雅思阅读 021 · Homing Pigeons: Are Their Livers the Compass?

改编自 Scientific American（Joseph Howlett, 2026）。题型：段落信息匹配 + T/F/NG + 填空。难度：较难（Passage 3，约 900 词）。

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正文 Passage

A. A long-standing mystery

How animals use Earth's magnetic field to navigate is one of biology's biggest unsolved mysteries. A new theory claims to have solved the long-standing conundrum of how homing pigeons find their way: the hypothesis is that pigeon livers act like compasses. Biologists have been unable to confirm the myriad previous theories for the pigeons' navigational abilities — from magnetite in their beaks to quantum entanglement in their eyes — and some experts are already saying the new theory does not stand up to scrutiny, either.

The study, published in Science, finds that homing pigeon livers are packed with magnetic immune cells containing a specific form of iron and that removing these cells messes with the birds' navigation.

B. The theory rests on macrophages

The new theory rests on macrophages in homing pigeons' livers — these immune cells are the body's garbage disposal. When a red blood cell dies, the iron it contains is recycled by macrophages. Periodically, macrophages empty their trash bags, recycling the iron back into the bone marrow to create new red blood cells. "But until then, the macrophages are full of iron," says Christian Kurts, an immunologist at the University Hospital Bonn.

In humans, these metal-rich immune cells congregate mainly in the spleen. But by sweeping homing pigeons' bodies with an extremely powerful magnetic field, the team found that the birds' livers were stuffed with them. And the magnetic immune cells also abutted nerve endings, suggesting a potential pathway to the brain.

C. The experiment

To test the hypothesis, the team turned to a fleet of pigeons trained to fly home to their aviary in Radolfzell, Germany, from nearly 20 kilometres away. The researchers gave the pigeons a drug that selected and killed macrophages and then tested their homing capabilities on a cloudy day. "They were completely lost," says immunologist Clivia Lisowski. "I mean, it was crazy — they were going in all directions."

But as to how these liver macrophages might respond to a bird's changing orientation in Earth's relatively weak magnetic field, let alone transmit that information to the brain, the paper does not have an immediate answer.

D. Critics are not convinced

A landmark 2012 Nature study suggests what the team has found may be impossible. It refuted earlier claims that pigeons' beaks contained compasslike neurons, by showing that these were actually iron-bearing macrophages. This and other past studies conclude that macrophages contain the wrong kind of iron to explain the pigeons' magnetic sense.

"I am not convinced," says Joe Kirschvink, a geobiologist at the California Institute of Technology, who has studied the relationship between Earth's magnetic field and animals for decades. "I am surprised this paper cleared the review process for Science."

The team points to "superparamagnetism," a quantum mechanical phenomenon that heightens the macrophage-borne iron's response. But there is no evidence that that would be enough for the cell to pick up and notify a neuron, says Carl Meyer, a biologist at the Hawaii Institute of Marine Biology. "Until then, I remain skeptical — not least because of all the previous declarations of victory."

E. A new direction

Neuroscientist Pascal Malkemper says that among the paper's more surprising findings is that killing off the pigeons' liver macrophages disrupted their navigation. Research shows that these birds use lots of visual and smell-based cues to navigate, as well as the magnetic field.

"The magnetic sense is usually the least important sense somehow. It's kind of the last resort," Malkemper says. He speculates whether the drug administered to kill the macrophages might have agitated the birds in some way.

Importantly, the new paper also shows that birds treated with the drug could find their way home on a sunny day, when they were able to use solar cues to navigate.

"Nevertheless, it is always good to have a new hypothesis on the table," says Malkemper. "We're looking in the inner ear, we're looking in the eye — now we might be looking in the liver as well."

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题目 Questions

Questions 1-5: Which paragraph mentions the following?

1. a 2012 study that refuted earlier claims about pigeon beaks

2. the experiment with a drug that killed macrophages

3. macrophages as the body's garbage disposal

4. pigeons finding their way home on a sunny day

5. Kirschvink's surprise that the paper was published

Questions 6-9: T/F/NG

6. Previous theories about pigeon navigation have all been confirmed.

7. In humans, metal-rich macrophages are found mainly in the spleen.

8. Pigeons whose macrophages were killed were completely lost on a sunny day.

9. Malkemper says the magnetic sense is usually the most important sense for pigeons.

Questions 10-13: Complete the notes. Choose NO MORE THAN TWO WORDS.

• The study was published in the journal (10) ____________.
• Macrophages recycle (11) ____________ back into the bone marrow.
• The pigeons were trained to fly home from nearly (12) ____________ away.
• Malkemper says researchers are now looking in the inner ear, the eye, and the (13) ____________.

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答案 Answers

题号	答案
1	D
2	C
3	B
4	E
5	D
6	FALSE（unable to confirm）
7	TRUE
8	FALSE（lost on cloudy day, found way on sunny day）
9	FALSE（least important sense）
10	Science
11	iron
12	20 kilometres
13	liver

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核心词汇

magnetic field / navigation / homing pigeon / macrophage / immune cell / iron / beak / magnetite / quantum / superparamagnetism / spleen / bone marrow / neuron / hypothesis / scrutiny