Vibrations in the ground are a poorly understood but probably widespread means of communication between animals.
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.
Almost all the research done into animal signaling 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 signaling) have been almost entirely over-looked. These re- searchers believe that such signals are far more common than biologists had realized—and that they could explain a lot of otherwise inexplicable features of animal behavior.
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 50kin when they trumpet, make mock charges or stomp their feet.
A seismic sense could help to explain certain types of elephant behavior, 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 traveling—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.
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 defense, 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 realized 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.
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