timberland sunglasses uk Animal magnetism how animals use earth
WHY CHICKENS? That was the question. To biologist Dr Ursula Munro, a fellow scientist’s suggestion they investigate whether chickens could sense the Earth’s magnetic field seemed nonsensical.
“I ridiculed the idea,” says Ursula, an ecologist and expert on animal behaviour at the University of Technology, Sydney. “A built in magnetic compass makes sense in migratory birds, which must find their way across huge distances. But chickens? They don’t move far; why would they need it?”
Since the 1960s, scientists have confirmed that more than 20 migratory species of bird use the Earth’s magnetic field to help them find their way. In the early 2000s, Ursula herself found that the tiny Tasmanian silvereye navigates magnetically during its annual migrations up the Australian coast.
It seemed that species that took part in long haul travel were uniquely equipped for this task. Which was why, in 2004, Ursula was dismissive when Dr Raf Freire, then at the University of New England, NSW, suggested testing chickens to see if they too had the knack. Nevertheless, she agreed to collaborate with Raf on a series of experiments using young chickens.
What emerged from that research astonished not only Ursula but also the wider scientific community. “I was gobsmacked,” she says.
Early theory of bird navigation
One of the first people to suggest that inanimate objects could exert a force on living creatures was Franz Mesmer, an 18th century Austrian physician, although his theory of ‘animal magnetism’ soon fell out of favour.
Then, in the 1950s, scientists noted that caged European robins, which normally migrate southward in autumn, would assemble at the southern end of their cages in the appropriate season.
This and subsequent investigations by him and his wife, Roswitha, proved beyond doubt that birds can detect magnetic fields and use them to orient themselves.
These days we know that birds are not the only creatures that can do this. Snails, fruit flies, bees, butterflies, salamanders, newts, lobsters, frogs, bats, salmon, trout, whales, sea turtles and the mole rat of East Africa can,
While the evidence that these creatures can sense and use the Earth’s magnetic field is pretty conclusive, how exactly they do it and what organs they use to do so are the subjects of fiercely competitive research around the world.
How birds use magnetism to navigate
Raf, who now lectures in animal behaviour at Charles Sturt University in Wagga Wagga, NSW, says this makes it an exciting field to be working in. “A lot of important discoveries are happening very quickly,” he says. “You can feel the race to try and nail down what the mechanisms are.”
Scientists speculate that animals have two distinct magnetic sensing mechanisms in their bodies, each with a different function. In birds, both of these magneto receptors are believed to be in the head. It’s not known if they work simultaneously, independently or jointly.
However, as the Wiltschkos suggested in a 2005 paper, one of the receptors probably acts as a magnetometer which measures magnetic intensity and the other as a compass. In animals, this action could be based on two magnetically sensitive forms of iron oxide magnetite and maghemite. Particles of these minerals in an animal’s body align themselves with a magnetic field, affecting cells around them and thus firing off signals to the brain.
Many bird species have tiny bundles of these minerals in the upper beak region, and Raf has found that anaesthetising the nerves in this region impairs a bird’s capacity to sense a magnetic field. Different geographic features and zones have differing magnetic intensities.
As a bird flies over them during a migration, its magnetometer may detect these anomalies and enable it to compile a mental map of magnetic signposts for future use.
The second mechanism, the compass, may indicate direction. But unlike a conventional compass, it doesn’t distinguish between north and south; it tells the bird only where a pole is and where the equator is.
As Ursula says: “It doesn’t matter whether a bird is in the Southern or Northern hemisphere, in autumn it knows it needs to go equator wards.”
How birds us magnetism as a compass
Here’s how scientists think the avian compass works. The organs for this mechanism are believed to be in the right eye, but perhaps in the left eye also. Research indicates this magneto receptor may be based on pigment proteins in the retina known as cryptochromes.
The Earth’s magnetic field seems to induce a chemical reaction in these proteins when certain light wavelengths (mostly blue) strike the retina. This results in signals being sent from the eye to the brain via the optic nerve. Some scientists believe this may mean a bird can actually see the magnetic field.
The Earth is one vast magnet, with its magnetic poles situated close to the geographic poles. Magnetic field lines extend away from the Earth at the South Magnetic Pole, travel north and plunge back into the planet at the North Magnetic Pole. So at the poles, the lines appear vertical, at the Equator they appear horizontal, and in between they align at varying inclinations.
The precise nature of the chemical reaction in cryptochromes is believed to vary according to the angle of the magnetic field lines their inclination as they pass through the eye. Inclination, therefore, is a strong pointer to direction. Angled lines may indicate that a bird is close to a pole; horizontal lines may mean the bird is at the Equator.
For the moment, what the magnetic field looks like to a bird is anybody’s guess. “The magnetic compass is a side function of the eye, and magnetic information is primarily mediated by the visual system,” Roswitha Wiltschko says. “Yet birds must separate the magnetic from the visual information somehow. How birds perceive this information is impossible to tell.”
Illustrators have tried different techniques to depict what they imagine a bird may see. Some have shaded parts of the visual image in grey. Others have varied the colour intensity, with some parts of the image being brighter than others.
Raf, on the other hand, suggests the information might appear on the image as dots or blotches. Overall, the effect might be like the heads up display projected onto the windscreen of a jet fighter to give the pilot vital information.
There’s a theory that by swinging its head from side to side and thus changing the angle between the magnetic field lines and its eye a bird generates a moving visual impression of the magnetic field. Some migratory birds scan the horizon in just this way before setting out on a long journey.