How Do Pigeons Find Their Way Home? New Research Suggests the Birds’ Remarkable Navigational Skills Come From Their Livers
The birds might use the organs’ iron-rich immune cells as internal compasses on overcast days, when they must rely on Earth’s magnetic field, instead of the sun’s light cues, for navigation
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Pigeons are famous for their uncanny ability to return home, even after traveling long distances. But how do they manage to accomplish this impressive navigational feat? Scientists say the answer might lie in their livers.
The birds might use the organ’s iron-rich immune cells as internal compasses on overcast days, when they must rely on Earth’s magnetic field, instead of the sun’s light cues, for navigation, researchers report in a study published on May 28 in the journal Science.
The findings offer fresh insights into the potential mechanics of magnetoreception, or the ability to sense Earth’s magnetic field, which numerous animals use for long-distance travel. Migratory birds, sea turtles, spiny lobsters, mole rats and gray whales are among the creatures scientists believe rely on this “sixth sense” for navigation.
Pigeons’ “sense of magnetism has been a mystery for a century, and nobody could solve where that sits and how that works,” study co-author Martin Wikelski, a zoologist at the Max Planck Institute of Animal Behavior in Germany, tells National Geographic’s Dino Grandoni. “Now, we think we have found, really, a workable solution.”
Researchers took cell samples from pigeons’ beaks, eyes, muscles, spleens and livers. They discovered that iron-rich immune cells, known as macrophages, in the birds’ livers can demonstrate a type of magnetism known as “superparamagnetism.”
“Once the pigeon passes through the Earth’s magnetic field, [negatively charged particles in the special cells] all arrange in the same direction, making them superparamagnetic,” says lead author Clivia Lisowski, an immunologist at the University of Bonn in Germany, to CNN’s Kasha Patel.
Additionally, the macrophages seem to cluster near nerve fibers, raising the possibility that the iron-rich liver cells might somehow relay directional information to the brain based on the Earth’s magnetic field.
To further test this hypothesis, the researchers trained 34 homing pigeons to fly a roughly 12-mile route to their home aviary in Radolfzell, Germany, while wearing GPS trackers. “Once they are trained, they are perfect,” Wikelski tells National Geographic. “They evade everybody. They fly home as if the devil is behind them.”
Then, they injected about half of the birds with a drug that killed most of the macrophages in their livers. On a completely overcast day, they drove the birds to the start of the route and released them one by one.
The birds with fully functional liver macrophages returned home within 70 minutes. Meanwhile, the birds that had received the cell-depleting treatment flew around aimlessly and did not come home until the sun popped out on a different day. “They were completely lost,” Lisowski tells Scientific American’s Joseph Howlett. “I mean, it was crazy—they were going in all directions.”
Scientists don’t know exactly how the birds’ liver macrophages might perceive the Earth’s magnetic field, nor how the cells may communicate that information to the brain. Nevertheless, some outside researchers find the evidence persuasive.
“I would never have guessed it, but once it was explained to me, it makes sense,” says Albert Kao, a behavioral ecologist at the University of Massachusetts Boston who was not involved with the research, to Adithi Ramakrishnan of the Associated Press.
Catherine Lohmann, a sensory ecologist at the University of North Carolina at Chapel Hill who was also not involved with the paper, calls the findings “mind-blowing,” telling Science’s Erik Stokstad the study provides “a new direction and a very fresh take on a controversy that’s been in the literature for a long time.”
Did you know? Humans’ relationship with pigeons goes way, way back
Pigeon bones found at an ancient Cyprus settlement suggest the birds were at least partially domesticated around 1400 B.C.E., researchers reported in May. The findings push back the direct evidence of domestication of common pigeons by around 1,000 years.
However, past research suggests that macrophages contain a type of iron that doesn’t respond much to Earth’s relatively weak magnetic field, reports Scientific American. More skeptical experts would like to see more direct evidence that the pigeons’ macrophages can detect the field.
“I am not convinced,” Joe Kirschvink, a geobiologist at Caltech who was not involved in the new study, tells Scientific American. “I am surprised this paper cleared the review process for Science.”
Other possible explanations for pigeons’ navigational abilities include light-sensitive proteins in their eyes, magnetic minerals in their beaks or a set of structures in their inner ears. The new study does not rule out these or other mechanisms, and pigeons might have multiple methods for getting around when the sun is obscured.
“Perhaps one process dominates for long-distance navigation, whereas another is used for more specific destination-finding, with both operating with different degrees of precision,” write Simon Spiro, a wildlife veterinary pathologist at the Zoological Society of London, and Hal Drakesmith, a biologist at the University of Oxford, both of whom were not involved with the paper, in an accompanying commentary. “Indeed, it could be prudent to have more than one way of getting home in the dark.”
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