I am reading Ed Yong’s “An Immense World” about how animals perceive the world. It feels like this is the fifth or sixth post on the topic. Currently, a very intriguing chapter about magnetoreception. The dull parts remain off-stage, I only share the most roof-shattering discoveries on the topic with you. Not always from “recent years,” but we didn’t cover biology very thoroughly in school.
Generally, everyone knows about pigeons that manage to arrive exactly where needed from any point. Interestingly, as of today, no one understands the physics and biology of this phenomenon definitively. There are three theories, which I’ll discuss later.
It turned out that quite a lot of animals, not just birds, can navigate by Earth’s magnetic fields. Even humans have cells potentially telling the brain what’s needed, but evolution, with our speeds and the distances we travel, deemed it unnecessary. But whales, turtles, all sorts of flying creatures, even mice, and foxes demonstrate it quite well.
Recall that only in 2014 did one of the Ig Nobel prizes go to Czech researchers who demonstrated that dogs prefer to defecate aligning themselves along geomagnetic field lines, from north to south. My observations don’t confirm this, so I don’t have a prize, but they do.
Testing on birds was the easiest because they are small and the phenomenon there is easily reproducible. It turned out that birds have a predictable moment, having a German name Zugunruhe, “migratory restlessness.” As the time approaches, they start going crazy and fly wherever they must go. It’s not a signal from the heavens, of course, but something like that. And at such moments, scientists come and start wrapping birds in magnetic coils (Hermholtz coils) and noting in which direction the legs left more traces (Merkel & Wiltschko).
But with whales, it doesn’t work like that. However, whales leave clear signs—carcasses on the shore. In English, this is called “to beach themselves.” So, it turned out after 33 years of observations (Granger/Walkowicz), that these self-killings happen during the peaks of solar storms. Although this doesn’t prove that whales have a compass, there is a sense in which forces generated by the planetary layer of molten metal collide with forces unleashed by the fiery star, jointly influencing the minds of wandering animals, and determining whether they find their path or lose it forever.
Something else turned out interesting about turtles. It’s known that sea turtles crawl up on the shore to lay eggs, far enough from the water, but so that the newborns still have a chance to make it to the ocean. And these little ones, once born, quickly figure out which direction to run. And they run not just towards the water—at least because they simply don’t see it at first. By the way, this run is quite dangerous, as predators know the spot well (it’s the same one every time), and estimates suggest that one in 10,000 turtles survives. Specifically, Florida ones head for the North Atlantic Gyre—a formation in the ocean spanning several million square kilometers, swirling clockwise. On this planetary-scale carousel, they ride for 5-10 years and grow into adults. And guess where they lay their eggs? In the same place! That is, after 5-10 years of wandering the vast ocean, they pretty well understand where their spot is, where they managed to survive at least once.
So, what was discovered (Lohmann, 1991)—the turtles have not just a compass. A compass alone would show the relative direction, but for what was described above, you need a full GPS. Well, they have one. Research has shown that their brains sense two parameters of the magnetic field— the inclination of magnetic lines and intensity. Overall, these two parameters are more or less enough to figure out “where I am.” For example, at the poles the inclination is 90 degrees, and at the equator – zero. Intensity is generally higher at the poles, lower at the equator, but there are also various anomalies in the Earth’s crust affecting it. In the end, turtles figured out how to use this, and experience has shown that even from a completely new place, they correctly determine where to crawl and then where to swim.
Meanwhile, over the last 83 million years, the Earth’s geomagnetic field has changed places 183 times. But the genetic programs, apparently, are adapted to this.
Since they don’t have maps of Earth in their heads, only rules on where to swim under various combinations of inclination and intensity, and because the magnetic field is very, very weak, turtles have to swim quite a long time not entirely in the right direction to figure out how to correct their course. But in the end, they arrive where they need to.
How these sensory organs work isn’t fully understood. There are three theories. They do not contradict each other, and most likely, all three are valid.
According to the first, sensitivity to the magnetic field may be provided by magnetite (Fe3O4)—a ferromagnet, meaning a substance in which the magnetic moments of atoms are ordered and persist in the absence of a significant magnetic field. Their direction depends on the polarity of such a field (if it exists). In 1970, bacteria were found containing crystals of magnetite. I don’t know why bacteria need the south, but this is definitely proven. Well, if such magnetite needles can be combined with sensory cells, it’d make a biological compass. But so far, such haven’t been found. There’s a noticeable amount of magnetite in the cerebellum and in the brainstem, but these are not receptors, and they are located deep in the tissue—whereas it would logically be best to place magnet-sensitive structures closer to the surface to increase the number of signals they can perceive. Regular domestic chickens have deposits of iron minerals in the dendrites at the top part of their beaks, enabling them to be capable of magnetoreception.
According to the second, the culprit is electromagnetic induction. This is closer to electroreception in sharks and rays—I wrote about this two days ago, take a look, it’s interesting. Essentially, it’s about the generation of induced electric signals in organisms moving in a magnetic field, signals that could potentially be read by cells. But this second theory doesn’t work anywhere except in water—induced electric fields are too weak for detection due to the lower conductivity of air compared to water.
The third theory is the most complex, but seemingly the main one. Here comes into play quantum physics (skip if scary). It involves the assumption that the geomagnetic field can influence biochemical reactions inside the retinal receptor cell, similar to those that lead to the detection of light by visual pigments. Cryptochrome cells are responsible for detection. (Interestingly, these blue light-sensitive proteins have the prefix “crypto” from cryptogamic organisms—ferns, on which many studies of blue light were done, and cryptogamic organisms are called so in Russian—Taynobrachnie—because Carl Linnaeus didn’t find flowers—sexual organs, and the name then stuck.) So, the theory is that cryptochromes, influenced by light, can form pairs of radicals—molecules with unpaired electrons. These unpaired electrons are sensitive to magnetic fields, allowing cryptochromes to act as magnetic receptors. Changes in the magnetic field can affect chemical reactions within these radical pairs, which in turn can trigger signaling pathways in cells, informing the organism about the direction of the Earth’s magnetic field. Also, research has shown that birds’ visual cortex gets excited when this “GPS” is turned on. Apparently, they conditionally “see” the magnetic field overlaid on what they see in the visible spectrum—RGB+ultraviolet. Of course, looking through their eyes and brain will never be possible, but there are indirect signs that this is the case.
Hi, I'm Rauf Aliev. 