In order to find their way home over long distances, pigeons orientate themselves, among other things, on the earth’s magnetic field. However, how they perceive this is still controversial. A study now suggests that iron-enriched immune cells in the liver serve as sensors. If the researchers paralyzed these cells, the pigeons’ internal compass failed. While they could continue to navigate by the sun when the weather was good, they were disoriented when the sky was cloudy.
People have been using pigeons as mail carriers for thousands of years. Thanks to their excellent sense of direction, pigeons can find their way home even over hundreds of kilometers. How exactly the birds navigate is still not fully understood. Previous studies have suggested that pigeons can virtually “see” the Earth’s magnetic field thanks to special receptors in their eyes. Others suggested a magnetic sense in the epidermis of the beak and in the inner ear.
Magnetic cells in the liver
Now a team led by Clivia Lisowski from the University of Bonn has discovered another possible location for pigeons’ magnetic sense: in the immune cells of the liver. For their study, the researchers examined various pigeon tissues for cells that respond to magnetic fields. They discovered that iron-containing cells are also found in the liver and are much more common there than, for example, in the eyes, the skull or the brain of birds.
A more detailed analysis showed that the iron ions in the liver accumulate primarily in a certain class of immune cells, the macrophages. These phagocytes break down pathogens and cell fragments – including the iron-containing remains of red blood cells. “The iron in the macrophages is crystallized in oxide nanoparticles, which makes the cells superparamagnetic and allows them to react to magnetic fields,” explains co-author Ulf Wiedwald from the University of Duisburg-Essen.
© Martin Wikelski / Max Planck Institute of Animal Behavior
Double protection for orientation
To find out whether the iron-containing macrophages actually play a role in the pigeons’ magnetic sense, the team trained some animals to return to their dovecote at the Max Planck Institute for Animal Behavior in Konstanz over a distance of around 20 kilometers. They tracked the animals using GPS. When all the pigeons had reliably found their way, they injected some of them with a poison that killed the macrophages and thus removed the suspected magnetic sensors. And indeed: When the researchers sent these pigeons on their journey again under cloudy skies, they flew disoriented in different directions and could not find their way back to their dovecote. When the weather was good, however, the birds apparently navigated according to the sun and returned to the dovecote just as safely as their counterparts from the control group.
“Our study reveals a previously unknown mechanism of magnetic perception in animals,” says co-author Martin Wikelski from the Max Planck Institute for Behavioral Biology. In search of a mechanistic explanation, the researchers discovered that the iron-rich macrophages are located near nerve fibers that transmit information to the brain. “These results provide the first evidence of how the Earth’s magnetic field can be sensed in the body and transmitted to the brain to control movement,” explains Lisowski. However, exactly how the cells encode the signals and how they are processed in the brain is still unclear.
In an accompanying commentary, Simon Spiro from the Zoological Society of London and Hal Drakesmith from the University of Oxford, who were not involved in the study, point out that several studies have already found different locations of the magnetic sense in pigeons. “Although each of them implies a distinct detection method and different anatomical locations, decades of research have failed to pinpoint the mechanisms underlying magnetoreception,” they write. From their perspective, this suggests that, depending on the situation, several complementary processes could be at play. “In fact, it might make sense to have more than one way to find your way home in the dark.”
Clivia Lisowski (University of Bonn) et al., Science, doi: 10.1126/science.ady2486