雅思阅读 110: The Narwhal Tusk Double Helix

Adapted from 'What makes a narwhals tusk so special?', Scientific American, August 2026.

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Reading Passage

Narwhals are known for their straight, spiral tusks, which can reach some 10 feet in length. These overgrown teeth are the only tusks in the animal kingdom known to grow in a completely straight line. Most male narwhals have one, and so do some females; other narwhals even have two.

Yet every tusk seems to spiral in one direction, left, and scientists have long wondered why. Now researchers are using advanced imaging technology to look at the tusk in unprecedented detail.

In a new study published in Nature Communications, researchers reveal the tusks building blocks, tiny threadlike structures of mineralized collagen, grow in a sort of double helix: on the outside of a tusk, the structures follow a left-handed helix, and inside, they grow in a right-handed helix. This is what enables the tusk to grow in such a straight line.

In nature, it is common to see swirling structures going in different directions, even in the same species. Some snail shells, for instance, spiral both right and left. But as far as scientists know, narwhals are the only creatures to have a singularly left-spiraling tusk.

Henrik Birkedal and his colleagues imaged narwhal tusks using advanced x-ray scanners, which he describes as turbocharged CT scanners. Those building blocks were bits of collagen, a protein found in skin and bones with a texture like half-cooked spaghetti. It is firm but flexible.

The team found that while the outer layer of the tusk, called cementum, has a structure that spirals left, the inner part, dentine, spirals right. These two opposing spirals help keep the tusk straight.

It is unclear why a narwhal needs a straight tusk while elephants have tusks that bend. The dominant hypothesis is that the narwhal tusk is for social display, to establish dominance or attract mates, rather than for hunting. It could also be that a straight tusk stays out of the way when narwhals suction-feed on fish and squid. Birkedal says a tusk functions like tree rings, growing in bands that reveal the animals age and life history.

Questions 1-5

Choose the correct letter, A, B, C or D.

1. How long can a narwhal tusk reach? A. About 3 feet B. About 10 feet C. About 30 feet D. About 1 foot
2. What is unique about narwhal tusks? A. They are white B. They grow in a completely straight line C. They are hollow D. They are shed annually
3. What direction does the outer cementum spiral? A. Right B. Left C. Up D. It does not spiral
4. What is collagen compared to? A. Cooked rubber B. Half-cooked spaghetti C. Hard plastic D. Wet paper
5. The dominant hypothesis about the tusk is that it is used for A. fighting B. social display C. digging D. breaking ice

Questions 6-10

Do the following statements agree with the information? Write TRUE, FALSE or NOT GIVEN.

6. All narwhals have two tusks.
7. The outer layer spirals left and the inner layer spirals right.
8. Snail shells always spiral in the same direction.
9. The tusk can reveal the animals age.
10. Narwhals use their tusks to catch fish.

Questions 11-13

Complete the summary. Choose ONE WORD ONLY.

The tusk building blocks are threadlike structures of mineralized 11. The opposing spirals of cementum and 12 keep the tusk straight. The dominant hypothesis is that the tusk is for social 13.

Answers

1. B 2. B 3. B 4. B 5. B
2. FALSE 7. TRUE 8. FALSE 9. TRUE 10. FALSE
3. collagen 12. dentine 13. display

Key Vocabulary

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