Magnetoception


From Wikipedia, the free encyclopedia

Magnetoception (or "magnetoreception") is the ability to detect changes in a magnetic field to perceive direction or altitude and has even been postulated as a method for animals to develop regional maps. It is most commonly observed in birds, though it has also been observed in many other animals including honeybees and turtles. Researchers have identified a probable sensor in pigeons: a small (dwarf), heavily innervated region of the skull, which contains biological magnetite. Humans have a similar magnetite deposit in the ethmoid bone of the nose, and there is some evidence this gives humans some magnetoception. [1][2]
Although there is no dispute that a magnetic sense exists in many avians (it is essential to the navigational abilities of migratory birds), it is a controversial and not well-understood phenomenon. Certain types of bacteria (magnetotactic bacteria) and fungi [3]are also known to sense the flux direction, these contain organelles known as magnetosomes for this purpose. In bees, it has been observed that magnetite is embedded across the cellular membrane of a small group of neurons; the theory is that when the magnetite aligns with the Earth's magnetic field, induction causes a current to cross the membrane which depolarizes the cell.


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Do humans have a compass in their nose?


By Dr Stephen Juan
Published Friday 17th November 2006 11:22 GMT
The Register

Who knows what there is to know about the nose? (http://www.theregister.co.uk/2006/11/17/the_odd_body_nose/)
Do humans have a compass in their nose?

Asked by Lee Staniforth of Manchester, UK


Some years ago scientists at CALTECH (California Institute of Technology in Pasadena) discovered that humans possess a tiny, shiny crystal of magnetite in the ethmoid bone, located between your eyes, just behind the nose.

Magnetite is a magnetic mineral also possessed by homing pigeons, migratory salmon, dolphins, honeybees, and bats. Indeed, some bacteria even contain strands of magnetite that function, according to Dr Charles Walcott of the Cornell Laboratory of Ornithology in Ithaca, New York, "as tiny compass needles, allowing them [the bacteria] to orient themselves in the earth's magnetic field and swim down to their happy home in the mud".

It seems that magnetite helps direction finding in animals and helps migratory species migrate successfully by allowing them to draw upon the earth's magnetic fields. But scientists are not sure how they do this.

In any case, when it comes to humans, according to some experts, magnetite makes the ethmoid bone sensitive to the earth's magnetic field and helps your sense of direction.

Some, such as Dr Dennis J Walmsley and W Epps from the Department of Human Geography of the Australian National University in Canberra writing in Perceptual and Motor Skills as far back as in 1987, have even suggested that this "compass" was helpful in human evolution as it made migration and hunting easier.

Following this fascinating factoid, science journalist Marc McCutcheon entitled a book The Compass in Your Nose and Other Astonishing Facts.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au (mailto:s.juan@edfac.usyd.edu.au)


Nature Reviews Neuroscience 6, 703-712 (2005); doi:10.1038/nrn1745


THE PHYSICS AND NEUROBIOLOGY OF MAGNETORECEPTION


Sönke Johnsen & Kenneth J. Lohmann about the authors
From Nature.com

Abstract

Diverse animals can detect magnetic fields but little is known about how they do so. Three main hypotheses of magnetic field perception have been proposed. Electrosensitive marine fish might detect the Earth's field through electromagnetic induction, but direct evidence that induction underlies magnetoreception in such fish has not been obtained. Studies in other animals have provided evidence that is consistent with two other mechanisms: biogenic magnetite and chemical reactions that are modulated by weak magnetic fields. Despite recent advances, however, magnetoreceptors have not been identified with certainty in any animal, and the mode of transduction for the magnetic sense remains unknown.

Summary

Behavioural experiments have shown that diverse animals can detect the Earth's magnetic field and use it as a cue for guiding movements over both long and short distances. However, whereas receptors for most other sensory systems have been characterized and studied, primary receptors involved in detecting magnetic fields have not yet been identified with certainty. This article reviews the three main mechanisms that have been proposed to underlie magnetoreception (electromagnetic induction, chemical magnetoreception and biogenic magnetite) and evaluates the evidence for each.

Electromagnetic induction involves detecting small electrical currents that are generated when an animal moves through the Earth's magnetic field. This requires a well-developed electrosense. Most induction models also require the animal to live in a conductive medium such as sea water. Although sharks and a few other electrosensitive marine fishes might plausibly rely on induction, no direct evidence has yet been obtained that they do so.

Chemical magnetoreception involves molecular reactions, the yields of which are modified by the direction and intensity of Earth-strength magnetic fields. All proposed reactions involve electron spins in pairs of radicals. At present, however, no radical pair reaction has been identified that is affected by magnetic fields as weak as the Earth's. Evidence consistent with a radical pair mechanism includes effects of light and radio-frequency fields on magnetic orientation behaviour.

The magnetite hypothesis posits that crystals of the magnetic mineral magnetite transduce magnetic field energy into physical forces that can be detected by the nervous system. In several animals, magnetite has been detected in anatomical locations that have been linked to magnetoreception, but unequivocal morphological or neurophysiological evidence for magnetite-based receptors has not yet been obtained.

All three of the proposed mechanisms are plausible from the standpoint of physics and, at present, the available data are insufficient to confirm or refute any of them. Different animals might rely on different mechanisms. Moreover, at least a few animals might use two different magnetoreception systems, one for sensing field direction and the other for sensing field elements useful for determining geographic position. Each system might be based on separate receptors with different underlying mechanisms.

Most magnetoreception research has been based on behavioural studies. Sustained efforts are now needed to exploit a wider range of modern neuroscience techniques. Such undertakings might be facilitated by the discovery that magnetic sensitivity exists in several favorable model systems, including zebrafish, the fruitfly Drosophila melanogaster and the mollusc Tritonia diomedea.

Author biographies
Sönke Johnsen is currently an assistant professor in the Department of Biology at Duke University, Durham, North Carolina, USA. He received his Ph.D. in 1996 from the University of North Carolina at Chapel Hill, USA, where he worked with William Kier on the optical design of decentralized visual systems in echinoderms. He then received postdoctoral fellowships from Woods Hole Oceanographic Institution, Massachusetts, USA, and Harbor Branch Oceanographic Institution, Florida, USA, where he studied the optical and sensory properties of oceanic species. His current interests include the interactions of light and magnetic fields with organic and non-organic materials, with an emphasis on their implications for sensory biology and ecology.
Kenneth Lohmann is a professor of biology at the University of North Carolina at Chapel Hill, USA. He received his Ph.D. from the University of Washington, Seattle, USA, and completed postdoctoral studies at the University of Illinois, USA, and the Marine Biological Laboratory in Woods Hole, Massachusetts, USA. He is trained as both a neuroscientist and a marine biologist, and has broad interests in neuroethology, sensory biology and animal behaviour. He is particularly interested in how sea turtles and other marine animals navigate long distances through the ocean, and how animals detect and exploit the Earth's magnetic field.

Cattle shown to align north-south


By Elizabeth Mitchell
Science reporter, BBC News
Link to BBC News

Have you ever noticed that herds of grazing animals all face the same way? Images from Google Earth have confirmed that cattle tend to align their bodies in a north-south direction. Wild deer also display this behaviour - a phenomenon that has apparently gone unnoticed by herdsmen and hunters for thousands of years.

In the Proceedings for the National Academy of Sciences, scientists say the Earth's magnetic fields may influence the behaviour of these animals. The Earth can be viewed as a huge magnet, with magnetic north and south situated close to the geographical poles. Many species - including birds and salmon - are known to use the Earth's magnetic fields in migration, rather like a natural GPS.

A few studies have shown that some mammals - including bats - also use a "magnetic compass" to help their sense of direction. Dr Sabine Begall, from the University of Duisburg-Essen, Germany, has mainly studied the magnetic sense of mole rats - African animals that live in underground tunnels. "We were wondering if larger animals also have this magnetic sense," she told BBC News.

Dr Begall and colleagues first decided to study the natural behaviour of domestic cattle. The researchers surveyed Google Earth images of 8,510 grazing and resting cattle in 308 pasture plains across the globe. "Sometimes it took hours and hours to find some pictures with good resolution," said Dr Begall. The scientists were unable to distinguish between the head and rear of the cattle, but could tell that the animals tended to face either north or south.

Their study ruled out the possibility that the Sun position or wind direction were major influences on the orientation of the cattle. Dr Begall said: "In Africa and South America, the cattle (were) shifted slightly to a more north-eastern-south-western direction. "But it is known that the Earth's magnetic field is much weaker there," she explained.

The researchers also recorded the body positions of 2,974 wild deer in 277 locations across the Czech Republic. Their fieldwork revealed that the majority of grazing and resting deer face northward. About one-third of the deer faced southward. "That might be some kind of anti-predatory behaviour," speculated Dr Begall.

Willy Miller - a Scottish cattle farmer - remarked: "I've never noticed that my cows all face the same way." Cows are social animals: "[They] all sit down before it rains [and] huddle together in a circle formation during blizzards. But from a cow's point of view, that's just sensible," he told BBC News.

Professor John Phillips, a sensory biologist from Virginia Tech University, US, commented that this sixth magnetic sense might be "virtually ubiquitous in the animal kingdom". He added: "We need to think about some really fundamental things that this sensory ability provides in animals." The challenge remains for scientists to explain how the animals behave in this way - and if Scottish cattle are the exception to the rule!

Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/7575459.stm

Published: 2008/08/25 21:04:01 GMT

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