Frequency shifting reduces but does not eliminate acoustic interference between echolocating bats: A theoretical analysis.

Bats have been observed to shift the frequency of their echolocation calls in the presence of other echolocating bats, ostensibly as a way to reduce acoustic interference. Few studies, however, have examined the theoretical efficacy of such jamming avoidance responses. The present study uses the wideband ambiguity function to analyze the effects of acoustic interference from conspecifics and congeneric heterospecifics on the target acquisition ability of Myotis californicus and Myotis yumanensis, specifically whether unilateral or bilateral frequency shifts reduce the effects of such interference. Model results suggest that in conspecific interactions, M. yumanensis recovers its target acquisition ability more completely and with less absolute frequency shift than does M. californicus, but that alternative methods of jamming avoidance may be easier to implement. The optimal strategy for reducing heterospecific interference is for M. californicus to downshift its call and M. yumanensis to upshift its call, which exaggerates a preexisting difference in mean frequency between the calls of the two species. Further empirical research would elucidate whether these species do in practice actively employ frequency shifting or other means for jamming avoidance, as well as illuminate the role of acoustic interference in niche partitioning.

Neural representation of the self-heard biosonar click in bottlenose dolphins (Tursiops truncatus).

The neural representation of the dolphin broadband biosonar click was investigated by measuring auditory brainstem responses (ABRs) to "self-heard" clicks masked with noise bursts having various high-pass cutoff frequencies. Narrowband ABRs were obtained by sequentially subtracting responses obtained with noise having lower high-pass cutoff frequencies from those obtained with noise having higher cutoff frequencies. For comparison to the biosonar data, ABRs were also measured in a passive listening experiment, where external clicks and masking noise were presented to the dolphins and narrowband ABRs were again derived using the subtractive high-pass noise technique. The results showed little change in the peak latencies of the ABR to the self-heard click from 28 to 113 kHz; i.e., the high-frequency neural responses to the self-heard click were delayed relative to those of an external, spectrally "pink" click. The neural representation of the self-heard click is thus highly synchronous across the echolocation frequencies and does not strongly resemble that of a frequency modulated downsweep (i.e., decreasing-frequency chirp). Longer ABR latencies at higher frequencies are hypothesized to arise from spectral differences between self-heard clicks and external clicks, forward masking from previously emitted biosonar clicks, or neural inhibition accompanying the emission of clicks.

Vocalizations associated with anxiety and fear in the common marmoset (Callithrix jacchus).

Vocalizations of common marmoset (Callithrix jacchus) were examined under experimental situations related to fear or anxiety. When marmosets were isolated in an unfamiliar environment, they frequently vocalized "tsik-egg" calls, which were the combination calls of 'tsik' followed by several 'egg'. Tsik-egg calls were also observed after treatment with the anxiogenic drug FG-7142 (20mg/kg, sc). In contrast, when marmosets were exposed to predatory stimuli as fear-evoking situations, they frequently vocalized tsik solo calls as well as tsik-egg calls. These results suggest that marmosets dissociate the vocalization of tsik-egg and tsik calls under conditions related to fear/anxiety; tsik-egg solo vocalizations were emitted under anxiety-related conditions (e.g., isolation and anxiogenic drug treatment), whereas a mixed vocalization of tsik-egg and tsik was emitted when confronted with fear-provoking stimuli (i.e., threatening predatory stimuli). Tsik-egg call with/without tsik can be used as a specific vocal index of fear/anxiety in marmosets, which allows us to understand the neural mechanism of negative emotions in primate.