Navigation is defined as the act of an animal "moving on a particular course, or toward a specific destination, by using sensory cues to determine direction and position" (Hill et al. 2016). Various navigational systems exist to explain the phenomenon of how animals are able to navigate and migrate long distances without getting lost. With these navigational systems, there exists sensory receptors that aid in navigation. One of these navigational systems include usage of the magnetic field; However, there is no primary receptor in detecting the magnetic field as there is still uncertainty around the subject (Johnsen & Lohmann 2005). Below the focus in on the different theories surrounding navigation using the magnetic field.
Types of Sensory Receptors
There are various types of receptors that aid in navigation and two commonly discussed examples include electroreceptors and magnetoreceptors. Electroreceptors refer to the receptors that are able to detect electric fields in water (Nießner et al. 2011). On the other hand, magnetoreceptors refer to the receptors that can detect the position or change of the animal relative to the magnetic field of the Earth.
Magnetoreceptors
There is a lot of debate and ongoing research around the idea of magnetoreception and electroreception and its uses for navigation in animals. With continuous changes in knowledge, there are main hypotheses that dominate the explanation for how some animals are able to navigate long distances. The presence of crytochrome in the retina of some birds is said to be able to detect magnetic field for compass orientation (Nießner et al. 2011). The ultraviolet and violet cones in the eyes are capable of detecting magnetoreceptors when the domesticated chicken (Gallus gallus) and robins (Erithacus rubecula) are in flight.
Another theory is the presence of iron in the animal's which help with magnetic field use for navigation. This theory has gained much support and evidence as many animals were said to have iron or magnetite compounds in their systems (Shaw et al. 2018). In the article released by Shaw et al. in 2018, there was evidence of iron present in the abdomen of the honeybee (Apis mellifera) which was determined to be crucial in their magnetic field navigation. On the abdomen of the honeybee, the magnetoreceptors were identified to be present in the "hairs on the anterior dorsal region" giving solidifying evidence for the presence of magnetoreceptors (Shaw et al. 2018).
This idea of having iron or magnetite present was further supported by Kirschvink et al. in 2001 where the authors state how magnetoreception is similar to the process of "other sensory modalities" but with an added complexity where magnetoreceptors are able to "monitor the direction of the magnetic field and "respond of the variations in field intensity". The author examined the idea of ferromagnetic, where a "magnetite or greigite (Fe3S4)" produces "a magnetic moment large enough to rotate the cells passively into alignment with the geomagnetic field" (Kirschvink et al. 2001). These magnetite containing cells then relate to the corresponding nerves which completes the sensory process. There is further evidence for this concept of the magnetoreceptors as mollusks are described as having hardened teeth that consists of magnetite (Kirschvink 1997). In further research, the author states that magnetite are naturally found in in “insects, birds, fish and even humans” as well as in bacteria and protists.
The mechanism behind this sensory process includes that of the magnetic stimuli being interpreted as having the capacity to "activate neurons in the caudal vestibular nuclei" which induces a voltage in the semicircular canal of the inner ear of pigeons (Nimpf et al. 2019). This aspect shows correlation between the magnetoreceptors and the hearing sensory organs, adding to the idea that sensory systems work in conjunction with other systems of physiology.
Some other examples of animals that partake in magnetoreception are "sharks, skates, [and] rays" (Molteno & Kennedy 2019). These animals "can detect changes in the geomagnetic field" by utilizing the "magnetite based magnetoreceptors whose primary function is to measure the geomagnetic field for the purposes of navigation" (Molteno & Kennedy 2019). These animals are also able to orientate themselves by magnetic induction in which the “movement through the geomagnetic field induces currents” in the sensory system. This allows for them to have a compass sense and with the "constant electrosensory 'chord'" of "vestibular frequency" the animal is able to swim consistently with reference to the magnetic field.
Electroreceptors
Electromagnetic detection consists of analyzing "small electrical currents that are generated when an animal moves through the Earth's magnetic field" (Johnsen & Lohmann 2005). This mode of navigation and magnetic field detection "requires a well-developed electrosense" and to live in sea water such as sharks and few marine fish. It has been experimentally determined that sharks react to electrical stimuli performed to imitate the behavior of "bioelectric field of a flatfish" which could potential be a method sharks catch their prey using electromagnetic detection (Kalmijn 1971). While the use of electromagnetic detection might vary in circumstances, the mechanism for this sensory process remains the same.
Electroreceptors are found in the vestibular hair cells, found in the larger system of the ear (Nimpf et al. 2019). This is the primary way pigeons detect the magnetic field. The "semicircular canals" rely on the "voltage-gated calcium channels in a population of electrosensory hair cells". These hair cells are then able to transfer this information to the brain where it is interpreted for various uses.