Have you ever anticipated an earthquake? Some people report that they have "sensed" a temblor before it struck. They may claim to have felt a "foreboding" that something was going to happen. When an earthquake then strikes, it is easy to retrospectively join the dots and attribute that vague sense of impending doom to the quake.
In some animals, however, there seems to be a genuine ability to sense the changes that occur before earthquakes.
Perhaps the first person to record this was the Greek historian Thucydides, in 373 B.C. Days before a massive earthquake hit the city of Helice, he says all manner of animals streamed out. Dogs, rats and weasels, they ran for the hills. (Snakes sensed the coming catastrophe too, and they slithered for the highlands.)
And so it goes on throughout history. In 2008, there was a big quake in Sichuan, southwest China. The dust had hardly settled before reports came out of animals "predicting" the quake. In this case — of course, it being China — it was giant pandas. The animals were said to have been acting strangely in the hours before the quake.
Let's take the reports at face value. What could be the scientific explanation for such changes in behavior?
A clue comes from a bit of good luck that befell some biologists who were studying toad populations in L'Aquila, in central Italy. To get an idea of what was driving the growth of toad populations, the researchers, from the Open University in Britain, were making careful daily census measurements of toad numbers.
Last year, on April 1, they were surprised to find that the numbers of male toads at their field site had suddenly dropped by 96 percent. At the breeding site on April 3, where males and females met to pair up, there were no toads at all.
Then, on April 6, there was a magnitude 6.3 earthquake whose epicenter was 74 km from the toads' breeding site.
The biologists carried on monitoring the area for toads. They found no fresh spawn at the site from the date that the earthquake struck to the date of the last significant aftershock. It was all very unusual for toads, who normally remain hanging around the breeding site like teenage boys around a girls' school.
Researcher Rachel Grant and colleagues made a series of measurements. They found that the change in the toads' behavior coincided with disruptions in the ionosphere, the uppermost electromagnetic layer of the Earth's atmosphere. Those disruptions were detected using very low-frequency radio soundings.
It seems that radon gas started seeping up from deep within the Earth's crust in the days leading up to the quake. This apparently disrupted the electromagnetic charges detected by radio waves in the ionosphere — and the change was seemingly sensed by the toads. The paper describing the finding has recently been published in the Journal of Zoology.
"Our study is one of the first to document animal behaviour before, during and after an earthquake," says Grant. "Our findings suggest that toads are able to detect pre-seismic cues such as the release of gases and charged particles, and use these as a form of earthquake early-warning system."
That's not going as far as to say that the toads knew there was an earthquake coming — but they did detect something, and that something might well have caused them to flee, so saving their lives.
Radon is what's known as a noble gas, like helium: it is colorless, tasteless and odorless. It is also radioactive, but it is hard to understand how the toads could have directly detected the gas.
Perhaps, then, they detected the perturbation of the ionosphere.
Animals do have a wide range of senses that we don't. Migrating birds, for example, use the Earth's magnetic field to navigate. Even some bats, more renowned for their ability to fly using sonar, can tap into the magnetic field — as can some fish, including sharks and, perhaps, catfish.
These fish, known in Japan as nekogigi, have "whiskers" through which they are able to sense the tiniest movements of prey. Japanese legends tell that catfish can sense a coming earthquake.
On the day of the Asian tsunami, Dec. 26, 2004, villagers in parts of Thailand noticed that buffalo on the beach suddenly pricked up their ears and stampeded inland and uphill. Could they have heard the approaching wave?
Dog owners attribute all sorts of abilities to their pets. But a scientific study of dogs in Vancouver, Canada, in 2000 found some evidence for a "supernatural" canine ability. In the study, conducted by Stanley Coren from the University of British Columbia, some 200 dog owners had been routinely recording their dogs' behavior every day. But then, on one day in particular, more than half of them reported that their dogs exhibited strangely hyperactive behavior.
It turned out that there had been an earthquake that day, some 240 km to the south, in Washington State. Perhaps the dogs had picked up high-frequency noise of the quake.
Most of the "evidence" for animals having been able to somehow predict earthquakes is anecdotal. But it's no less intriguing for that, and certainly worth investigating. If it turns out that, say, toads can detect changes in the magnetic field of the ionosphere, then it might give us some new ideas for earthquake prediction. However, I can't imagine that everyone will start keeping toads as predictive pets . . .
Follow Rowan Hooper on Twitter at twitter.com/rowanns. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is "Hito wa Ima mo Shinka Shiteru (The Evolving Human)." Readers with any light to cast on animals' ability to sense imminent earthquakes are welcome to tweet their observations and/or information.