Distress vocalization sequences broadcasted by bats carry redundant information

Abstract

Distress vocalizations (also known as alarm or screams) are an important component of the vocal repertoire of a number of animal species, including bats, humans, monkeys and birds, among others. Although the behavioral relevance of distress vocalizations is undeniable, at present, little is known about the rules that govern vocalization production when in alarmful situations. In this article, we show that when distressed, bats of the species Carollia perspicillata produce repetitive vocalization sequences in which consecutive syllables are likely to be similar to one another regarding their physical attributes. The uttered distress syllables are broadband (12-73 kHz) with most of their energy focussing at 23 kHz. Distress syllables are short (~4 ms), their average sound pressure level is close to 70 dB SPL, and they are produced at high repetition rates (every 14 ms). We discuss that, because of their physical attributes, bat distress vocalizations could serve a dual purpose: (1) advertising threatful situations to conspecifics, and (2) informing the threatener that the bats are ready to defend themselves. We also discuss possible advantages of advertising danger/discomfort using repetitive utterances, a calling strategy that appears to be ubiquitous across the animal kingdom.

Vocal sequences suppress spiking in the bat auditory cortex while evoking concomitant steady-state local field potentials

“…1b–c). Distress sequences uttered by C. perspicillata are composed of syllables repeated at rates of ~14 ms4. This repetition rate is faster than that observed while C. perspicillata is calling in other scenarios, such as echolocation, in which the shortest calling intervals are above 26 ms425253.…”

“…When under duress or when in the presence of danger, most vertebrate species produce sounds that (among other functions) serve to advertise the ongoing discomfort situation. Calls uttered in distress circumstances are known as “distress calls”, “alarm calls”, or “screams”234567891011, and they have profound effects in the listeners’ physiology and behaviour, such as boosting gene expression, elevating heart-rate, activating the neuroendocrine axis, and evoking exploratory and mobbing behaviours in conspecifics5679101213141516171819202122.…”

“…The physical attributes of distress vocalizations have been intensively studied (see346161923242526272829) but, at present, comparably little is known about how distress signals are processed in the listeners’ brain, especially at the level of the auditory cortex (AC). Distress vocalizations are produced in the form of “sequences” composed of individual sound units defined as “syllables”41924253031.…”

“…Distress vocalizations are produced in the form of “sequences” composed of individual sound units defined as “syllables”41924253031. In the AC, representing the information carried by single syllables embedded within a vocal sequence can be affected by phenomena such as forward suppression and adaptation.…”

“…C. perspicillata - the bat species used as animal model for this study - is known to produce two basic types of distress sequences (see4): short sequences containing a single group of syllables (Fig. 1a); and long sequences containing multiple syllable groups defined as multi-syllabic bouts (see Fig.…”