Extra Sensory Perception

The Economist

Vibrations in the ground are a poorly understood but probably widespread means of communication between animals

IN 1975, tens of thousands of people were evacuated from Haicheng, in China, a few hours before a large earthquake struck the city. Western scientists regard earthquakes as unpredictable, and pre-emptive evacuations such as this as therefore impossible. What gave the game away, according to the Chinese authorities, was the strange behaviour of animals such as rats, snakes, birds, cows and horses.

It could have been a lucky coincidence. It seems unlikely that these animals could have detected seismic “pre-shocks” that were missed by the sensitive vibration-detecting equipment that clutters the world’s earthquake laboratories. But it is possible. And the fact that many animal species behave strangely before other natural events, such as storms, and that they have the ability to detect others of their species at distances which the familiar human senses could not manage, is well established. Such observations have led some to suggest that these animals have a kind of extra-sensory perception. What is more likely, though, is that they have an extra sense—a form of perception that people lack. The best guess is that they can feel and understand vibrations that are transmitted through the ground.

Good vibrations
Almost all the research done into animal signalling has been on sight, hearing and smell, because these are senses that people possess. Humans have no sense organs designed specifically to detect terrestrial vibrations. But, according to researchers who have been meeting in Chicago at a symposium of the Society for Integrative and Comparative Biology, this anthropocentric approach has meant that interactions via vibrations of the ground (a means of communication known as seismic signalling) have been almost entirely overlooked. These researchers believe that such signals are far more common than biologists had realised—and that they could explain a lot of otherwise inexplicable features of animal behaviour.

Until recently, the only large mammal known to produce seismic signals was the elephant seal, a species whose notoriously aggressive bulls slug it out on beaches around the world for possession of harems of females. But Caitlin O’Connell-Rodwell of Stanford University, who is one of the speakers at the symposium, suspects that a number of large terrestrial mammals, including rhinos, lions and elephants, also use vibration as a means of communication. At any rate they produce loud noises that are transmitted through both the ground and the air—and that can travel farther in the first than in the second. Elephants, according to Dr O’Connell-Rodwell, can transmit signals through the ground this way for distances of as much as 50km when they trumpet, make mock charges or stomp their feet.

Seismic vibrations do not qualify as signals unless they are being received and understood. But it has already been shown that some smaller animals, such as frogs and crickets, pick up information from the seismic part of what everybody had assumed to be simple acoustic (ie, airborne) signals. One way this was found out was by vibrating whole frogs while recording the electrical impulses from particular cells in their inner ears that were suspected of responding to seismic stimulation. Frogs, of course, are easily manipulated. Doing something similar to an elephant requires a higher degree of co-operation from the subject. Dr O’Connell-Rodwell is, however, trying. She is attempting to train several tame elephants to respond to such signals by shutting them inside a gently vibrating truck.

Even without this evidence, it seems likely that elephants do make use of seismic communication. They have specialised cells that are vibrationally sensitive in their trunks. And vibrations transmitted through their skeletons may also be picked up by their exceptionally large middle-ear bones.

A seismic sense could help to explain certain types of elephant behaviour. One is an apparent ability to detect thunderstorms well beyond the range that the sound of a storm can carry. Another is the foot-lifting that many elephants display prior to the arrival of another herd. Rather than scanning the horizon with their ears, elephants tend to freeze their posture and raise and lower a single foot. This probably helps them to work out from which direction the vibrations are travelling—rather as a person might stick a finger first in one ear and then in the other to work out the direction that a sound is coming from.

According to Peggy Hill, a biologist from the University of Tulsa who organised the symposium, work on seismic signalling is blossoming. Part of the reason is that the equipment needed to detect seismic vibrations (and thus short-circuit human sensory inadequacies) has become cheap. Geophones—which transform vibrations into electrical signals—were once military technology. They were developed by the American army to detect footsteps during the Vietnam war. Now, they can be picked up for as little as $40.

In the past decade many insects, spiders, scorpions, amphibians, reptiles and rodents, as well as large mammals, have been shown to use vibrations for purposes as diverse as territorial defence, mate location and prey detection. Lions, for example, have vibration detectors in their paws and probably use them in the same way as scorpions use their vibration detectors—to locate meals.

Dr Hill herself spent years trying to work out how prairie mole crickets, a highly territorial species of burrowing insect, manage to space themselves out underground. After many failed attempts to provoke a reaction by playing recordings of cricket song to them, she realised that they were actually more interested in her own footfalls than in the airborne music of their fellow crickets. This suggests that it is the seismic component of the song that the insects are picking up and using to distribute themselves.

Whether any of this really has implications for such things as earthquake prediction is, of course, highly speculative. But it is a salutary reminder that the limitations of human senses can cause even competent scientists to overlook obvious lines of enquiry. Absence of evidence, it should always be remembered, is not evidence of absence.