Animal Magnetism Susanne Mesoy investigates the mechanisms of magnetoreception AS SPRING takes hold, I enjoy the warmth of sunlight on my skin, the sound of birdsong, and the fresh scent of grass outside. But I am entirely insensible to an enormous grid of energy that runs right through me on its path between the poles: the Earth’s magnetic field. Many mammals, birds, fishes, insects, plants, and even some bacteria can detect the Earth’s magnetic field. It is clear how this sense serves migratory species like birds and fish, though the advantage for plants is less obvious. Exactly how it works though, is widely debated. Determining the mechanisms of natural magnetoreception would not only increase our understanding of animal physiology and behaviour, but also pave the way for technological advances from the laboratory to the clinic: possible applications include biological manufacture of extremely precise magnets, and the development of noninvasive tools to monitor and control cells in the human body. Challenges of Magnetoreception | The Earth’s magnetic field strength is about 20-60 microtesla (less than 100 times the strength of a fridge magnet). The energy of a molecule interacting with this field is less than a millionth of its thermal energy at body temperature, and therefore far too small to affect normal chemical reactions. Even if the Earth’s field were stronger, most biological tissue is unaffected by magnetic fields, and so cannot detect them at all. This makes potential magnetosensory organs almost impossible to find, as they could be distributed throughout an animal’s body, rather than being restricted to the surface. To detect the tiny signal of the Earth’s magnetic field, an organism must either have hypersensitive detectors, be able to amplify the signal, or somehow circumvent the noise of thermal energy in the body. Three main mechanisms have been proposed for biological magnetoreception, each answering to one of these conditions. The first one has been observed in some marine animals, the second in bacteria, and the third has never been proven, but is theoretically appealing, and the hunt is on as we speak. Delicate Detectors | As some of us may remember from high school physics, a conductive wire moving through a magnetic field induces a current in that wire. Sharks and rays (together termed ‘elasmobranchs’), among others, exploit this principle to build exquisitely sensitive organs capable of picking up the tiny signal of the Earth’s magnetic field. These organs consist of hundreds of long tubes running from tiny skin pores into their body, filled with a conductive jelly. These act as wires, and at the end of each are the ‘ampullae of Lorenzini’ — collections of cells sensitive to voltage changes. These tubes are sufficiently sensitive to detect the voltage generated by a shark swimming through the Earth’s magnetic field, but that 6
Animal Magnetism
alone is insufficient for functional magnetoreception. One complication is that swimming forward would generate a DC current (electrons flowing one way) in these jelly tubes, but elasmobranch electroreceptors can only detect AC currents (where electrons flow back and forth). They might solve this by swaying their heads back and forth as they swim, thereby reversing the electron flow at every head turn, to generate AC currents. This might also filter out noise from ocean currents (which are also fundamentally conductive ‘wires’ moving through a magnetic field). To test these theories, scientists stuck small magnets (and nonmagnetic brass as a control) up the noses of captive rays, which gave some indications to the nature of their magnetoreception but was not definitively conclusive; as one review (Johnsen and Lohmann, 2008) sardonically noted, ‘sharks and rays are not ideal experimental animals’. Regardless, this mechanism is unworkable for most magnetoreceptive insects and animals, as it would require internal organs filled with conductive liquid that have not yet been observed outside marine animals. Strengthened Signals | Phytoplankton and bacteria have gone another way, building signal amplifiers that augment the Earth’s field strength. These amplifiers consist of chains of magnetic material (magnetite or greigite) that amplify the Earth’s magnetic field until it’s large enough to rotate the entire organism. The individual ‘magnets’ must be about 0.1–1 µm in diameter to fulfil their purpose, but one magnet that size cannot amplify the Earth’s field sufficiently: hence daisychains of exquisitely-sized magnetic particles. There are also reports of microbes acquiring magnetoreception by consuming magnetoreceptive bacteria, and of sustainable symbioses between magnetoreceptive bacteria and unicellular flagellates. Could this mechanism be used by insects and larger animals? While magnetite has been detected in many magnetoreceptive species, including honeybees, salmon, sea turtles, and some birds, specific magnetoreceptors have so far only been conclusively found in microorganisms. The discovery of these ‘mini-magnets’ in animals would be extremely technically challenging, if they do exist: they are too small to be seen with light microscopy, are dissolved by many common tissue preservatives, and their constituent iron is commonly found outdoors, in labs, and in organs. Creative Chemistry | The final option for sensing the Earth’s magnetic field is so far only theoretical, but the circumstantial evidence is undeniably attractive. The theory is this: when two transient radicals (atoms or molecules with an unpaired outer electron) are created at the same time, the spins of the two electrons are correlated. The chemical properties of these kinds Easter 2021