Lewis, E.R. R. Lewis; Narins, P.M.*: Do White-lipped Frogs Use Seismic Signals for Intraspecific Communication

In the inner ear of a modern frog, the volume occupied by the saccule is approximately equal to that occupied by all of the seven other sensory endorgans taken together. Unlike that of any other terrestrial vertebrate, the sensory surface (macula) of the modern-frog saccule is detached from the wall of the otic capsule and suspended on a pool of periotic fluid. The sensory hair bundles are those of an acoustic sensor rather than a sensor of slow motion or orientation. The primary afferent axons are responsive to both airborne sound and substrate vibration– being exquisitely sensitive to the latter, especially in the dorsoventral plane. Their spikes are phase-locked to a linearly-filtered version of the stimulus waveform. The filter dynamics are remarkably complex, providing excellent temporal resolution, passing energy in the frequency range (10-150 Hz) appropriate for local seismic signals in the ground, and strongly rejecting energy in the frequency range (below 10 Hz) occupied by typical microseismic background noise. For stimuli in the middle of the filter pass band, many individual axons exhibit conspicuous responses to vibrational accelerations of the order of 0.5 x 10^-6g. In the field, the calling behavior often reflects this sensitivity, even very gentle footfalls producing a local area of silence among a chorus of male frogs. The airborne calls of the males of one species, Leptodactylus albilabris, often are accompanied by a strong seismic component whose spectrum lies in the middle of the saccular-filter passband. Hoping to obtain definitive evidence of involvement of this component in male-male communication, we undertook what has evolved into a long series of behavioral studies in the field. These have centered about a timing gap in the male frog’s advertisement call, produced in response to calls of conspecic males.