Regularities in spectral entropy variations within humpback whale song sessions

Abstract

Consecutive songs produced by humpback whales recorded off the coast of Hawaii show recurrent patterns of acoustic variation. Analyses of spectral entropy, a measure of the uniformity of energy distribution in the frequency domain, revealed that spectral entropy cycles in predictable patterns within song sessions. At time scales spanning tens of minutes, variations in spectral entropy appear to track changes in received intensity levels, which are correlated with song duration and may reflect the diving behavior of singers. Intra-individual variations in spectral entropy suggest that singing humpback whales may also vocally control entropic features of songs, both by gradually modulating the acoustic characteristics of units and by varying the amount of time they spend producing specific patterns of units. Preliminary analyses suggest that variations in spectral entropy within song cycles are indicative of the amount of time and energy singers spend producing constant-frequency (CF) elements versus more frequency-modulated/broadband elements. CF elements of units are generally more difficult to spatially localize, but easier to detect over long distances, suggesting that singing humpback whales may be varying the production of consecutive songs in ways that affect auditory spatial processing.