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RSS SubscribeSunday 13th May 2012
Magnetic navigation – Putting the cat among the pigeons
Homing pigeons are well known for their extraordinary sense of direction, thought to result from a specialised sense that allows them to navigate using the Earth’s magnetic field.
But new research by a team of Australian, Austrian and British scientists has revealed that cells previously thought to be the centre of the magnetic sense in birds are instead non-magnetoreceptive macrophages, or white blood cells.
Macrophages are found in the spleen, skin and lungs of many animals and play a vital role in defending against infection and in the recycling of iron from red blood cells. In addition to in the beak, similar iron-rich cells were found in the pigeons’ feather follicle and skin cells, showing such cells are widely distributed through the body.
The finding, published in Nature, has reset current thinking in avian magnetic navigation and moves the search to finding the true location of the magnetic sense in birds.
The scientists are also investigating if similar navigational tools are used by other animal species such as bees, rainbow trout and sea turtles.
“Using a three-dimensional blueprint of the pigeon beak created by magnetic resonance imaging and computed tomography, we mapped the location of iron-rich cells, revealing unexpected variation in their distribution and number – an observation that is inconsistent with a role in magnetic sensation,” the authors say in their report.
“Ultrastructure analysis of these cells, which are not unique to the beak, showed that their sub-cellular architecture includes ferritin-like granules, siderosomes, haemosiderin and filopodia – characteristics of iron-rich macrophages.
“Our conclusion that these cells are macrophages and not magnetosensitive neurons is supported by immune-histological studies. Our work necessitates a renewed search for the true magnetite-dependent magnetoreceptor in birds.”
Among the 12 scientists involved in the project, Dr Jeremy Shaw of the University of Western Australia, or UWA, worked with Dr David Keays, an Australian now at the Institute of Molecular Pathology in Vienna. Shaw assisted with the characterisation of the macrophage cells as part of the international study.
Keays said it took a team of Australians and Austrians to show that the established dogma in the field was completely wrong. He said the mystery of how animals detect magnetic fields “has just got more mysterious”.
Shaw and his team at UWA used transmission electron microscopy analysis – a beam of high-energy electrons passing through thin samples of pigeon tissue – to characterise the distribution and type of iron minerals present.
“Our contribution confirmed that the iron in the macrophage cells in the pigeon beak were normal iron deposits composed primarily of protein-bound iron in the form of ferrihydrite, similar to that found in many other animals. Magnetite is thought to be the likely candidate magnetic material,” Shaw said.
