| Little brown bats (Myotis lucifugus) have an exquisite ability to echolocate; they can utilize their sonar system to navigate, and detect, identify, locate and track prey, in darkness. During echolocation, these bats emit downward frequency modulated (FM) signals, which are situation-, colony-, and individual-specific. Their auditory systems are tailored to processing signals used in echolocation. For example, in the inferior colliculus (IC), neurons tuned to frequencies found in their echolocation signals are sensitive to pulse-echo combinations with specific echo delays (the critical information for target range). The sonar signals of echolocating bats have been studied in detail, but their communication calls are less well characterized despite the widespread use in their social interactions, e.g. in mother-young interactions, and in defending or advertising feeding areas.;The goals of my research were to characterize, and determine the behavioral relevance of, sound communication signals of little brown bats, and investigate the anatomical substrates for processing of these signals. In chapter 2, I recorded the communication calls of M. lucifugus from individual adult, male bats overnight, and developed an automatic classification algorithm to objectively classify the recorded calls. The automatic classification algorithm employed 5 acoustic features to extract each communication signal: center frequency, kurtosis (frequency), duration, skew (duration), and standard deviation (duration). Based on these features, I found 5 distinct communication signals: downward FM, steep FM, constant frequency, broadband noise burst, and broadband click train.;In chapter 3, I investigated the behavioral relevance of the communication signals. First, I used focal pair sampling to develop an ethogram for M. lucifugus. The ethogram revealed 5 solitary behaviors (approach, break contact, contact, head scan, leave) and 5 social behaviors (head scan, kicking, locomotion, torso shake, and wing movement) in which bats emitted vocal communication signals. The two dominant communication calls displayed individual signatures and were associated with various behaviors. Specifically, steep FM calls were associated with (social) approach, contact, head scan, leave and (solitary) head scan and locomotion. Broadband noise bursts were associated with (social) approach, contact, and leave.;In chapter 4, I investigated subcortical structures (i.e., thalamus and IC) involved in the processing of communication signals, using egr1 (an immediate early gene) as a molecular marker for brain activation. Three species-specific signals were included -- downward FM (DFM), courtship, and echolocation. Reversed DFM and silence groups were used as a comparison for basal activity. Call-dependent egr1 expression was found in all subdivisions of the thalamus and IC. Specifically, the central nucleus of the IC showed robust expression for DFM and echolocation calls, followed by courtship calls. The external nucleus and dorsal cortex of the IC also showed robust expression for DFM and echolocation calls, but not for courtship calls. The medial geniculate body of the thalamus showed generally weaker expression compared to the IC, with higher expression levels for DFM and echolocation signals compared to reversed calls and silence. Results of the present egr1 studies suggest that these subcortical auditory regions are shared for processing echolocation and communication signals. |
