By detecting the Earth’s magnetic field, numerous animals are known to navigate, including birds, bats, eels, whales and, according to some reports, maybe even humans. The precise mechanism at play in vertebrates is, however, not well understood. It is the product of a symbiotic relationship between animals and magnetic field-sensing bacteria, one theory suggests.
The leading theory entails chemical reactions by what is called the radical pair process induced in cells. Essentially, electrons will hop between them and their neighbors if those molecules are excited by light. That can produce pairs of molecules, known as a radical pair, with a single electron each. If the electrons have corresponding spin states in certain molecules, they will undergo, slowly, chemical reactions occur, and if there are opposites reactions will occur more rapidly. Because electron spin states can be changed by magnetic fields, they could cause chemical reactions that alter the behavior of an animal.
In the living cells of animals with magnetoreception, proteins called cryptochromes are thought to be the molecules that undergo this radical pair mechanism. And now, researchers at the University of Tokyo have observed cryptochromes responding to magnetic fields for the first time.
Ikeya and Woodward, PNAS
The researchers zapped the cells with blue light so that they fluoresced, then swept a magnetic field over them every four seconds. Each time it went over them, the fluorescence of the cells dropped by about 3 1/2 percent.
The team says that this lowering of the observable glow is evidence of the radical pair mechanism at work. When flavin molecules are excited by light they either produce radical pairs or fluoresce. The magnetic field influences more of the radical pairs to have the same electron spin states, slowing down their chemical reactions and dimming the overall fluorescence.
Magnetite, which is similiar, has turned up in turtles and fish brains and even in the human brain, as Joe Kirschvink of the California Institute for Technology in Pasadena reported in 1992, and it is very sensitive to magnetic fields. Kirschvink and other fans say it can tell an animal not only which way it is heading (compass sense) but also where it is, like looking at a map. “A compass cannot explain how a sea turtle can migrate all the way around the ocean and return to the same specific stretch of beach where it started out,” says neurobiologist Kenneth Lohmann of the University of North Carolina, Chapel Hill. A compass sense is enough for an animal to figure out latitude, based on changes in the inclination of magnetic field lines (flat at the equator, plunging into the earth at the poles). But longitude requires detecting subtle variations in field strength from place to place—an extra map or signpost sense that magnetite could supply, Lohmann says.
Goes to show how little we understand of our world still, and how much there is still to find.