138
Peter M. Narins
3.4 Crotalus viridus- the Western Rattlesnake
It has long been known that snakes are extremely sensitive to ground vibrations,
but this was not quantified until the classical work of Hartline (1971). In this
study, he demonstrated that snakes possess two sensory systems which respond to
both airborne sound and substrate vibration: one subserved by the eighth nerve
and inner ear (auditory), and the other mediated by the spinal cord and cutaneous
mechanoreceptors (somatic). Based on multiunit evoked responses, it was
concluded that the auditory system of the western rattlesnake (Crotalus viridus) is
not particularly sensitive to airborne sound; its airborne sound threshold between
200 and 400 Hz is about 20 dB higher than the human auditory system in that
frequency range. Nonetheless, it showed remarkable sensitivity to seismic
vibrations: a 1-A peak-to-peak vibration amplitude was suprathreshold (Hartline
1971 ). Evoked potential recordings failed to demonstrate that either the somatic or
auditory system can distinguish between vibrations from the air and from the
substrate. It was postulated that intensity information from the auditory and the
somatic system could be compared centrally to determine the relative amplitudes
of airborne and substrate-borne vibration in an unknown natural stimulus.
However, although it is presumed that vibration detection may subserve prey
detection in snakes, the function of their remarkable vibration sensitivity has not
been confirmed behaviorally.
3.5 Scincus scincus - the Sandfish Lizard
The sandfish lizard (Scincus scincus, family Scincidae) spends most of its time
buried in the Sahara Desert. Experiments with this animal in captivity demonstrate
that Scincus is quite adept at detecting insects on or below the surface of the sand,
and it appears to do so by reception of vibrations produced by the insects'
movements (Hetherington 1989). When live potential prey were placed in a sand
tank on one side of a partition composed of two sheets of Plexiglas separated by I
em, the buried lizard on the other side of the partition did not respond to the
motion of either mealworms or crickets. It was also shown in this study that the
location of the lizard relative to the sand surface was not critical for this behavior.
Lizards buried in the sand generally responded to mealworms and crickets
walking on the surface at distances of up to 15 em.
The buried lizard's almost complete indifference to dead insects at the same
distance supports the hypothesis that Scincus responds to signals produced by
moving insects rather relying strictly on olfactory cues. Moreover, Hetherington
(1989) measured the angle of lizard's head orientation upon its emergence from
the sand following prey detection and demonstrated that, far from being random,
the average emergence angle was not significantly different from the expected
angle (that angle corresponding to the prey's actual location). Thus, remarkably,
Peter M. Narins
3.4 Crotalus viridus- the Western Rattlesnake
It has long been known that snakes are extremely sensitive to ground vibrations,
but this was not quantified until the classical work of Hartline (1971). In this
study, he demonstrated that snakes possess two sensory systems which respond to
both airborne sound and substrate vibration: one subserved by the eighth nerve
and inner ear (auditory), and the other mediated by the spinal cord and cutaneous
mechanoreceptors (somatic). Based on multiunit evoked responses, it was
concluded that the auditory system of the western rattlesnake (Crotalus viridus) is
not particularly sensitive to airborne sound; its airborne sound threshold between
200 and 400 Hz is about 20 dB higher than the human auditory system in that
frequency range. Nonetheless, it showed remarkable sensitivity to seismic
vibrations: a 1-A peak-to-peak vibration amplitude was suprathreshold (Hartline
1971 ). Evoked potential recordings failed to demonstrate that either the somatic or
auditory system can distinguish between vibrations from the air and from the
substrate. It was postulated that intensity information from the auditory and the
somatic system could be compared centrally to determine the relative amplitudes
of airborne and substrate-borne vibration in an unknown natural stimulus.
However, although it is presumed that vibration detection may subserve prey
detection in snakes, the function of their remarkable vibration sensitivity has not
been confirmed behaviorally.
3.5 Scincus scincus - the Sandfish Lizard
The sandfish lizard (Scincus scincus, family Scincidae) spends most of its time
buried in the Sahara Desert. Experiments with this animal in captivity demonstrate
that Scincus is quite adept at detecting insects on or below the surface of the sand,
and it appears to do so by reception of vibrations produced by the insects'
movements (Hetherington 1989). When live potential prey were placed in a sand
tank on one side of a partition composed of two sheets of Plexiglas separated by I
em, the buried lizard on the other side of the partition did not respond to the
motion of either mealworms or crickets. It was also shown in this study that the
location of the lizard relative to the sand surface was not critical for this behavior.
Lizards buried in the sand generally responded to mealworms and crickets
walking on the surface at distances of up to 15 em.
The buried lizard's almost complete indifference to dead insects at the same
distance supports the hypothesis that Scincus responds to signals produced by
moving insects rather relying strictly on olfactory cues. Moreover, Hetherington
(1989) measured the angle of lizard's head orientation upon its emergence from
the sand following prey detection and demonstrated that, far from being random,
the average emergence angle was not significantly different from the expected
angle (that angle corresponding to the prey's actual location). Thus, remarkably,