Student Theses and Dissertations
Date of Award
1968
Document Type
Thesis
Degree Name
Doctor of Philosophy (PhD)
Thesis Advisor
Donald Griffin
Keywords
Vampyrum spectrum bat, echolocation, target discrimination, frequency-dependent echoes, pulse structure, sensory cues
Abstract
Five specimens of the neotropical false-vampire bat, Vampyrum spectrum, were studied in an attempt to decide whether they were capable of discriminating between targets of differing shapes and sizes using echolocation alone. The study commenced with the demonstration that the bats could capture live mice, items similar to their normal prey, without recourse to visual or olfactory cues. The subsequent use of dead mice in place of live ones indicated that target identification was much more difficult for the bats when the mice were not moving, but could still be effected by some of them. Bats were then trained to discriminate between two inedible targets of differing shape or size, for which they received food as a reward. Initial discrimination problems began with targets which were very disparate, both in size and shape, and progressed to two targets which differed very little. Disparate targets, such as a 15 cm. diameter ball and a 2.5 X 10.5 cm. wooden "sausage", were easily discriminated by all the bats and clearly by echolocative cues alone. Discrimination scores for more similar targets, such as a 6 cm. diameter sphere and a 2.5 X 7.0 cm. sausage were not as high as the previous series, but considerably greater than chance. Since the bats could clearly discriminate between rather similar targets using echolocation alone, an attempt was made to design targets which, if discriminated, would allow for some interpretations regarding the actual echo parameters used by the bats. In particular, the targets and flight paths were adjusted so as to discourage the use of overall intensity cues, but to encourage the use of the frequency dependent properties of the echoes. Of the five bats begun on this problem, two learned the discrimination with an accuracy of 70% or better. Because the elimination of overall intensity differences was contingent upon the point at which the bats selected a target and the angle at which they approached it, actual discrimination flights of the two best performers were submitted to extensive photographic analysis. This suggested that one of the bats selected the targets and approached them along a path that was consistent with the use of the frequency dependent properties of the target echoes, but not with the use of their overall intensities. The opposite was true of the second animal. Further proof of this point came from the discovery that the threads supporting the targets were contributing to the frequency dependence of the echoes, the armplitude of this frequency dependence being a direct function of the diameter of the thread. When threads of small diameter were used, (thereby reducing the frequency dependence of the echoes), the first bat was unable to discriminate above the chance level while the second animal's performance was quite high. When larger diameter threads were used, both animals were able to discriminate. This was thus taken as additional proof of the use of the frequency dependence of the echoes by the first bat, and of the use of other cues, presumably overall intensities, by the other. Since echolocation is effected by the use of emitted pulses of sound, the structure of the pulses emitted during discrimination flights was examined in detail. From this examination it was possible to show that at different stages along each flight, differently structured pulses were emitted. In addition, the two bats which apparently were using different echo cues to discriminate produced pulses of different structure at the points of target selection. No obvious explanations for the use of any given type of pulse at any given point in a flight could be given, but some relevant speculations were presented. In summary: it was shown 1) that these bats could discriminate between very similar targets using echolocation, 2) that at least one of the animals was probably using as a cue the frequency dependent properties of received echoes, and 3) that there may be some correlations between the kinds of pulses these bats emitted and the kinds of problems they encountered.
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Recommended Citation
Bradbury, Jack Winningham, "Mechanisms of Target Discrimination by the Echolocating Bat, Vampyrum Spectrum" (1968). Student Theses and Dissertations. 562.
https://digitalcommons.rockefeller.edu/student_theses_and_dissertations/562
Comments
A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy