Acoustic Signaling by Singing Humpback Whales (Megaptera novaeangliae): What Role Does Reverberation Play?

Abstract

When humpback whales (Megaptera novaeangliae) sing in coastal waters, the units they produce can generate reverberation. Traditionally, such reverberant acoustic energy has been viewed as an incidental side-effect of high-amplitude, long-distance, sound transmission in the ocean. An alternative possibility, however, is that reverberation actually contributes to the structure and function of songs. In the current study, this possibility was assessed by analyzing reverberation generated by humpback whale song units, as well as the spectral structure of unit sequences, produced by singers from different regions. Acoustical analyses revealed that: (1) a subset of units within songs generated narrowband reverberant energy that in some cases persisted for periods longer than the interval between units; (2) these highly reverberant units were regularly repeated throughout the production of songs; and (3) units occurring before and after these units often contained spectral energy peaks at non-overlapping, adjacent frequencies that were systematically related to the bands of reverberant energy generated by the units. These findings strongly suggest that some singing humpback whales not only produce sounds conducive to long-duration reverberation, but also may sequentially structure songs to avoid spectral overlap between units and ongoing reverberation. Singer-generated reverberant energy that is received simultaneously with directly transmitted song units can potentially provide listening whales with spatial cues that may enable them to more accurately determine a singer’s position.

Chickadee songs provide hidden clues to singers’ locations

“…Animals that live in highly reverberant, low visibility environments and that often coordinate their movements (e.g., whales and dolphins) would be the most likely to benefit from the use of such cues. Consistent with this possibility, recent analyses of songs produced by humpback whales revealed that individual sounds within their songs also generate reverberant tails that overlap with subsequent sounds, and that ratios of frequencies across consecutive sounds are similar to those found in chickadee songs (Mercado, 2016). Closer examination of correlations between the spectra of sequential sounds produced by other species that acoustically communicate over long distances may further clarify the extent to which such auditory distance cues enable organisms to better coordinate their actions when visual inputs are degraded.…”

The Sonar Model for Humpback Whale Song Revised

“…Bats and dolphins make use of a fixed set of sonar signals with specific acoustic properties that have been highly refined through evolutionary processes to yield predictable echoic features ( Simmons and Stein, 1980 ). Singing humpback whales, in contrast, seem to use a more varied repertoire of units, which they produce in an exceptionally wide range of soundscapes; the acoustic properties of units within a song can vary from highly stable to highly dynamic ( Mercado et al, 2010 ; Mercado, 2016 ). Use of ultrasound by bats and dolphins enables them to rapidly extract fine-resolution spatial details from echoes.…”

“…Singing humpback whales produce large numbers of units with CF or quasi-CF (minimally FM) components ( Ou et al, 2013 ). In at least some environments, these units can generate spectrally narrow, reverberant bands (overlapping environmental echoes) that persist 10 s or more, such that acoustic energy within the band lasts for as long as the singer repeats a phrase ( Mercado, 2016 ). Units immediately following CF units often contain acoustic energy focused at frequencies just above or just below the CF band ( Figure 5 ).…”

“…For example, the frequency content of units often gradually shifts to lower frequencies as a song cycle progresses ( Mercado et al, 2010 ; Mercado and Handel, 2012 ; Kowarski et al, 2018 ). Although gradual shifts in phrase structure are evident within and across years ( Payne et al, 1983 ; Payne and Payne, 1985 ), the acoustic relationships between consecutive units appear to be stable across years and populations ( Green et al, 2011 ; Mercado, 2016 ). In fact, objective analyses of phrase structure show several commonalities (e.g., prototypical modes of unit alternation) across years and populations ( Mercado et al, 2003 ; Mercado, 2016 ).…”

“…Although gradual shifts in phrase structure are evident within and across years ( Payne et al, 1983 ; Payne and Payne, 1985 ), the acoustic relationships between consecutive units appear to be stable across years and populations ( Green et al, 2011 ; Mercado, 2016 ). In fact, objective analyses of phrase structure show several commonalities (e.g., prototypical modes of unit alternation) across years and populations ( Mercado et al, 2003 ; Mercado, 2016 ). In short, sequencing of both units and themes within songs is more consistent over time and locales than past literature suggests.…”