Meet the songbirds of the Arctic Sea: The bowhead whales | @GrrlScientist

The Critically Endangered bowhead whale is unusual amongst mammals because they sing a diverse repertoire of many distinct and complex songs throughout the months-long polar night

by GrrlScientist for Forbes | @GrrlScientist

NOTE: originally published under the title: “Bowhead Whales: Songbirds Of The Arctic Sea”

A bowhead whale (Balaena mysticetus) swims just under the water’s surface with its mouth open, feeding, in Fram Strait, northwest of Norway.
(Credit: Kit M. Kovacs / Norwegian Polar Institute)

Although singing complex songs is common amongst songbirds, most mammals lack this ability. But two species of baleen whales — the humpback, Megaptera novaeangliae, and the bowhead, Balaena mysticetus — are amongst the rare exceptions.

The more ubiquitous humpback is better known than the mysterious bowhead whale, whose geographic range is restricted to the Arctic. In fact, bowhead whales got their name from their massive, dome-shaped skull that they use to bash through sea ice that may be as much as 60 centimeters (2 feet) thick. They have the largest mouth of any animal, festooned with two rows of about 300 vertical baleen plates, which at 300–450 centimeters (9.8–15 feet) long are the largest of any whale. Bowhead whales are insulated against frigid Arctic waters with an exceptionally thick layer of blubber.

Bowhead whales are slow swimmers, with a top speed of about 6mph, and they don’t tend to make deep dives. This, combined with their economically valuable blubber and baleen almost pushed them into extinction. Worldwide, there were probably more than 50,000 bowhead whales, but this species was hunted until fewer than 3,000 animals remained alive in the early 1920s (link). There are four distinct populations, some of which are recovering better than others from the ravages of hunting. The study whales were part of the Spitsbergen bowhead whale population, one of the smaller groups, which was hunted almost to extinction in the 1600s and was recently estimated to number roughly 200 animals.

Serendipitous discovery

Oceanographer Kate Stafford, in the Applied Physics Laboratory at the University of Washington, is an expert on using sound to track and study marine mammals. She first identified bowhead whales singing in some audio recordings made near Greenland in 2007.

In this new study, Dr. Stafford and her collaborators, Christian Lydersen and Kit Kovacs of the Norwegian Polar Institute, and Øystein Wiig of the Natural History Museumat the University of Oslo, expanded the original five-month song collection from that earlier study (ref) to cover more than three years.

“We were hoping when we put the hydrophone out that we might hear a few sounds,” Dr. Stafford said in a statement. “When we [listened], it was astonishing: Bowhead whales were singing loudly, 24 hours a day, from November until April.”

To make these audio recordings, Dr. Stafford and her collaborators placed special underwater recording equipment in the western Fram Strait, a frigid water channel that runs north-south between Greenland and the Svalbard archipelago.

Map of Fram Strait. Green dot shows where the hydrophone was located from 2010–2014. This icy region is only accessible to ships in late summer. The underwater microphone (a hydrophone) was moored about 80 meters below the surface in water 1,020 meters deep.
(Credit: Kate Stafford /
University of Washington)

The Fram Strait, the only deep water connection between the Arctic Ocean and the other oceans of the world, connects the Arctic and the Greenland Seas. The hydrophone was located roughly 80 meters (262 feet) below the surface of the sea in the Fram Strait in water that was 1,020 meters (3346 feet) deep. The hydrophone remained in place from 2010 to 2014.

Bowhead whales sing many intricate but ephemeral songs

The research team recorded bowhead whale songs 24 hours per day throughout the polar winter, and recorded the greatest number of songs in December and January (Figure 1):

Figure 1. Total numbers of bowhead whale song types recorded 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.
(doi:
10.1098/rsbl.2018.0056)

These recorded bowhead whale songs, the largest collection amassed so far, revealed that these marine mammals have a surprisingly varied and constantly changing vocal repertoire.

“If humpback whale song is like classical music, bowheads are jazz,” Dr. Stafford explained. “The sound is more freeform.”

“With bowheads, you never know where they’re going,” Dr. Stafford added.

During the three year study period, a total of 184 song types were recorded — a number that increased each year during the three-year study. The fewest song types were recorded in 2010–2011 (39 song types total, 895 recordings). Both 2012–2013 (69 song types total, 1338 recordings) and 2013–2014 (76 song types total, 998 recordings) had approximately twice as many different song types.

The variety of bowhead whale songs was so surprising that the researchers compared them to songbirds (ref).

No, that’s not a distant duck!
A bowhead whale (Balaena mysticetus) surfaces among ice floes in Fram Strait.
(Credit: Kit Kovacs / Norwegian Polar Institute)

As many bird watchers know, songs are distinct from calls because of their auditory complexity: songs contain intricate and unique phrasing that must be learned. Birds use song to identify themselves as individuals as well as to impress any lady birds that may be listening in, and to defend a territory.

But whales are not songbirds, nor do they use sound in identical ways.

“For marine mammals, acoustics is how they do everything,” Dr. Stafford elaborated. “Humans are mostly visual animals, but marine mammals live in a three-dimensional habitat where sound and acoustic information is how they navigate, how they find food, how they communicate.”

For these reasons, the complexity and distinct musical phrasing of bowhead whale songs was remarkable. Yet most bowhead whale songs were performed over a very short time period — sometimes just a few hours or days and were never heard again — and most songs were seldom recorded over time periods of longer than one month. Yet, at the same time, a few song types were recorded throughout the entire winter (Figure 2):

Figure 2. Total number of hours and months during which each song type was recorded by year. 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. (a) 2010–2011: 38 song types were recorded; (b) 2012–2013: 69 song 157 types recorded; © 2013–2014: 76 song types were recorded.
(doi:
10.1098/rsbl.2018.0056)

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. Of the 3231 song recordings made during the three year study, slightly over half (53%) contained only a single song type, while two different songs occurred in 37% of the recording periods. Only a few recording periods included song diversity of 3 (9%) or 4 (less than 1%) distinct songs.

“[W]hen we looked through four winters of acoustic data, not only were there never any song types repeated between years, but each season had a new set of songs,” Dr. Stafford said.

This contrasts with what we’ve learned about humpback whales. That species, which breeds off the coasts of Hawaii and western Mexico, has a common song shared by males within each population. Although this song changes from one breeding season to the next, it basically remains the same, with only slight changes, throughout each breeding season.

“It was thought that bowhead whales did the same thing, based on limited data from springtime,” Dr. Stafford explained. “But those 2008 recordings were the first hint, and now this data confirms that bowhead whale songs are completely different from the humpbacks’.”

This study generates more questions than it answers

Like all good research, this study provides the scientific basis for asking a plethora of new questions about bowhead whales. For example, it’s suspected that the Arctic winter is bowhead whale breeding season, and that male whales sing to impress the ladies or to compete in acoustic competitions with other males. Or both. But at this time, no one knows whether each whale performs just one special melody or selects specific tunes — or mixes and matches short phrases — from a larger oeuvre.

“Why are they changing their songs so much?” Dr. Stafford said. “In terms of behavioral ecology, it’s this great mystery.”

This study suggests bowhead whales may be similar to cowbirds and meadowlarks — songbirds that learn and perform a diverse and dynamic repertoire of songs — maybe because novelty offers some advantage.

Of course, it’s conceivable that females sing. It’s also possible that the four different bowhead whale populations maintain their genetic isolation from each other through their songs. For example, previous research found that the two North Pacific populations — the Okhotsk Sea population and the similarly-sized study population, the Spitsbergen bowheads — are genetically isolated from each other, indicating that individuals rarely roam between these two populations (link). But considering that these four bowhead whale populations will not remain isolated from each other as Arctic sea ice grows ever thinner, this raises questions about whether song can maintain these populations’ genetic isolation, particularly in view of this species’ extraordinary diversity in song types.

Answers to these questions would provide a reasonable biological context for this vast annual Arctic concert, but because it’s impossible to track or observe whales during the Arctic winter using current technologies, it’s impossible to know the reasons that govern bowhead whale singing.

“Bowhead whales do this behavior in the winter, during 24-hour darkness of the polar winter, in 95 to 100 percent sea ice cover. So this is not something that’s easy to figure out,” Dr. Stafford said. “We would never have known about this without new acoustic monitoring technology.”

Already, Dr. Stafford and her collaborators are developing plans and new technologies so they, say, can attach acoustic tags to individual bowhead whales that will record its songs and journeys. Not only would this help them answer the many questions they have about singing, it will also give them more data they need to make a more accurate population estimate. Further, such technology may help Dr. Stafford and her collaborators get a realistic idea of how thinning Arctic sea ice is affecting these animals.

“Bowheads are superlative animals: they can live 200 years, they’ve got the thickest blubber of any whale, the longest baleen, they can break through ice,” Dr. Stafford said. “And you think: They’ve evolved to do all these amazing things. I don’t know why they do this remarkable singing, but there must be a reason.”

Source:

Stafford KM, Lydersen C, Wiig Ø., and Kovacs KM. (2018). Extreme diversity in the songs of Spitsbergen’s bowhead whales, Biology Letters, 20180056 | doi:10.1098/rsbl.2018.0056

Also cited:

Kathleen M. Stafford, Sue E. Moore, Catherine L. Berchok, Øystein Wiig, Christian Lydersen, Edmond Hansen, Dirk Kalmbach, and Kit M. Kovacs (2012). Spitsbergen’s endangered bowhead whales sing through the polar night, Endangered Species Research, 18:95–103, | doi:10.3354/esr00444


Originally published at Forbes on 6 April 2018.

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