周越

In 2004， a tsunami triggered by a 9.1 magnitude undersea quake off Indonesia decimated coastal communities around the Indian Ocean， killing at least 225000 people across a dozen countries. The huge death toll was in part caused by the fact that many communities received no warning.

Local manmade early warning systems， such as tidal and earthquake sensors， failed to raise any clear alert. Many sensors were out of action due to maintenance issues， while many coastal areas lacked any tsunami siren warning systems. Haphazard communication also failed to provide warnings， with many text messages failing to reach mobiles in threatened areas or going unread.

Yet in the minutes and hours before surging walls of water up to 9m （30ft） high smashed through coastlines， some animals seemed to sense impending peril and make efforts to flee. Elephants ran for higher ground， flamingos abandoned low＼|lying nesting areas， and dogs refused to go outdoors. In the coastal village of Bang Koey in Thailand， locals reported a herd of buffalo by the beach suddenly pricking their ears， gazing out to sea， then stampeding to the top of a nearby hill a few minutes before the tsunami struck.? Survivors also reported seeing animals， such as cows， goats， cats and birds， deliberately moving inland shortly after the earthquake and before the tsunami came. Many of those who survived ran along with these animals or immediately after.

Similar stories happened in the 2010 tsunami generated by a subsea quake near Sumatra， which killed nearly 500 people on the Mentawai Islands. Here too， however， some animals， such as elephants， were reported to have responded as if possessing some kind of early knowledge of the event. Just days ago， a newly re＼|released turtle made a sudden U＼|turn two days before January's volcanic eruption in Tonga.

These accounts about animal behaviour before disasters have prompted some researchers to devote serious scientific attention to the theory that animals may have inbuilt systems which alert them to impending natural disasters. It raises an intriguing question ＼| could animals provide natural early warning systems for humans？

The researchers found evidence that the farm animals began to change their behaviour up to 20 hours before an earthquake. Whenever the monitored farm animals were collectively 50% more active for more than 45 minutes at a stretch， the researchers predicted an earthquake with a magnitude above 4.0. Seven out of eight strong earthquakes were correctly predicted in this way.

The closer the animals were to the epicentre of the impending shock， the earlier they changed their behaviour. This is exactly what you would expect when physical changes occur more frequently at the epicentre of the impending earthquake and become weaker with increasing distance.

Over in South America， behavioural ecologists has found similar results. They carried out biologging of animal movement patterns using motion＼|triggered cameras inside Yanachaga National Park in the Peruvian Andes over a period which included the magnitude 7.0 Contamana earthquake in 2011. The number of animals recorded on the camera traps started to decrease about 23 days before the earthquake ＼| with the decrease accelerating eight days prior to the earthquake. And on the day of the earthquake， no animal movements were recorded， which is highly unusual.

Scientists are now exploring whether these electromagnetic perturbations in the atmosphere prior to earthquakes could be a warning sign of impending quakes which animals may be sensing.

Earthquakes are invariably preceded by a period when severe stresses arise in deep rock ＼| stresses known to create electronic charges called "positive holes". These highly mobile electronic charge carriers can flow quickly from the crust to the Earth's surface， where they ionise air molecules above where they appear. Such ionisation has been noted prior to quakes across the globe. As these positive holes flow， they also generate ultra＼|low frequency electromagnetic waves， providing an additional signal that some animals may be able to pick up.

Earthquake precursors aren't well documented scientifically. But some scientists theorise that animals could have evolved a seismic escape mechanism. Perhaps they detect pressure waves before earthquakes arrive， perhaps they detect changes in electric field as fault lines when rock starts to compress. Animals also contain a lot of iron， which is sensitive to magnetism and electric fields.

Positive holes could also cause certain toxic chemicals to appear before quakes. For example， if they come into contact with water， they can trigger oxidation reactions which create the bleaching agent hydrogen peroxide. Chemical reactions between the charge carriers and organic matter in the soil could trigger other unpleasant products such as ozone.

Many animals， of course， are equipped with highly developed sensory apparatus that can read an array of natural signals on which their lives may depend ＼| so it seems perfectly possible that some animals may be able to pick up any earthquake precursors. Unpleasant chemicals could be sniffed out， low frequency waves picked up， and ionised air sensed by sensations in fur or feathers.

Not all experts think that animal early warning systems are a viable option for predicting disasters. And even if they do help， animal movements alone are unlikely to be enough to provide： people will need to rely on a combination of early warning signals to get the full picture.

Still， while we may not be able to talk to animals quite yet， perhaps it's time to pay more attention to their warnings.