Not only migratory birds, but also turtles and salmon safely find their way over thousands of kilometers to their destination. Animals owe this ability, among other things, to their magnetic sense, with which they can orient themselves in the Earth’s magnetic field. An interdisciplinary collaboration between biologists and physicists has now shown that a metal in animal cells is responsible. In an interview with Welt der Physik, Uwe Hartmann of the University of Saarland reported how researchers discovered and how animals’ sense of direction varies.
Physicist: How can animals orient themselves using the Earth’s magnetic field?
Uwe Hartmann: It has long been suspected that magnetic particles in animal cells are responsible for the sense of direction. The mineral magnetite in particular has been an interesting candidate for decades, as it enables animals to sense magnets. Magnetite nanocrystals are found in a variety of life forms, from simple bacteria to insects and fish to mammals. However, the way in which this mineral eventually leads to a nerve impulse within the sensory cells that tells the animal: “This is north” has not yet been elucidated. This is currently the subject of extensive research. However, my colleagues and I have now demonstrated that magnetite indeed plays the critical role.
how did you know?
We worked with a multidisciplinary team that included researchers from sensory physiology, genetics, and developmental biology as well as geophysicists and nanophysicists. We have been searching for a long time about the magnetism of salmon, which is located in the olfactory organ of animals. We first succeeded in separating those cells in tissues with a high concentration of magnetite from other cells. We then specifically analyzed the magnetite crystals in these cells using highly sensitive physical methods, including magnetic resonance, atomic force and magnetic force microscopy.
What did this analysis reveal?
We found that magnetite is present as nanocrystals, which are about 30 nanometers in size in salmon. In other animal species it can also be somewhat smaller and in some bacteria it is only about ten nanometers in size. Interestingly, our analysis showed that the crystals in salmon differ from the shape of the so-called magnetic bacteria. In the latter, they are arranged along a chain, allowing the bacteria to orient themselves in murky waters and seek refuge in the mud if they are stirred up. In contrast, the magnetite crystals in salmon are arranged in grape-shaped groups. We don’t know yet why. Nor is it ordinary magnetite, which requires temperatures as high as several hundreds of degrees Celsius to produce. Instead, these particles are permeated with proteins. So these crystals are formed through a certain type of biomineralization that takes place at ambient temperature.
Is there a genetic relationship between different species that take their directions from the Earth’s magnetic field?
Genetic analysis has revealed surprising evolutionary links between living things. Because all organisms that use these magnetite nanocrystals — from archaea and bacteria to mammals — have the same genes. Archaea and bacteria are considered prokaryotes, which means that they do not have a cell nucleus. However, all higher organisms are eukaryotes and have a cell nucleus. Presumably, early eukaryotes evolved to include magnetically sensitive prokaryotes, or at least their genes. Thus, the magnetic sense of salmon and other higher animals did not develop genetically by itself, rather the genes to produce magnetite nanocrystals arose in prokaryotes sometime over a billion years ago, and then picked up by eukaryotes.
And how do animals use magnetite crystals for navigation?
How it all works in terms of sensory physiology within cells is not entirely clear yet. Because the forces to which magnetic nanoclusters are subjected in the Earth’s magnetic field are small and difficult to distinguish from thermal noise. An exciting thesis is that magnetic nanocrystals can activate ion channels in the cell membrane and thus generate nerve signals. However, it must also be said that it is not only the magnetic sense that allows complex species of animals to orient themselves over thousands of kilometers, so that salmon, for example, can safely find their spawning places in the upper reaches of the river. This is where the special memory ability comes into play, which also needs to be researched.