According to new research published in the European Physical Journal Special Topics, microscopic hair cells in animals' inner ears could act as effective biological compass needles. The development proves one of the leading theories behind animals' incredible ability to tap into Earth's magnetic field as a source of locational information.

Until now, this sixth sense, known as magnetoreception, remained one of the greatest mysteries to sensory biologists.

Using statistical analysis, Dr. Kirill Kavokin at St Petersburg State University, Russia, demonstrated that animals use about one hundred of these hair cells to perceive Earth's magnetic field. The fascinating result could provide a boost in knowledge for biologists to understand the origins of magnetoreception and finally identify the mechanisms behind this nature.

The new study supports one of two leading theories of magnetoreception. This is the presence of 'stereocilia,' which are bundles of hair cells in the inner ear. Scientists propose that magnetite nanocrystals are attached to the stereocilia. Because this iron-based mineral can become permanently magnetized, it aligns itself with Earth's magnetic field. As the animal's orientation changes, magnetite forces changes in the orientations of the stereocilia attached to it.

And that's not all. The theory is also based on how mechanoreceptors i.e. nerves cells, which can detect mechanical pressure, pick up these changes in stereocilia orientation. In this way, the animal gains a physical sense of a magnetic field. Whether these nerve cells are sensitive enough to detect such subtle changes remains uncertain- until now.

Under the new study, Kavokin used statistics to prove the fluctuations of stereocilia and demonstrated that these microstructures could indeed act as susceptible compass needles. To go one step further, the researcher even added a number to how many of these structures are needed for mechanoreceptors to sense their fluctuations (i.e., 100).

Despite the mysteries surrounding how animals can sense the Earth's magnetic field, the list of animals that use magnetoreception to navigate is vast. Take the adult loggerhead sea turtles, for example, who utilize their well-developed magnetic sense to return to their breeding grounds.

Evidence suggests that homing pigeons use various biological compasses in sync with the Earth's magnetic field to guide themselves home.

Previously, we covered a study that suggested that humans, too, inherited a biological sense of the Earth's magnetic field. Participants in a 'faraday cage' equipped with a magnetic field revealed that human brains collected and selectively processed directional input from magnetic field receptors.

With that said, there is still much to learn about the mechanisms of magnetoreception, and not a great deal is highlighted in the limitations of the study.

Still, witnessing how sensory biologists continue to scratch the surface (and their heads!) of this extraordinary evolutionary ability to navigate and survive will always make for an intriguing topic.

The possibility of realization of magnetoreception in vertebrates with chains of magnetite nanocrystals (magnetosomes) attached to hair cells of the inner ear is evaluated. To this end, statistical mechanics is applied to analyze fluctuations of stereocilia bundles. Correlation functions of fluctuations of the bundle position and of the number of open mechanoreceptor channels are derived. The sensitivity threshold of the hair cell to applied forces is calculated. Its comparison with the force couple exerted by the magnetosome in the geomagnetic field suggests that a compass magnetoreceptor can be realized with ~ 100 specifically adapted hair cells. To the opposite, no viable magnetic map receptor is possible within this system.