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First published online February 4, 2005
Journal of Experimental Biology 208, 647-659 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01396

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Evidence for the use of reflected self-generated seismic waves for spatial orientation in a blind subterranean mammal

Tali Kimchi^1,*, Moshe Reshef² and Joseph Terkel¹

¹ Department of Zoology, Faculty of Life Sciences, Tel-Aviv University, Tel Aviv 69978, Israel
² Department of Geophysics and Planetary Sciences, Faculty of Exact Sciences, Tel-Aviv University, Tel Aviv 69978, Israel

View larger version (26K): [in a new window]   Fig. 1. (A) Schematic side view drawing of the obstacle (ditch) created across the mole-rat tunnel (upper part), and an overhead photo of a bypass burrowed by a mole-rat around a ditch (lower part). The location of the recording geophones used to record the seismic waves produced by the mole-rat while it dug the bypass tunnel are marked schematically (filled circles): a, small ditch; b, location of the original tunnel; c, bypass tunnel around the ditch. (B) Typical seismic signal and amplitude spectrum generated by the mole-rat during bypass burrowing recorded by the geophones in the field. (C) Synthesized wavelet pulse that matched the amplitude spectrum of a typical mole-rat's seismic signal recorded in the field, used in the computer simulations.

View larger version (31K): [in a new window]   Fig. 2. Schematic drawings and photos (insets) of the set-up used to test whether the mole-rat can perceive and localize seismic waves only through the paws. (A) Top view: ST, supporting table anchoring the suspended maze; T, suspended T-maze; CT, central table; D, doors controlling access to the entrance maze tube; C, clamp arms suspending T maze 1 cm above the central table; S, vibratory stimulus (mechanical shaker or stimulus mole-rat) source producing seismic waves on the central table; MR, mole-rat. (B) Back view of the entrance tube: SL, slit (3.5 cm width) in maze tube, enabling the experimental animal to move in the tubes with only its feet in contact with the board surface; X, gap of 1 cm between the suspended maze tube and the board surface; other letters are as in A.

View larger version (16K): [in a new window]   Fig. 3. A simulation model, consisting of a single-source/single-receiver (the mole-rat's body) located at 30 or 50 cm distance D from two different reflecting interfaces (upper part: soil-air interface; lower part: soil-stone interface). The scheme presents the polarity, amplitude and relative time delay of the reflected wave returning to the mole-rat (the source location) from the two interfaces.

View larger version (39K): [in a new window]   Fig. 4. A series of four photos from a simulation of seismic waves: T1 (the time that the seismic pulse was generated): the source is located 30 cm from an asymmetrically aligned open ditch, where the left corner of the ditch is closer to source than the right corner. T2 (4 ms after the pulse was generated): a seismic wave reflected from the nearest obstacle wall. T3 (5.5 ms after time the pulse was generated): a seismic wave diffracted from the obstacle's closest corner. T4 (8 ms after the pulse was generated): a seismic wave diffracted from the obstacle's more distant corner. Stars indicate the location of the source and receiver (individual mole-rat).

View larger version (29K): [in a new window]   Fig. 5. Illustration of the computer simulation demonstrating how the mole-rat might detect the diffraction waves from the obstacle corners. In this simulation the mole-rat generates at least two seismic waves (the source was at a distance a=30 cm and b=35 cm from the obstacle boundary) and perceives the reflection and diffractions waves using receptors in the forepaws and hindpaws. (A) The mole-rat's position relative to the obstacle boundary while it generates the two seismic waves. (B) The relative time delay of the reflected and diffracted waves as they should reach the mole-rat's left forepaws and left hindpaws from each of the two sources.

View larger version (11K): [in a new window]   Fig. 6. Success (%) in selecting the correct maze arm leading to the vibratory source when the generated vibratory waves could only reach the mole-rat's feet (Experiment) or did not reach any part of the mole-rat's body (Control). Values are means ± S.E.M. (N=7 subjects in each of two stimulus sources). Broken line represents random choice (50%) of maze arm.

View larger version (141K): [in a new window]   Fig. 7. Lamellate corpuscle mechanoreceptors found in both the forepaws and hindpaws of the mole-rat, possibly used to detect seismic waves. (A) Cluster of three corpuscle structures. Scale bar, 10 µm. (B) Close-up of the corpuscle marked with arrow in A.