Reports of pets vanishing or wildlife disappearing ahead of large earthquakes had previously been anecdotal, but an international study
has for the first time documented changes in wild animal behavior before a seismic event.
Using motion-triggered cameras located in the Yanachaga National Park in Peru, scientists found significant changes in animal behavior more than three weeks before a magnitude 7 earthquake struck the region in 2011.
On a typical day the cameras recorded 5 to 15 animal sightings, but within the 23-day period in the run-up to the earthquake, they recorded five or fewer sightings.
For the five to seven days immediately before the earthquake, no animal movements were recorded at all -- an unusual phenomenon in a mountainous rainforest region normally teeming with wildlife.
Dr Rachel Grant, lecturer in animal and environmental biology at Anglia Ruskin University in the UK, told CNN the study is looking at the possibility that positive ions in the air -- generated in large numbers when rocks below the surface are stressed in the build-up to an earthquake -- may hold the key to the animals' behavior.
"Animals don't like positive ions and humans also get adverse symptoms," she said.
The imbalance related to positive ions is known as "serotonin syndrome," after the hormone that regulates mood.
"People can get headaches, nausea, anxiety and restlessness. We think the animals are moving away from this concentrated source of positive ions.
"Animals, in general, will move away from unpleasant stimuli."
She said that positive ions tend to collect in hilltops and that it was possible the animals were moving to lower ground to get away from them.
Far from ground zero
"Over the past two years I analyzed data to see whether there were any earthquakes in the vicinity that were large enough to be likely to cause some kind of reaction," Dr Grant said. "I didn't think there'd be much reaction from the animals because the national park was about 350 kilometers from the epicenter.
"But when I looked at the data I was quite surprised -- there was a big decline in animal numbers before the earthquake."
As well as the camera traps, researchers recorded the reflection of very low frequency (VLF) radio waves above the area surrounding the epicenter, to detect disturbances in the ionosphere.
A particularly large fluctuation was recorded eight days before the quake, coinciding with the second significant decrease in animal activity observed in the pre-earthquake period.
Science, not sixth sense
While science stops short of calling it a sixth sense, wild animals are generally more sensitive than people when it comes to responding to their environment.
"Humans are generalists, we are not specialized ecologically," she said. "We don't live in contact with the soil or the ground. We've insulated ourselves into concrete buildings in cities.
"While there have been reports of people displaying medical symptoms ahead of earthquakes, I think the effect would be negligible.
"However, I'm prepared to keep an open mind on that subject."
Rats on the run
Of all the species in the rainforest, Dr Grant said that rats had the most finely attuned early warning system.
"What was interesting was that rodents were the first to disappear -- they were nowhere to be seen eight days before the earthquake. Normally, rodents are everywhere and in very large numbers in tropical forests. That they should completely disappear was amazing."
"It tallies with these ancient stories of all the rats fleeing a city before an earthquake."
She said the findings backed earlier studies from Japan and China that showed that lab rats displayed problems with their circadian rhythms -- the biological patterns that regulate sleep -- ahead of earthquakes.
Ground-dwelling birds also showed acute sensitivity and armadillos emerged within sight of the cameras very soon after the earthquake had passed.
"It was almost as if they were coming out of hiding," she said.
The next step: underwater
The study, which was co-authored by Professor Friedemann Freund of the SETI Institute and Professor Jean-Pierre Raulin of the Centre of Radio Astronomy and Astrophysics Mackenzie (CRAAM) in Brazil, is hoping to look at biological changes in plankton ahead of major seismic shifts.
"I want to measure lake water and that could potentially be a bio-indicator of earthquakes -- these marine animals are very sensitive to chemical changes in the water," Dr Grant said.
But while animals may one day be a helpful early warning system, researchers say that animal behavior alone may not be a wholly reliable indicator of an approaching tremor.
"Animal behavior is not always predictable -- for the earthquake forecasting side of things we would still need to have geophysical measurements in combination with animal measurements."