Deep Tracks: Researchers Dive Into Bowhead Whale Song

A bowhead whale swims in the Arctic. (Photo by Vicki Beaver/North Slope Borough via NOAA)

(CN) – Giving some airtime to an often-overlooked arctic crooner, oceanography research published online Wednesday breaks down for the first time the diversity of the bowhead whale’s song catalog.

“Because bowhead whales sing underwater, in heavy ice during the polar night, a nuanced understanding of the variable syntax of this species will be difficult to obtain,” the article published in Biology Letters by the Royal Society states. “Nevertheless, the singing behavior of Spitsbergen bowhead whales, in which tens of distinct song types are produced annually, makes them remarkable among mammals.”

While most research on baleen whale songs to date has focused on the humpback whale, University of Washington oceanographer Kate Stafford and fellow researchers concluded after their three-year study of bowhead whales that they are rivaled only songbirds in the diversity and interannual variability of songs.

Recording the whales with omnidirectional hydrophone equipment in an area of the northeast Atlantic called the western Fram Strait, the research team captured 184 different song types from the Spitsbergen bowhead whale population over the course of three years.

“While most song types were short-lived — from hours to days — and seldom lasted longer than a month, every year a few song types persisted throughout the winter,” the article states. “The overall trend for all years was a progression of song types appearing and then disappearing over time, with the greatest within-year diversity occurring in January for all three years examined.”

Though the researchers deployed their recording equipment on moorings from 2010 to 2010, the mooring deployed in 2011 was never recovered.

The study notes many mammals produce repetitive calls but complex “song” in the class is rare.

“Among other mammalian singers, mice and gibbons tend to produce highly stereotyped and repetitive songs with few elements,” the article states. “Variation in rock hyrax and bat songs is primarily through changes in the arrangement of units. Humpback whales produce complex songs that are similar within a year. Although the repertoire of any one individual bowhead whale in this study cannot be determined, the catalogue of song types (184) is remarkably varied.”

This graph from Biology Letters shows the total numbers of bowhead whale song types recorded by researchers in each month (bars) and cumulative number of song types (dashed lines) by year. The greatest number of different song types occurs in December and January, presumed to be the peak of mating season for bowhead whales.

The study also looks at mammalian singing by gender, noting that complex songs in mammals are thought to be produced only by males, “with the exception of gibbons, in which males and females duet.”

“Male mammals are thought to sing to defend territories, advertise their quality, attract mates or some combination of these functions,” the article states.

If a diverse song repertoire attracts more mates, increased song complexity might confer reproductive advantages, the research suggests.

The study notes that one recent study of howler monkeys showed something a trade-off in male reproductive characteristics: Groups with fewer males tended to show males investing “more in vocal displays as a reproductive tactic.”

“Normally, testes size and vocal repertoire (or other reproductive displays) are considered evolutionary trade-offs: depending upon social context, one of these may provide a selective advantage for individuals within a population over the other,” the article states. “For example, humpback whales have relatively small testes, and engage in physical competition as well as producing complex song displays, while right whales (Eubalaena spp.) have enormous testes, are drawn to ‘surface active groups’ by a vocalizing female, and lack any apparent male acoustic display.”

Male bowhead whales are unique, however, in that they “have both large testes and large vocal repertoires.”

Recounting what has been observed in humpback whales, the article notes that males in this species sing similar songs within a season across a whole population.

“The structure of that song gradually evolves over the season in unison,” the article states.

This graph from Biology Letters shows, by year, the total number of hours and months during which researchers recorded each song type by bowhead whales. In most cases, a song type was only recorded in one month, though in some instances the same song type was recorded in two to four different months.

Bowhead whale songs by contrast appear to “generally consist of a single phrase that includes amplitude- and frequency-modulated elements repeated in bouts, with two different sounds often produced simultaneously.”

Researcher Stafford noted that before her project, a pilot study from the Fram Strait in 2008–09 provided the first indication that tens of song types were produced by bowhead whales in this region within a single overwinter period.

“No year-round studies of song diversity exist for other bowhead whale populations although multiple song types in a single year have been documented for two other populations,” the article states.

Among other conclusions still yet to be made, the researchers called it unknown whether individual bowhead whales sing multiple song types in a season, though some are known to share the same song type in the same period in the Bering–Chukchi–Beaufort population.

“Nor is it known if individual bowhead whales maintain the same song throughout their lifetime or if they switch within and/or between years,” the article states.

Furthermore the mating explanation for song diversity is one of several possible theories researchers are floating.

The article notes that “extreme declines in sea ice extent and thickness may have facilitated contact between these populations” may have facilitated contact between two populations of bowhead whale: the Bering–Chukchi–Beaufort population and the eastern Canada–western Greenland population.

“However, even if this region contains bowhead whales from multiple populations, this does not fully explain the high numbers of different song types recorded in this study or the lack of recurrence of song types from year to year,” the article states.

“It is plausible that the bowhead whales in the Fram Strait are simply a remnant of the original Spitsbergen population that survived the extreme historical levels of exploitation,” the article continues. “The influence of small population size on song diversity is conflicted; some studies suggest song diversity increases in smaller populations, although others have found that reduced or isolated populations exhibit a reduction in song diversity and produce simpler songs.”

The article emphasizes as well that bowhead whales are the only High Arctic resident baleen whale.

“Thus, interspecific identification via song may not confer the same selective advantage for bowheads that it might for other cetacean species,” it states. “This could reduce selection pressure on song stereotypy, allowing for greater variation in song types as a result of a long-term cultural mutation in songs, or song novelty itself might confer an advantage.”

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