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When J.A. Randall (at
Her study site was in the strip of Chihuahuan Desert lying between the
Chiricahua Mountains in extreme eastern Arizona and the town of Rodeo in
extreme western New Mexico, about fifty miles north of the Mexican border. This
is a fascinating place for a biologist to work at night, especially during
monsoon season. The mammals and arthropods on the desert floor at night are
amazing-- the stuff of a Disney film. The presence of Mojave rattlesnakes keeps
one always alert. Ned Lewis and I had worked close to this site on a windless
night several years earlier, recording acoustic output from three neighboring
Couch's spade-foot toads. With respect to seismic background, it was the
quietest place either of us had encountered (much quieter than the Namib proved
to be). We were able to turn the gains on our geophone preamps to their highest
levels. Listening to the preamp output through headphones, with the geophones
10 meters away, gently rubbing one's finger over the surface of the sand
produced thunder in his ears. We inadvertently left the headphones connected as
we recorded the toads' calls. Suddenly, one of them fell silent. A few minutes
later, when Ned checked the cassette recorder, he motioned me over to see a
spade-foot toad sitting directly between the earpieces of the headphones. We
supposed it had come over to inspect the new toad voice that had joined the
chorus.
The banner-tailed kangaroo rat uses foot drumming for acoustic communication,
and this of course sets up sound waves propagating in the air and vibrational
waves propagating in the ground. Except for females with pups, each rat lives a
solitary life in a large burrow, over which the excavated dirt produces a
single, conspicuous mound. A herd of wandering steers occasionally will flatten
the mound to the ground, confirmation of a conservation principle that would be
comforting to a physicist. When a rat completes its foraging and retreats into
its burrow, it seals the burrow entrance. Jan had used geophones to record the
foot drumming of rats in their sealed burrows. With pairs of geophones on
neighboring burrows, separated by 10 meters or more, Jan had detected the clear
back-and-forth foot drumming interactions between the occupants. This occurred
even on very windy nights, when all but the most intense airborne sound was
masked by the wind. Even on a windless night, foot drumming sounds from the
inside of a sealed burrow are subtle. This was her evidence for communication
through the seismic channel, vibrational waves in the ground. It was
compelling.
The acoustic measurements and simple behavioral studies that she and I carried
out confirmed her hypothesis. Kangaroo rats show no evidence of special seismic
senses, but they don't need them. Their sealed burrows are very quiet places,
even on windy nights. The wall of the burrow serves, in a formal sense, as a
transducer. It transfers the power of the seismic waves from the neighboring
rat's foot drumming to easily audible air-borne sound inside the burrow.
In the process of determining these things, we drilled a small hole through the
roof of an occupied burrow and lowered a tiny electret microphone into the
hole. The first time we tried this, we lowered the microphone too far-- all the
way through the hole, into the burrow. It stopped operating very quickly, and
when we pulled the lead out of the hole, it was cut as cleanly as one could do
with a sharp pair of diagonal cutters. We were more careful after that.