Sonic Patterns III: Sounds and Vibrations from Soils - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 210

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maximum Fig. 8.2 Relationship between number of elephants and Area = (amplitude envelope)

(Reproduced with permission from Wood et al. 2005 )

8.4 Seismic Signals from Arthropods

Seismic vibrations cause the Diplocardia earthworm to emerge from soil. This
phenomenon is well known and is utilized to harvest earthworms. This practice is
called “worm grunting” and is used in the Southeast United States by local families
(worm grunters) and is well diffused around Florida's Apalachicola National
Forest. The earthworms are forced to emerge from the soil by the use of a very
peculiar technique named grunting, which consists of scraping a wooden stack
driven into the soil with a flat metal object. The sound when produced at regular
intervals causes the worm Diplocardia mississippiensis to emerge in a few minutes
from the soil. A broadband frequency is produced by this grunting; the number of
emerging worms decreases according to increasing distance from the origin of the
vibration. Strokes produced for a duration of approximately 760 ms and repeated
every 1.2 s produce the emergence of worms. The seismic signals are broadband,
below 500 Hz with a dominant frequency of 97 Hz. The worms emerge 54-131 s
after the onset of the vibrations. The number of emerging worms is positively
correlated with the signal strength.
Some animals use the technique of grunting to capture worms. It has been
observed in wood turtles ( Clemmys insculpta ), which stomp their front feet with a
frequency of once per second, but also birds such as gulls have a paddling behavior,
as observed by Tinbergen ( 1960 ).