by Hinal Kharva – Have you ever played with a magnet and coins? Do you know how many animals can detect the magnetic field and why they do?
When I hear of the magnetic field detection in animals, birds come first to my mind. Some of them are known for their ability to use Earth’s magnetic field in navigation and orientation. In addition to birds, sea turtle and freshwater fish (trout) also use magnetic cues for migration and foraging (search for food).
Very few invertebrates like the fruit fly, monarch butterflies, cockroaches and honeybees are known to perceive magnetic cues. The question is why do some insects have this ability? In the case of honey bees, it’s been studied for many years that they use different senses to navigate and come back to their hives after foraging for long distances. One of the exceptional senses is their property of magnetoreception. They were also reported to use magnetic cues in comb building in a new hive (DeJong, 1982). One of the biggest challenges in neural and sensory ecology is to study and understand the mechanisms by which animals sense magnetic fields.
Recent studies suggest that there are two models of the mechanism of magnetoreception system in terrestrial animals. One is a light-dependent magnetic sense also known as the chemical magnetoreception system, where the signal is transmitted through a protein called cryptochrome (which are ultraviolet/blue light photoreceptors). Cryptochromes are usually present in the eye of an animal and they are known to have a role in circadian clock (day-night cycle). From the available limited information, it is suggested that cryptochrome can absorb photons of certain wavelengths and generate radical pairs, which in principle, form the basis of a compass magnetoreceptor. Some of the migratory birds, American cockroach, and monarch butterfly are few examples, which uses light-dependent magnetic detection. The second model suggests that animals can detect magnetic fields through the ferromagnetic crystals of magnetite Fe3O4 located in their bodies, light independent, also known as magnetite-based magnetoreception system. These magnetic crystals change their distribution under the influence of magnetic fields. Magnetotactic bacteria, algae and some of the migrating birds are reported to use this type of magnetite system (Hsu, C.Y 2007).
The magnetoreception of the honey bee has been studied for a long time. However, the exact route of the magnetic signals is still not clear. Researchers have identified that cryptochromes are present in honeybee brain as well but it has not been well studied that they use it for magnetoreception. To further investigate they dissected different body parts of the bee mainly head, thorax and abdomen and check out their magnetic properties using a technique called superconducting quantum-interference device magnetometers(SQUID)(Gould J, 1978). Which revealed that abdomen of bees was more magnetized than other parts. Studies show that there are iron granules present in abdomen region which can detect magnetic fields. To understand the source of the signal scientist trained honeybees with classical conditioning. Here bees were trained by associating the magnetic stimulus with a sugar reward. Results of this experiment show that when there was no neural connection between these granules and brain, bees couldn’t respond to external magnetic stimulus. (Liang C.H., 2016).
These observations open up new questions of how this signal gets transmitted and interpreted in the neural system. Can honeybee be a good model to study magnetite-based magnetoreception? Till further research unravel the pathways of magnetoreception, it will remain a fascinating question “why do bees carry magnetic granules in their butt”.