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Deep mystery: why sea turtles plumb the depths

Researchers say they have figured out why sea turtles that normally feed and breed in shallow water or on land will, very rarely, go deep sea diving: the reptiles are on reconnaissance.

Scientists have long puzzled over why leatherbacks are built to plumb the icy depths.

Imagine someone donning a complete set of scuba gear -- tanks, buoyancy compensator, regulator -- only to paddle about the surface of a shallow lagoon. What's the point?

The mystery deepens. Not only are the turtles equipped with myoglobin-rich blood ideal for stocking oxygen, they sometimes plunge more than a kilometre below the surface.

Jonathan Houghton and colleagues from the University of Swansea in Britain conducted experiments to find out why the lumbering sea creatures make these rare forays.

Of more than 26,000 dives logged all across the North Atlantic Ocean, only 95 -- less than half of one per cent -- went below three hundred metres.

Several theories have competed to explain these out-of-character deep dives.

Some researchers argue that the egg-laying reptiles go below to escape predators, while others speculate they simply want to cool off.

A third hypothesis is that the turtles are on the hunt for deep-sea delicacies.

But Houghton's findings suggest all these theories are off the mark.

A turtle trying to avoid becoming some fish's lunch would surely swim a bit more vigorously than usual, but the data collected indicate they were in no hurry as they plunged.

Moreover, turtles spent several hours at the surface just before deep diving, probably to boost oxygen efficiency.

"Hanging out at the surface would be a daft strategy for avoiding predators, because that is where they can spot your silhouette," said Houghton.

As for keeping cool, temperatures don't drop much after the 350-metre mark, so there's little incentive to go any deeper.

But the food hypothesis, the study found, may be at least half right: even if the turtles don't eat the food they find at extreme depth, they probably find the food they will eat -- later on.

Leatherbacks like to dine on surface-dwelling jellyfish, but during the months spent travelling from their tropical breeding grounds in the Caribbean to cooler waters, they rely on jellyfish-like animals that form long colonies during the day at depths of about 600 metres

The turtles, Houghton speculates, dive when the sun is out to find the colonies, and then wait for them to surface at night to begin feasting.

This would explain why the leatherbacks often loiter in the same area for days or weeks after such a deep dive, he said.


to figure out, v: to work out; to find out

to puzzle, v: to think hard over a confusing problem

to don, v: to put on clothes

shallow, adj: not deep

to plunge, v: to go down very quickly

to lumber, v: to move clumsily

predator, n: animal that kills and eats other animals for food

delicacy, n: special food that is very tasty and delicious

off the mark, expression: not correct; not accurate; not true

to loiter, v: to stay in one place with no apparent aim


1. Where do leatherbacks normally feed?

a. on high trees

b. in the deep sea

c. in deep tunnels

d. in shallow water

2. What is puzzling about the leatherbacks' anatomy?

a. They taste delicious.

b. They are built to dive very deep.

c. They can swim extremely swiftly.

d. They need to stay in groups for protection.

3. Which of the following is NOT a theory why leatherbacks dive so deep?

a. to mate

b. to cool off

c. to escape predators

d. to hunt for deep-sea delicacies

4. What do leatherbacks feed on?

a. krill

b. sea anemones

c. kelp and small fish

d. jellyfish and similar animals

5. What do leatherbacks often do after a deep dive?

a. roll over and die

b. stay in the same area

c. leave the place quickly

d. breach the surface for a leap


Which of the following words or phrases replace the ones from the passage best?

1. reconnaissance

a. flight

b. grazing

c. investigation

d. reproduction

2. plumb

a. fall

b. hurl

c. dash

d. explore

3. foray

a. delay

b. meeting

c. incursion

d. postponement

4. vigorously

a. lazily

b. carefully

c. randomly

d. energetically

5. daft

a. novel

b. stupid

c. clever

d. alternative


Questions           1. d, 2. b, 3. a, 4. d, 5. b

Synonyms           1. c, 2. d, 3. c, 4. d, 5. b

By Ajarn Horst Baelz

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