Biology Reference
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infrastructures reduce the free surface of soils in forests and in rural and urban
areas.
The organisms living in the soils produce numerous sounds, vibrations, and
noises. In fact, most of the sounds produced in the soil are represented by vibrations
of the stridulating apparatus of insects and other arthropods (Cocroft and Rodriguez
2005
) or by the mechanical vibrations produced by fossorial animals (earthworms,
mole crickets, moles) during their digging activity or root scraping. Soil is a
granular and not a continuous medium, so acoustic transmission across interstices
(intergranular passages) is reasonable. Sound attenuation is higher in the soil
(600 dB/m) than in the air (0.008 dB/m), but sounds with a frequency
<
5 kHz
can be detected, at least in sandy soil, over a range of 5-50 cm.
Every organism that digs in the soil for creating refuges or to search for prey
produces unwanted vibrations that can be eavesdropped by other organisms.
Indeed, the soil strata are not a silent environment but an ambient rich in sonic
information for its dwellers.
The majority of the sounds from soil are used by soil fauna to communicate,
although these sounds are not easily captured and interpreted by the human ear. For
this reason, the study of arthropod vibrational communication represents an inter-
esting innovative field of interest for the future. Acoustic techniques are very useful
to detect, to investigate, and to assess the amount of life present in the soils,
avoiding laborious and destructive techniques.
Evaluating underground biodiversity and monitoring the changes that occur in
the soil, especially after human disturbances, by using acoustic cues, represents a
new chapter of the soundscape ecology agenda, although “sounds from soil” from
the literature still represent a marginal argument in soundscape ecology.
8.2 Seismic Signals from Reptiles
In snakes there are two sensory systems reacting to airborne sound and to substrate
vibration. In Colubridae, Crotalidae, and Boidae, the auditory system has a
U-shaped frequency-threshold curve that ranges from 150 to 600 Hz. The auditory
system of snakes is 20 dB less sensitive than the human auditory system to stimuli
produced on the head and along the body. The lung system seems an important part
in the auditory system of snakes, representing a novelty in land vertebrates, as
experimentally proved by Hartline (
1971
).
In Saharan sand vipers (
Cerastes cerastes
), Young and Moran (
2002
) have
experimentally demonstrated that mice are captured using the vibrations produced
by the prey. In fact, this species has been observed to also launch its strike when its
mouth and nostrils are under the sand, and without a tongue-flick. Experimental
animals deprived of chemosensory information and temporarily blinded were able
to freely locate mobile prey in every experimental trial, proving that vibrational
stimuli are used to locate prey (Fig.
8.1
).
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