Senior Lecturer, School of Physics, University of Melbourne

David Simpson received funding from the Australian Research Council.

The University of Melbourne provided funding as a founding partner of The Conversation AU.

Carrier pigeons are known for their incredible ability to find their way home-navigating in a complex and ever-changing landscape. In fact, they do a very good job in this regard and were used as a source of secure communications more than 2,000 years ago.

According to reports, Emperor Caesar sent the news of his conquest of Gaul back to Rome via pigeons, as did Napoleon Bonaparte after he was defeated by England at the Battle of Waterloo in 1815.

We know that pigeons use visual cues and can navigate based on landmarks on known travel routes. We also know that they have a kind of magnetic induction called "magnetic induction", which allows them to use the earth's magnetic field for navigation.

Read more: Interpreter: How do carrier pigeons navigate?

But we don’t know exactly how they (and other species) do this. In a study published today in the Proceedings of the National Academy of Sciences, my colleagues and I tested a theory that attempts to link the magnetic perception of homing pigeons with small iron-rich substances found in their inner ears. stand up.

By using a new type of magnetic microscope, we confirmed that this is not the case. But this technology opens the door for us to study this phenomenon in several other species.

Scientists have spent decades exploring the possible mechanism of magnetic sensation. There are two mainstream theories at present.

The first one is the "free radical pair" model based on vision. The eye retinas of homing pigeons and other migratory birds contain a protein called "cryptochrome". These generated electrical signals vary according to the strength of the local magnetic field.

This may allow birds to "see" the Earth's magnetic field, although scientists have not yet confirmed this theory.

The second suggestion on how homing pigeons navigate is based on the blocks of magnetic material inside them, which can provide them with a directional compass based on magnetic particles.

We know that magnetic particles exist in nature, in a group of bacteria called magnetotactic bacteria. These bacteria produce magnetic particles and orient them along the earth's magnetic field lines.

Scientists are now looking for magnetic particles in a range of species. Potential candidates were discovered in the upper beak of homing pigeons more than ten years ago, but subsequent work has shown that these particles are related to iron storage, rather than magnetic induction.

Read more: New evidence of human magnetism allows your brain to detect the Earth’s magnetic field

New research is now being carried out in the inner ears of pigeons, where iron particles called "keratinocytes" were first discovered in 2013.

The individual epidermal bodies are located in different areas of the inner ear of the pigeon, where there are other known sensory systems (such as hearing and balance during flight). In theory, if there is a magnetic induction system in the pigeon, it should be located close to other induction systems.

But to determine whether iron keratinocytes can act as magnetic receptors for pigeons, scientists need to determine their magnetism. This is no easy task, because the stratum corneum is 1,000 times smaller than a grain of sand.

More importantly, they are only present in 30% of the hair cells in the inner ear, so they are difficult to identify and characterize.

To solve this problem, our team at the University of Melbourne, together with colleagues from the Vienna Institute of Molecular Pathology and the Max Planck Society in Bonn, turned to a new imaging technique to explore the magnetism of the iron keratin in the inner ear of pigeons.

We have developed a magnetic microscope that uses a diamond-based sensor to visualize the subtle magnetic field emitted by tiny magnetic particles.

We took a closer look at the thin slices of pigeon inner ears placed directly on the diamond sensor. By applying magnetic fields of different strengths to the tissue, we can measure the magnetic susceptibility of a single epidermal body.

Our results indicate that the magnetic properties of the epidermal bodies are insufficient to enable them to act as magnetoreceptors based on magnetic particles. In fact, these particles need to be 100,000 times stronger to activate the sensory pathways required for pigeon magnetoreception.

However, although research on the elusive magnetic receptors has failed, we are very excited about the potential of this magnetic microscopy technology.

We hope to use it to study a large number of magnetic candidates for various species including rodents, fish and turtles. By doing this, we can not only focus on the epidermal bodies, but also a series of other potential magnetic particles.

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