Immune Cells in Pigeon Livers May Act as a Biological Compass
Key Insight: A study suggests that immune cells in the liver of homing pigeons, which accumulate iron, exhibit magnetic properties that could help the birds navigate when visual cues like the sun are absent.
A study published in the journal Science reveals a surprising new candidate for magnetic field detection in homing pigeons: immune cells in the liver. Researchers from the Max Planck Institute of Animal Behavior and the University of Bonn conducted experiments showing that pigeons lacking these specific cells struggled to navigate under overcast skies but oriented normally when the sun was visible.
The Discovery: Iron-Rich Macrophages
Scientists identified macrophages in the pigeon liver—immune cells that accumulate iron by breaking down old red blood cells—as exhibiting superparamagnetic properties. These cells are located near nerve fibers, which researchers propose could transmit directional information to the brain.
- Field Test Results: In experiments, 34 homing pigeons were released 19 kilometers from their home loft under overcast skies. Of these, 18 were treated with clodronate, a drug that eliminates macrophages.
- Stark Contrast: Untreated pigeons returned home within approximately 70 minutes. None of the treated pigeons returned the same day.
- Control Test: When released under sunny conditions, treated pigeons navigated normally. This suggests the liver-based mechanism is primarily used when visual cues like the sun are unavailable.
Scientific Context
Pigeons can navigate distances of up to about 1,000 kilometers. For roughly a century, scientists have known that magnetoreception—the ability to sense magnetic fields—is part of their navigation system.
Expert Perspective: "We didn't expect immune cells to act like sensors for magnetic fields at all." — Christian Kurts, University Hospital Bonn
Previous hypotheses suggested magnetic sensing mechanisms might be located in the eyes, beak, or inner ear. The identification of iron-rich liver cells represents a potential new mechanism for magnetic perception. The study used physical, morphological, functional, and genomic assays to identify these superparamagnetic macrophages.
Looking Ahead
The research team plans to investigate how these immune cells communicate with nerves to send signals to the brain. They also aim to establish a satellite system for tracking pigeons globally to further study navigation.
Key Quote: "It could be prudent to have more than one way of getting home in the dark." — Veterinary pathologist Simon Spiro and biologist Hal Drakesmith, in an accompanying editorial
Expert Reactions and Further Research
- Co-author Clivia Lisowski (University of Bonn) stated that the proximity of the cells to nerve fibers "might be how they transmit their 'magnetic sense' to the brain and help the pigeons to navigate."
- Senior co-author Martin Wikelski (Max Planck Institute of Animal Behavior) noted that precise direction-keeping is important for birds migrating at night and for pigeons in poor weather conditions.
- Joseph Kirschvink, a geophysicist at Caltech, urged caution, stating that more direct evidence is needed to confirm these particles sense magnetic fields. He noted that similar materials exist in other animals and even in Alzheimer's patients' brains, where their function might be different.
Implications for Other Species
Outside experts say more work is required to confirm the mechanism and verify how signals reach the brain. The discovery may have implications for understanding navigation in other species, including migratory birds, sharks, bats, and blind mole rats, though further research is needed to determine if other animals use similar mechanisms.