Every dog owner knows how seriously their pet takes the decision of where to do its business. First it will pace in circles, nose to the ground, searching intently for the perfect spot. Once selected, there are another few turns and a shimmy until, finally, it is ready to commit. If you’ve ever wondered what’s going through a dog’s mind as it does this, the answer could be something like the spinning of a compass needle. Dogs prefer to defecate with their spine aligned in a north-south position, and they use Earth’s magnetic field to help them position themselves.
It has long been suspected that animals such as turtles and birds use magnetoreception to navigate, but it is becoming increasingly obvious that many other animals sense magnetism too, seemingly when they’re doing very little. Insects like to align their bodies along a north-south axis, as do sleeping warthogs, fish in tanks, nesting and foxes on the hunt. So how are they are doing it and, more head-scratchingly, why?
Advertisement
Early evidence that animals align themselves in a particular direction was dug up more than 50 years ago by entomologist Purushottam Deoras. Called to a farm in a suburb of Mumbai with a severe termite infestation, including several mounds that had erupted inside the farmer’s hut, Deoras searched high and low for the queen – the key to eradicating the infestation. He eventually found her lying with her head pointing north, beneath a smattering of mounds that were orientated east-west. When Deoras looked at other colonies, he spotted that in almost every case, the queen aligned herself in this same direction.
more species exhibiting the same behaviour. That led Hynek Burda, a zoologist at the University of Duisberg-Essen in Germany, to wonder if larger animals might do something similar.
“Cattle across six continents showed a roughly north-south alignment”
In 2008, Burda and Jaroslav CervenĂ˝ at the Czech University of Life Sciences in Prague analysed Google Earth images of 8000 cattle across six continents and concluded that the animals tend to show a roughly . The finding was challenged in 2011, when Lukas Jelinek and his team at the Czech Technical Institute in Prague analysed a different set of Google Earth images and found no such alignment. Further research by Burda and others, however, concluded that alignment only shows up in low density herds, where there was less jostling and they were making their own decisions about which way to face. The team went on to find a similar phenomenon in deer, kept in plastic tubs at a Christmas market in Prague, and in sleeping .
Taking direction
Why they are aligning themselves in this way is still a mystery. “It could be that these animals just feel more comfortable lined up,” says Roswitha Wiltschko, an ornithologist at Goethe University in Frankfurt, Germany, whose work in the early 1970s confirmed that birds possess a magnetic compass.
A more interesting possibility, however, is that they have this compass for a reason. Like birds and sea turtles, a preference for north could come as part of a general navigation system. Or, perhaps, a natural alignment could streamline a herd in an emergency, or, in the case of predators, keep the pack running together. The same sort of logic might apply to the festive carp.
Red foxes, however, seem to have a more precise use for their magnetic sense. In 2011, Burda and CervenĂ˝ noticed that red foxes seem to prefer to face in a particular direction , regardless of wind direction. On closer inspection, it turned out that foxes not only favoured a pounce in a north-easterly direction, but were significantly more successful in catching their prey when they did so: 72 per cent of the time compared with just 18 per cent. The study concluded that the foxes may be using the relative strength of the magnetic field to help them compute the distance between themselves and their prey.
How animals sense the magnetic field is more difficult to explain, despite 50 years of research into the abilities of migrating birds. There are, however, several candidates. One is magnetite, a naturally occurring iron oxide that has been discovered in bacteria, and in birds’ beaks and just happens to be the most magnetic mineral on Earth. If this is a magnetic sensor then animals may literally feel the pull of north.
David Keays at the Research Institute of Molecular Pathology in Vienna, Austria, has a different explanation. In 2013, he searched for magnetite in the nerve cells of pigeons’ beaks, from where they were widely presumed to wire magnetic information to the brain. Instead, he found magnetite clustered in certain kinds of white blood cells. Even if these cells were able to detect a magnetic field, there would be no way that they could convey information to the brain on a useable timescale, he concluded. Keays’s team suggested another possible compass: in the inner ears of pigeons that contain large iron-rich granules.
“We are trying to work out whether these balls of iron have anything to do with the detection of magnetic fields,” says Keays. But he is cautious. “A bunch of people have claimed to have discovered the magnetoreceptor and they are all wrong,” he says.
It is also possible that animals sense the magnetic field with their eyes. Many species have proteins called in the retina, molecules that are light-sensitive, and may have pairs of electrons whose reactivity is affected by the magnetic field. If this is correct, the magnetic field may be something that animals see – perhaps as a shadow on their field of vision. This has been put forward as a potential explanation for how foxes sense the field, too. Perhaps the foxes turn until the magnetic field “looks” right before taking a leap at covered prey.
“Foxes that pounce towards the north-east catch prey 72 per cent of the time”
Last year, Can Xie at China’s Peking University and his colleagues added another element to this theory. A protein they called . “We believe the MagR/cryptochrome complex must play key roles in magnetoreception, magnetic alignment and navigation in animals,” says Xie.
It’s possible that everyone is right. “There are at least two mechanisms of how animals can sense the geomagnetic field,” says Wiltschko. Xie agrees. “Multiple mechanisms, including magnetite, radical pairs and MagR-based biocompass are certainly possible,” he says.
With no consensus yet on how various species are sensing the magnetic field, the list of species that can do it just keeps growing. Progress is slow, however, because magnetic alignment isn’t easy to spot, even if you know it is there.
In the dog study, for example, “there were some days where the alignment was totally messed up and other days where the alignment was really strong”, says Pascal Malkemper, a zoologist at the University of Duisburg-Essen who was part of the team that did the research. A closer inspection of the data revealed that the dogs faced north or south when the magnetic field was completely stable. If it was in flux, because of solar activity, their positioning looked random.
The same is true of migrating species. “If you change the magnetic intensity [in the lab] rapidly, the birds are disorientated at first,” says Wiltschko. Similarly, the north-south orientation of cattle when they graze beneath high-voltage power lines, presumably because the electricity interferes with their powers of magnetoreception.
Factors like these confuse the data. “You really need hundreds of thousands of measurements to get this alignment effect,” says Malkemper. But once these factors are controlled for, he says, there is no doubt that the effect is there.
If magnetic sense is common in the animal kingdom, then what about humans? It’s a controversial field, but some researchers suggest that our health and behaviour is affected by Earth’s magnetic field without us feeling a thing. One study, from 2008, found a correlation between a peak in suicides during periods when the magnetic field is most in flux, and another, in 2004, found that blood pressure varies with .
So far, the evidence is circumstantial and many remain sceptical about human magnetoreception, but we are certainly getting closer to solving the mystery of animal magnetism. “There is something big and exciting here that remains to be discovered,” says Keays. “I never know whether it’s going to happen tomorrow or if it’s going to be in 10 years’ time.”
This article appeared in print under the headline “Animal magnetism”
