Scientists in Germany have found a surprising solution to one of biology’s big mysteries — how birds and other animals sense Earth’s magnetic field to help them navigate accurately for thousands of miles.
The team from Germany’s University of Bonn and Max Planck Institute of Animal Behaviour discovered that pigeons have iron-rich immune cells in their liver, which detect geomagnetism. The study was published on Thursday in the journal Science.
Decades of experiments have shown that many migratory and homing species rely on an internal magnetic compass for guidance, as well as using the sun, stars, visual landmarks and even smells.
But there is much uncertainty about the nature and position of the avian compass. Scientists have produced evidence to support the involvement of various magnetically sensitive cells in birds’ eyes, inner ears and beaks, with little consensus about which are active in real life.
The latest contenders are macrophages — immune cells in the liver that accumulate iron as they break down old red blood cells. They are very sensitive to small changes in the external magnetic field.
“It is really exciting that we have found a physical basis for what looks like a ‘gut feeling’ in bird navigation,” said Martin Wikelski, director at the Max Planck Institute of Animal Behaviour.
The conclusions were based on lab tests and behavioural experiments. First, the researchers screened pigeons’ bodies to find the organs that showed the strongest magnetic response and found that the liver stood out. Further investigation identified macrophages as the cells responsible. These incorporate an iron-rich protein called ferritin that acts as a very sensitive nano-magnet.
Next, the team experimented with homing pigeons. When the birds were given a drug (clodronate) that removes macrophages from the liver, tracking showed that they lost their sense of direction and flew in random directions in overcast weather. On cloudless days, the treated pigeons navigated successfully, using the sun as a cue.
Electron microscopy then revealed that nerve fibres in the liver run past the macrophages, suggesting how magnetic signals could reach the brain.
Clivia Lisowski, who led the team’s immunological work, said: “These findings provide the first concrete evidence of how the Earth’s magnetic field can be perceived within the body and passed on to the brain to guide movement.”
Chemistry professor Christiane Timmel is leading a large interdisciplinary project at Oxford university to investigate a different hypothesis, also rooted in quantum physics.
A protein in the eye called cryptochrome responds to light by creating free radicals — shortlived molecules with unpaired spinning electrons. These respond to the strength and direction of the magnetic field with reactions picked up by nerves in the retina. “In a sense, the animal would see the magnetic field,” she said.
Commenting in Science on the German team’s “provocative study”, two scientists not involved in the research, Simon Spiro of London Zoo and Hal Drakesmith of Oxford university, suggested that in magnetic guidance “multiple complementary processes could be at play, depending on circumstance. Perhaps, one process dominates for long-distance navigation, whereas another is used for more specific destination finding.”