Scientists can't predict earthquakes. But toads might be able to.
In the spring of 2009, Rachel Grant, a doctoral candidate in life sciences at London's Open University, was studying a population of toads in a large dry lake in central Italy. Common toads reproduce once a year, sometimes traveling great distances to gather at their breeding grounds, and Grant was looking at whether her subjects were using the cycles of the moon to coordinate their romantic encounters.
In the previous three years, she had watched the toads increase in number with the waxing of the moon. But last year was different. The moon grew from crescent to gibbous, and suddenly the toads were gone. "It went from there being 90 to 100 toads down to six, and then to one, and then zero," says Grant. A few mating pairs hung around, but after two days, they too left. "It was so dramatic, I was trying to think of reasons why they might have gone," she says. "I was at a loss. Did somebody come and disturb them? Did somebody run through with a tractor? But that didn't seem right. Toads get run over by cars all the time, and that doesn't make them run away."
Five days after the toads disappeared, she had a possible answer: an earthquake struck in the middle of the night. The 6.3-magnitude quake was the deadliest to hit Italy in nearly 30 years, killing roughly 300 people and leaving tens of thousands homeless. The medieval city of L'Aquila, which lay near the epicenter, was devastated. Villages nearby were also reduced to ruins. Grant, sleeping in a country home 45 miles (about 70 km) away, awoke to the walls of her room shaking. "Things were falling down, cracking. Everything was rattling," she recalls. The next day, her adviser, a professor of biology at Oxford University named Tim Halliday, e-mailed to make sure she hadn't been hurt. "I wrote to him, 'I'm O.K., but the toads are gone,' " she says. "He wrote back, 'This could be interesting. Why don't we look at it further?' "
It was one of the strokes of luck from which science is made. The seismic shift continued to set off aftershocks almost every day, but Grant stayed to count her toads. When a full moon rose three days after the quake, a few toads risked a return. But then their numbers dropped again, remaining low until two days after the last major aftershock a full 10 days after the first tremor. "It's never been reported to have happened before," says Grant. "Once they're breeding, then they're breeding. That's it."
It's not clear what caused the toads to scatter. Grant, whose research was published this week in the Journal of Zoology, says the toads could have reacted to changes in the earth's magnetic field, alterations in the ionosphere or spikes in the amount of radon gas in the water. "Toads are very sensitive to their environment," she says. Now that at least one potential connection has been drawn between toads and earthquakes, she says, scientists could look for similar reactions in other toad populations that live in seismic areas and are being monitored by conservationists. She suggests exploring how toads respond to some of the changes she believes could have triggered their mass exodus.
There are many anecdotal accounts of animals acting strangely before tremors, but there's little hard science. "This is the first study that monitors unusual behavior in animals for a significant period before and after a major earthquake," Grant says. In the 1st century, the Greek historian Diodorus recounted how rats, centipedes and snakes escaped from the city of Helice in 373 B.C. a few days before an earthquake dropped it into the Corinthian Gulf. After an earthquake struck China's Sichuan province in 2008, killing 68,000 people, residents in the city of Mianzhu said they had seen a mass migration of toads precede the tremor a sign, perhaps, that the amphibians had known what was coming.
There was a brief boom of research into whether animals could be used to predict earthquakes in the 1970s, when a few scientists documented changes in the behavior of birds, mice and domestic animals immediately before the earth's beginning to shake. But the idea never gained much traction. The very difficulty of predicting earthquakes makes it hard to study how animals react to them. "This was a completely fortuitous event," says Halliday. "It would be practically impossible to plan research like this. You'd spend a lot of time watching toads with nothing happening."
Some seismologists have doubts about Grant's research, saying the toads' behavior could have just been a fluke. After all, critics say, the L'Aquila earthquake was preceded by minor shocks that also worried the city's residents. "If there was a fright among the toads, it would have been a reflection of the fright that was happening among the people," says Pascal Bernard, a seismologist at the Institute of Earth Physics in Paris. "People were afraid, but nobody knew for certain that something was going to happen."
Grant, though, is convinced there's a connection. The question now, she says, is whether seismologists can do anything with her research to try to predict the next big one. "A lot of people are asking, 'Can we use them as a kind of monitoring tool, keep one at home and watch to see if they run away?' " says Grant. "That's obviously not going to work." But she's hoping something might.