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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.
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