Magnetoreception

How do migrating species find their way to a specific beach or island or cove or rock hundreds of miles away? One possibility is by using the Earth’s magnetic field as a guide. It’s a controversial theory because the magnetic field is weak and scientists haven’t unambiguously determined if organisms have the appropriate magnetoreceptors. But it means a wide-open field of study and some clever ideas have been posited. I have no expertise in this area so I’m learning all this from Chapter 12 of Ed Yong’s marvelous An Immense World.

 

We tool-inventing humans made the compass. It can be as simple as a lodestone composed of the natural mineral magnetite (Fe₃O₄). Modern compasses are composed of a combination of materials – what goes into making them is quite interesting (but not the subject of this post)! But the lodestone or compass is a tool separate from our body and our evolutionary instincts. For an organism to utilize magnetoreception, the detector must be an integral part of the body and influence behavior directly.

 

There is evidence that at migration time, birds in captivity try to move in a particular direction. By placing their enclosures within an artificially-generated magnetic field, they can be fooled into moving in a different direction. Other organisms that seem to show magnetoreceptive behavior include monarch butterflies, brown bats, mole rats and sea turtles. How these senses have evolved even as Earth’s magnetic field has ‘flipped’ multiple times remains a mystery.

 

Loggerhead turtles seem able to ‘read’ a ‘magnetic map’ of their surroundings. Impressively, they can detect both the inclination and the intensity of Earth’s magnetic field. As Yong writes: “… most spots in the ocean have a unique combination of the two. Together they act like coordinates, much like latitude and longitude.” The turtles have “both a compass to tell them which way to go and a map to tell them where they were. Only with both senses can they change direction at the appropriate places.” But because the field changes, an organism has to keep moving because “magnetic information isn’t especially accurate over short [distances]” but can be effectively used over long distances.

 

Yong discusses the three possible hypotheses for magnetoreception. The first is magnetite, and it’s known that some bacteria utilize them for orientation. But there isn’t any smoking gun evidence for appropriate receptors in more complex organisms. The second is via electromagnetic induction. Electric fish can induce an electric current. If there were receptors that can detect the interaction between the generated current and Earth’s magnetic field, it could be a compass of sorts. But while this might work in water (a conductive fluid), it wouldn’t work in air. How might birds, bees or bats find their way? One suggestion is that the inner ear might contain detector proteins, but once again no smoking gun.

 

The third hypothesis is intriguing. When photons hit certain molecules such as flavins, they can create a radical pair – two unpaired electrons. The progress of this reaction, more specifically the spin-flip rate, is affected by the presence of a magnetic field. As a quantum chemist, I find this idea attractive. I was pleasantly surprised to learn that this was proposed by Klaus Schulten (a famous computational biophysicist, i.e., essentially an interdisciplinary chemist) whose idea was ignored for twenty years until cryptochromes were discovered in animal eyes and there seems to be a link between certain photon wavelengths to the visual centers of the brain in songbirds. Yong writes: “Songbirds might be able to see Earth’s magnetic field, perhaps as a subtle visual cue that overlays their normal field of view.” Still, no smoking gun.

 

Yong ends the chapter with a cautionary note: “The study of animal behavior is also plagued by human behavior. People tend to see the patterns they want to see… Scientists are no less prone to such biases… but they do have ways of preventing those biases from interfering with their work… Making matters worse, the quest to find the elusive magnetoreceptor has become a race. The promise of glory and prizes for the winner has created incentives for fast research and big claims, rather than careful and methodical work… Even if scientists do everything right, they might still flounder because magnetic fields are imperceptible… You might be exposing animals to erratic or unnatural fields, and you’d have no idea unless you were constantly doing checks with the highest-quality [and very expensive] equipment.”

 

It turns out to be very difficult to replicate magnetoreception studies because the detected field is weak, and the process is inherently very ‘noisy’. We don’t even know the appropriate window of time to make these measurements since we don’t know how the organisms are detecting and responding to the signal. Do they time-average the signals? We don’t know. It’s very likely that migrating animals use more than one sense. Magnetoreception may play a role but so would other environmental cues. That’s a good reminder to us: Except for the blind, we humans rely dominantly on vision. But often below our conscious notice, our brain is integrating signals from sound, smell, touch and other cues. We’ve evolved to do so for good reason. And to detect magnetic fields? We make tools!