REVIEW The adaptive value of increasing pulse repetition rate during hunting by echolocating bats Philip H.-S. JEN (✉) Division of Biological Sciences, University of Missouri, Columbia, MO 65211, USA © Higher Education Press and Springer-Verlag Berlin Heidelberg 2012 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. Keywords bat, echolocation, inferior colliculus, multiple-parametric selectivity, pulse repetition rate Introduction During hunting, insectivorous bats such as the big brown bat, Eptesicus fuscus, emit ultrasonic pulses and listen to the returning echoes as they search, approach and finally catch the localized insects or avoid obstacles (Griffin, 1958; Simmons et al., 1979; Jen and Kamada, 1982; Surlykke and Moss, 2000). Previous studies have shown that insectivorous bats prepare their auditory system to analyze changing echo parameters for successful orientation and prey capture. They progressively shorten the pulse duration to avoid the overlap between the outgoing sounds and returning echoes and they systematically decrease pulse intensity to compensate for progressively increasing echo intensity so as to ensure the echoes reaching the ear at an optimal level (Novick, 1971; Schnitzler and Henson, 1980; Jen and Kamada, 1982; Kobler et al., 1985; Hartley, 1992a, b; Smotherman and Metzner, 2003; Hiryu et al., 2007). They also contract their middle ear muscles and send inhibitory signals from their vocalization center(s) to suppress the sensitivity of midbrain auditory neurons during pulse emission in order to protect their auditory system from overstimulation by the intense self- emitted pulses but to maintain high sensitivity to weak Received February 24, 2012; accepted March 12, 2012 Correspondence: Philip H.-S. JEN E-mail: jenp@missouri.edu Front. Biol. 2013, 8(2): 198–215 DOI 10.1007/s11515-012-1212-4