth’s magnetic field

D. To provide evidence showing that hatchling do not automatically follow their geomagnetic compass if other cues are available

 

 

7. The word “switch on” in the passage is closest in meaning to

A. follow

B. change

C. control

D. activate

 

 

8. According to paragraph 5, all the following claims about the geomagnetic compass of sea turtle hatchings are true EXCEPT

A. In some species it is switched on almost as soon as a turtle hatches.

B. It helps keep the hatchlings heading in a direction that it is initially determined by some cue such as light or sound.

C. Its direction is followed automatically to swim toward open water.

D. It can be used to navigate around islands when the waves stop indicating the direction of the open sea.

 

 

【Paragraph 5】

The farther a hatchling gets from shore, the less reliable wave direction becomes as a pointer to the open sea. Researchers have shown that hatchling green sea turtles released from a hatchery in Borneo, East Malaysia, are able to navigate around small islands and keep swimming offshore, even when there are few waves to guide them. They may be relying on yet another internal compass this time oriented to Earth’s magnetic field. Recent experiments suggest that leatherback and olive ridley hatchlings “switch on” their geomagnetic compass almost as soon as they are out of the nest. Though the hatchlings position themselves geomagnetically as soon as they leave the nest and appear to be able to use that position as a reference point, they will not follow it automatically if other cues, such as light and sound, are available. ■Hatchlings find their geomagnetic compass useful only after they have already been able to determine the direction they should swim. ■A simple directional compass—one that always sent the turtles westward, for instance—would be useless if the open sea lay in some other direction. ■Therefore, a magnetic compass does not so much tell a hatchling turtle which way to go as keep it on course once it has determined the direction it should swim from some other cue. ■

 

9. Look at the four squares [■] that indicate where the following sentence could be added to the passage

This preference for other cues points to the special role of the geomagnetic compass in hatchling navigation.

Where would the sentence best fit?

 

 

10. Directions: An introductory sentence for a brief summary of the passage is provides below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This questionis worth 2 points.

 

To minimize attacks from predators, turtle hatchlings must quickly find their way to the water’s edge.

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Answer Choices

A. When light cues are absent, hatchlings use other navigational strategies, such as following the downward slope of the beach to find the water’s edge.

B. If the incoming waves are too strong to allow hatchlings to reach the open sea, they are carried back to the beach where they wait for calmer waters.

C. Once hatchlings reach the open sea, they begin to use their geomagnetic compasses as simple directional compasses.

D. Hatchlings use light and cues such as shapes to help them locate the sea, though strong artificial light on land may lead them in the wrong direction.

E. After they reach the water, hatchlings rely on incoming waves to indicate the direction in which the open sea is located.

F. Hatchlings’ geomagnetic compasses can help keep hatchlings on course toward the open sea, once other cues have been used to get them headed in the right direction.

 

 

 

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