Vibration Communication in Vertebrates
129
Two types of well-studied surface waves are Love waves and Rayleigh waves,
which differ in both their plane of polarization and their velocity. The plane of
polarization of a seismic wave is that plane containing the particle motion, and the
direction (or axis) of propagation is the direction of energy flow for the wave.
Displacements of the substrate particles in Love waves are perpendicular to, and
in Rayleigh waves parallel to, the vertical plane containing the axis of wave
propagation. Love waves cause particle displacement to occur in a direction
transverse to the axis of energy flow. In contrast, Rayleigh waves result in no
transverse vibration, but impart a strong vertical component to individual substrate
particles. Both categories of propagated mechanical waves are illustrated in Fig. I.
Primary and Shear waves are high-velocity (body) waves that travel through the
earth but are not known to be important for animal signaling. Only surface (R- and
L-) waves have been shown to be used in animal communication signals- L-waves
for desert-dwelling scorpion signaling and R-waves for signaling not only by the
white-lipped frog, but for a variety of other terrestrial vertebrates as well.
Some variation in these ideal polarization patterns occurs due to both local
inhomogeneities in the substrate and proximity to the source. Surface waves
dissipate their energy in an ever expanding circular pattern concentric with the
source; as a result, the amplitude of a Rayleigh or Love wave decreases as 1!r 112 ,
where r is the distance from the source. Thus, far from the source, the major
remaining component of the seismic disturbance is the surface wave. Rayleigh
waves propagate significantly slower than body waves; typical Rayleigh wave
velocities in moist soil are 100 ms- 1 (Lewis and Narins 1985) and in loose sand,
40-50 ms- 1 (Brownell 1977).
Recent studies in our laboratory and in others have revealed that, at least in one
group of animals (frogs), a population of inner ear sensors that respond to lowfrequency sound are also exceptionally sensitive to whole-body vibrations. Upon
reflection, this is not surprising, given the striking similarity of the anatomy and
physiology of the sensory cells of the auditory and vestibular systems (Lewis and
Narins 1999). Thus, for some of the inner ear sensors in the frog, sound and
vibration appear to converge. Clearly, in this case, we might adopt the view that
auditory and vibratory signals differ principally in the pathway traveled to the
inner ear sensors. Thus, the tympanic auditory pathway describes vibratory stimuli
reaching the inner ear receptors via the tympanic membrane [and thence the
middle ear ossicle(s)]. This pathway is often used for the detection of airborne or
water-borne vibrations, then referred to as sound. The extra-tympanic auditory
pathway conducts signals to the inner ear through any route in which energy does
not directly excite the tympanic membrane (e.g., most modes of bone conduction).
This pathway is often used for the detection of vibrations traveling in a solid
substrate, for example, soil(= seismic channel).
A totally independent somatosensory pathway, mediated by specialized
cutaneous mechanorereceptors, may also be used to detect seismic communication
signals (Nevo et al. 1991). It follows, therefore, that the few vertebrate organisms
known or presumed to exploit vibratory communication signals all inhabit media
129
Two types of well-studied surface waves are Love waves and Rayleigh waves,
which differ in both their plane of polarization and their velocity. The plane of
polarization of a seismic wave is that plane containing the particle motion, and the
direction (or axis) of propagation is the direction of energy flow for the wave.
Displacements of the substrate particles in Love waves are perpendicular to, and
in Rayleigh waves parallel to, the vertical plane containing the axis of wave
propagation. Love waves cause particle displacement to occur in a direction
transverse to the axis of energy flow. In contrast, Rayleigh waves result in no
transverse vibration, but impart a strong vertical component to individual substrate
particles. Both categories of propagated mechanical waves are illustrated in Fig. I.
Primary and Shear waves are high-velocity (body) waves that travel through the
earth but are not known to be important for animal signaling. Only surface (R- and
L-) waves have been shown to be used in animal communication signals- L-waves
for desert-dwelling scorpion signaling and R-waves for signaling not only by the
white-lipped frog, but for a variety of other terrestrial vertebrates as well.
Some variation in these ideal polarization patterns occurs due to both local
inhomogeneities in the substrate and proximity to the source. Surface waves
dissipate their energy in an ever expanding circular pattern concentric with the
source; as a result, the amplitude of a Rayleigh or Love wave decreases as 1!r 112 ,
where r is the distance from the source. Thus, far from the source, the major
remaining component of the seismic disturbance is the surface wave. Rayleigh
waves propagate significantly slower than body waves; typical Rayleigh wave
velocities in moist soil are 100 ms- 1 (Lewis and Narins 1985) and in loose sand,
40-50 ms- 1 (Brownell 1977).
Recent studies in our laboratory and in others have revealed that, at least in one
group of animals (frogs), a population of inner ear sensors that respond to lowfrequency sound are also exceptionally sensitive to whole-body vibrations. Upon
reflection, this is not surprising, given the striking similarity of the anatomy and
physiology of the sensory cells of the auditory and vestibular systems (Lewis and
Narins 1999). Thus, for some of the inner ear sensors in the frog, sound and
vibration appear to converge. Clearly, in this case, we might adopt the view that
auditory and vibratory signals differ principally in the pathway traveled to the
inner ear sensors. Thus, the tympanic auditory pathway describes vibratory stimuli
reaching the inner ear receptors via the tympanic membrane [and thence the
middle ear ossicle(s)]. This pathway is often used for the detection of airborne or
water-borne vibrations, then referred to as sound. The extra-tympanic auditory
pathway conducts signals to the inner ear through any route in which energy does
not directly excite the tympanic membrane (e.g., most modes of bone conduction).
This pathway is often used for the detection of vibrations traveling in a solid
substrate, for example, soil(= seismic channel).
A totally independent somatosensory pathway, mediated by specialized
cutaneous mechanorereceptors, may also be used to detect seismic communication
signals (Nevo et al. 1991). It follows, therefore, that the few vertebrate organisms
known or presumed to exploit vibratory communication signals all inhabit media