Magnetoreception in animals

mals can detect Earth’s magnetic field has traveled the path from ridicule to well-established fact in little more than one generation. Dozens of experiments have now shown that diverse animal species, ranging from bees to salamanders to sea turtles to birds, have internal compasses. Some species use their compasses to navigate entire oceans, others to find better mud just a few inches away. Certain migratory species even appear to use the geographic variations in the strength and inclination of Earth’s field to determine their position. But how animals sense magnetic fields remains a hotly contested topic. Whereas the physical basis of nearly all other senses has been determined, and a magnetoreception mechanism has been identified in bacteria, no one knows with certainty how any animal perceives magnetic fields. Finding this mechanism is thus the current grand challenge of sensory biology. The problem is difficult for several reasons. First, humans do not appear to have the ability to sense magnetic fields. Whereas most nonhuman senses, such as polarization detection and UV vision, are relatively straightforward extensions of human abilities, magnetoreception is not. As a result, neither intuitive understanding nor the medical literature on human senses provides much guidance. Another complicating factor is that biological tissue is essentially transparent to magnetic fields, which means that magnetoreceptors, unlike most other sensory receptors, need not be located on an animal’s surface and might instead be anywhere in the body. That consideration transforms a routine two-dimensional visual inspection into a three-dimensional search requiring advanced imaging techniques. Another impediment is that large accessory structures for focusing and otherwise manipulating the field—the analogs of eardrums and lenses—are unlikely to exist because few materials of biological origin affect magnetic fields. Indeed, magnetoreception might be accomplished by a small number of microscopic, possibly intracellular structures scattered throughout the body, with no obvious structure devoted to magnetoreception. Finally, the weakness of the interaction between Earth’s field and the magnetic moments of electrons and atoms, roughly one five-millionth of the thermal energy kT at body temperature, makes it difficult to even suggest a feasible mechanism. The weakness of the field does provide one major advantage to researchers: It greatly limits the list of possible physical detection mechanisms. Any suitable mechanism would presumably have to involve a very sensitive detector, amplification of magnetic interactions, or isolation from the thermal bath. Interestingly, the three main mechanisms that have so far been proposed—electromagnetic induction, ferrimagnetism, and chemical reactions involving pairs of radicals—are each based on one of those designs. The electromagnetic induction hypothesis, for example, is based on the extremely sensitive electroreceptive abilities of some marine species. The various hypotheses involving magnetite or other ferrimagnetic materials are based on the powerful interaction of such materials with magnetic fields. Finally, the radicalpair mechanism relies on the relatively efficient isolation of electron and nuclear spins from other degrees of freedom. Different animals may detect magnetic fields in different ways, and behavioral experiments and microscopic examinations of possible magnetoreceptors have both yielded results that are consistent with all three mechanisms. Nevertheless, a magnetoreceptive organ has not yet been identified with certainty in any animal. In this article we discuss the physics of the three main mechanisms that have been proposed and highlight some of the critical evidence in support of each.