Turtles travel thousands of miles back across the sea just to lay eggs in the same sand?! Not only Turtles, many other animals and birds like Homing Pigeons, Fruit flies, many Mammals, even certain Bacteria and Molluscs have these awesome navigation skills. We don’t even remember where we had dinner last weekend.
Wondering how they do it? Let’s see.

Magnetoreception

Magnetoreception (also magnetoception) is a sense which allows an organism to detect a magnetic field to perceive direction, altitude or location. This sensory modality is used by a range of animals for orientation and navigation, and as a method for animals to develop regional maps. For the purpose of navigation, magnetoreception deals with the detection of the Earth’s magnetic field. – Wikipedia

How does magnetoreception work

Magnetite Infusion – The Mechanical Sensor

Crystals of Magnetite which are able to detect the Earth’s magnetic field have been found in the beaks of many birds, but whether these crystals are Superparamagnetic or just have single domain ferromagnetism is still unknown. They act as tiny magnets inside the body and just as the animal travels, they increase or decrease the force of magnetism they exhibit depending on the intensity of the external magnetic field. Still, with only this type of navigation, the bird remains unaware of the direction of the poles. Theoretically, it was assumed that these Magnetite crystals can help birds to detect directions and have an overview of their position on the geological map as well. But with certain experiments, it has been found that these Magnetite crystals transmit information related to the intensity of the external magnetic field only.

Superparamagnets behave much like standard paramagnets but with a much larger magnetic susceptibility. Superparamagnets have single domains, i.e. they have a uniform orientation throughout in a single direction. Whenever they experience an external magnetic flux, they tend to align with it and add up to that field. They have a magnetic permeability greater than one but not very high. In the absence of the external flux, they lose the magnetization because the aligned electronic spins are disrupted by the thermal motion in the lattice and their net field turns to zero. Whereas, single domain ferromagnets can retain the magnetization for a long time even when the external magnet is removed and are much difficult to remagnetize unless a strong magnetic field is applied.

The experimental results till now debunk the theoretical assumptions and state that the Magnetite crystals perform only one of many roles involved in navigation and just convey information regarding the magnetic intensity of the external field rather than its direction.

This makes us think of another mechanism which has to be working underneath because birds such as Homing Pigeons or animals like Turtles always end up in the exact same place where they started from.

Reason For A Separate Direction Indicator

In an experiment, alterations were made in the external magnetic field in order to observe if birds will move in the direction of the altered magnetic field or that of the original field which they would follow for actual migration. As the nature of the Magnetite crystals is not discovered yet, a strong enough magnetic field was applied in order to make sure that the single domain ferromagnets get remagnetized because superparamagnets can be remagnetized easily by a weak field too. The result was that the birds followed their original paths and were not deviated by any disturbances. If the directional analysis was based on the Magnetite crystals, then the birds would have changed their direction because the crystals are magnetic and align the direction of their magnetic field with that of the external field to some extent.

Traces have been found that the Ophthalmic branch of the Trigeminal nerve has been involved in perceptions of changing magnetic field and is still under research. It is observed to be connecting the Magnetite mechanism to the Brain. Look here.

Under the above-mentioned experiment, an anesthetic was also injected into the Ophthalmic nerves of some Homing Pigeons who were trained to react to a strong external magnetic pulse in a certain way. After injecting, the birds stopped responding to the magnetic field in the way they were taught. However, this did not cause any change in the direction they were following. This was an indication that the system for navigating direction functioned independently of the Magnetite-based mechanism.

Here is the complete experiment.

Protein Cryptochrome – The Biochemical Sensor

Where Magnetite detection is just a way to determine that the external field is changing, photochemical reactions are more accurate in acting as a compass to determine the poles. Cryptochrome is a flavoprotein found in the retinas of some birds and other animals. It is a magnetic molecule which is capable of absorbing blue light and producing Radical Pairs.

A singlet state of electrons is when the two unpaired electrons spin antiparallel to each other and nullify each other’s magnetic moment. Whereas, in a triplet state electrons have three possibilities to spin that can be either both electrons spinning upwards, both downwards or both in opposite directions, depending on the change in external Magnetic Flux. While singlet state is similar to that of the paired electrons, triplet state also has both the electrons having an angular momentum in the same direction which makes them exhibit a magnetic field. The external magnetic field influences the states of radical pair spin by causing transformations between the singlet and the triplet states while the direction of the field changes.

The radical pairs are formed when photons hit the respective molecules (here, Cryptochrome). Initially, these pairs are in a singlet state and have no net magnetic field. But, the transformation to triplet states occurs when these pairs interact with the external magnetic field. When the magnetic field is not that strong, these radical pairs remain in the singlet state because they do not get enough energy to maintain the magnetic moment in a certain direction by making their spins parallel. As soon as the external flux increases, the spins switch to triplet state and have parallel orientations.

Inclination Rather Than Polarity

 

The radical spinning is dependent on the direction of the magnetic field lines. It does not indicate in a straightforward manner about which pole to approach. Looking at the direction of the magnetic field lines of Earth, we find that they enter from the North in a downward direction and emerge out of the South in an upward direction and as we come near the Equator, these field lines tend to become horizontal. The spinning orientation of the radical pairs changes with a change in the direction of the external magnetic field. If the bird is traveling to the North, then it will develop a positive Magnetic Inclination in the radical pairs, whereas a negative Magnetic Inclination when it will travel toward South.

Different organisms with Magnetoreception and why it lacks definite explanation.

How It All Works Together

Magnetoreception is mostly based on hypotheses to think of the possible mechanisms that could be in function during navigation in birds and other animals. The brain interpretation system for magnetic detection has not yet been identified but it is believed that the brain has a specific region for this purpose. Similarly, even the two theories for Magnetoreception are not concrete and it is unknown how they coordinate in the process. This is because the Magnetite theory is just a positional analysis in the navigational process and does not indicate the direction. Also, the effects of the three spinning states of radical pairs  (parallel in the upward direction, parallel in the downward or antiparallel) are different from each other and the brain response is different too.
For the sake of knowledge, we have predicted how the magnetic fields enter the navigational system in animals but we still do not know how their Brains tell them to move in a certain way.

How Sea Turtles Navigate Across Vast Oceans

Sea turtles’ paths across oceans are one of the bizarre cases of animal navigation. In accordance with the recent experiments and observations, sea turtles migrating through open seas for various purposes have confirmed to be using geomagnetic fields to travel. While several groups of turtles were monitored after altering their magnetic sensation in different ways, although the paths they took disrupted minorly as compared to those of the unaffected turtles, there was no significant deflection whatsoever.  It was even after applying strong magnetic fields, depicting that turtles turned to other external cues when their magnetic reception was being altered showing that magnetoreception is just one of many strands involving navigation among sea turtles. And this applies to many other migratory animals as well.

Although magnets impaired navigational performance in the new study and perhaps in the earlier one, the magnets did not, in either case, prevent the turtles from eventually reaching their goals. This implies that, when magnetic cues are disrupted, the turtles can fall back on other sources of information such as celestial compasses, wave direction, or olfactory cues, in much the same way that blind and blindfolded people are often able to use non-visual cues to guide themselves~ Source: Science Direct

Other experiments:

Magnet-induced disorientation in hatchling loggerhead sea turtles

 

Humans and Magnetoreception

The same protein, Cryptochrome found in animals has been found in human retinas as well and is also observed to be highly active. But the problem is that humans do not have the pathway to transmit this magnetic reception to the Brain. Which is why the current results show that humans cannot possibly respond to Magnetic Fields although they have the chemicals that detect them.
Plants have the same case. They too possess Cryptochrome but do not have a noticeable response machinery. One more reason for no magnetic response in plants is that they do not show any complex or mobile processes such as Migration. Plants, however, use the Earth’s Magnetic Field for various other evolutionary purposes.

How Do Animals Find Their Way?
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