Mid-frequency song and low-frequency calls of sei whales in the Falkland Islands

Abstract

Although sei whales ( Balaenoptera borealis ) are distributed throughout the globe, their behaviour and vocal repertoire are poorly described. We used passive acoustic monitoring to describe the vocal behaviour of sei whales in the Falkland Islands, between December 2018 and April 2019. We isolated more than 2000 low-frequency calls for manual classification, of which 510 calls with high signal-to-noise ratio were quantitatively measured. Five categories of stereotyped call types in the 15–230 Hz range were described, some with multiple subcategories. These included some similar to previously described calls (e.g. downsweeps), but others that were novel in acoustic structure and frequency band. In the mid-frequency range, we documented a highly stereotyped, hierarchically structured and rhythmically repetitive song display. Songs were arranged in phrases with a structure composed of repetitive sub-phrases, and a diverse variety of sounds in the 1–5 kHz range. Singing commenced in late February, despite the presence of whales and calls since early December, and continued through April. These acoustic properties and behavioural characteristics indicate that this is likely a male breeding display similar to songs and singing of other balaenopterids. This is the first detailed description of a song display for sei whales, highlighting the importance of the Falkland Islands.

Greater Horseshoe Bats Recognize the Sex and Individual Identity of Conspecifics from Their Echolocation Calls

“…For example, when more individuals of R. clivosus were included, the classification success decreased because of the increasing overlap of the pulses of the individuals [ 27 ]. In addition, many mammals encode their acoustic individual characteristics with source characteristics, such as frequency, amplitude contours, and harmonic structure [ 60 , 63 , 64 ]. Therefore, the 13 acoustic parameters that we measured in this study may not truly represent the full range of acoustic differences among individuals.…”

Cetaceans are the next frontier for vocal rhythm research

“…Heterochrony is not as well documented but is present in the songs of at least six mysticetes: blue ( Balaenoptera musculus ) ( 27 , 86 , 87 ), fin ( Balaenoptera physalus ) ( 88 ), humpback ( 4 , 91 ), dwarf minke ( Balaenoptera acutorostrata ) ( 28 ), North Pacific right ( 96 ), and sei ( Balaenoptera borealis ) ( 84 ) whales. It typically manifests as two or three distinct, repeated intervals within single songs ( Table 2 and SI Appendix , Table S2 ).…”

“…Some promising avenues for future research include the following: Rhythm in hierarchical vocal displays: Most analyses of temporal structure focus only on the most granular level, even though many species produce hierarchically organized vocal displays. For mysticetes with hierarchical songs, like humpback ( 74 ), bowhead ( 98 ), North Pacific right ( 96 ), and sei ( 84 ) whales, intervals are often measured at the “call” level but rarely at higher “phrase” or “song” levels. Traditional analyses accordingly fail to capture the full rhythmic complexity of these displays ( 62 , 63 ).…”