The adaptive value of increasing pulse repetition rate during hunting by echolocating bats

Abstract

During hunting, bats of suborder Microchiropetra emit intense ultrasonic pulses and analyze the weak returning echoes with their highly developed auditory system to extract the information about insects or obstacles. These bats progressively shorten the duration, lower the frequency, decrease the intensity and increase the repetition rate of emitted pulses as they search, approach, and finally intercept insects or negotiate obstacles. This dynamic variation in multiple parameters of emitted pulses predicts that analysis of an echo parameter by the bat would be inevitably affected by other co-varying echo parameters. The progressive increase in the pulse repetition rate throughout the entire course of hunting would presumably enable the bat to extract maximal information from the increasing number of echoes about the rapid changes in the target or obstacle position for successful hunting. However, the increase in pulse repetition rate may make it difficult to produce intense short pulse at high repetition rate at the end of long-held breath. The increase in pulse repetition rate may also make it difficult to produce high frequency pulse due to the inability of the bat laryngeal muscles to reach its full extent of each contraction and relaxation cycle at a high repetition rate. In addition, the increase in pulse repetition rate increases the minimum threshold (i.e. decrease auditory sensitivity) and the response latency of auditory neurons. In spite of these seemingly physiological disadvantages in pulse emission and auditory sensitivity, these bats do progressively increase pulse repetition rate throughout a target approaching sequence. Then, what is the adaptive value of increasing pulse repetition rate during echolocation? What are the underlying mechanisms for obtaining maximal information about the target features during increasing pulse repetition rate? This article reviews the electrophysiological studies of the effect of pulse repetition rate on multiple-parametric selectivity of neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicus fuscus using single repetitive sound pulses and temporally patterned trains of sound pulses. These studies show that increasing pulse repetition rate improves multiple-parametric selectivity of inferior collicular neurons. Conceivably, this improvement of multiple-parametric selectivity of collicular neurons with increasing pulse repetition rate may serve as the underlying mechanisms for obtaining maximal information about the prey features for successful hunting by bats.

Key words： bat; echolocation; inferior colliculus; multiple-parametric selectivity; pulse repetition rate

Acknowledgments

The research works described in this review article have been supported by the National Science Foundation and National Institute of Health of USA, Human Frontier Science Program of International Brain Research Organization, the Research Board, Research Council, College of Arts and Sciences, and Division of Biologic Sciences of University of Missouri-Columbia, USA. I particularly like to express my sincere thanks to my former coworkers in participation of all the studies described in this review. They are Drs, Qi-Cai Chen, Rui-Ben Feng, Tsutomu Kamada, Erika Hou, Ruihong Luan, Toshio Moriyama, Danial Pinheiro, Peter Schlegel, Xinde Sun, Haibing Teng, Min Wu, Chung Hsin Wu, Ji Ping Zhang and Xiaoming Zhou.

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