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Tuesday September 28th, 2021
Environment

Song Sung Blue*

The biggest creature ever to live, perhaps the largest that ever could live, abounds on Sri Lanka’s porches. This past decade has established Sri Lanka as one of the world’s premier locales for seeing and studying blues. Beneath shallow waters outward from Lankan shores lies continental shelf: a gently sloping sea floor stretching to an edge where the bottom drops steeply into the deep. Blues enjoy cruising deepening waters at shelf edges. They find lots to eat there and they may use the shelf edges in navigating. The southernmost wedge of the Indian (sub)continental shelf happens to ‘pinch in’ close to the south Lankan shore, precisely where blues migrating back and forth between the Arabian Sea and the Bay of Bengal find their shortest route. Some of them sing along the way.

Blues emit extremely loud moans so low in frequency as to be near or below the lowest edge of unaided human hearing, comparable to the deepest pipes of large cathedral organs. They could pop your eardrums were you to be swimming nearby even if you couldn’t ‘hear’ them. These songs probably come only from males and likely play a role in locating or attracting mates. It appears plausible that blues hear each other calling for hundreds if not thousands of miles, perhaps across entire ocean basins. This means they could be ‘in touch’ with one another far more intensively than would seem to be the case from the fact that they do not seem to cluster that much.

Blues subsist in perhaps a dozen relatively distinct population groups spread across the world’s oceans. Though there is overlap, there is also a fair degree of geographic separation. Each population (some would say ‘subspecies’) has a singing style common to all members but slightly different from that of other populations. A previously undetected song population has recently been identified in the western Indian Ocean from recordings near Madagascar, Oman and the mid-ocean Chagos Archipelago. If it gets as far as the Chagos, this population might reach Sri Lanka as well. Someone should be listening.

Recordings reveal the startling fact that the sound pitch of blue songs across all populations has dropped just a bit every year for the past several decades, as long as we have been recording. Since the 1960s, this pitch drop adds up to the equivalent of three white piano keys. Populations must be following one another’s songs so that they all move in the same direction: slightly lower year by year.

In previously posted pieces, I speculated that this rapid pitch change represents ‘fashion’ among the blues. (For example, see: Sri Lanka’s Blue Whale is a Superb Athlete. Is She Also a Scholar?, EconomyNext, Jan. 9, 2021.) By ‘fashion’ I meant arbitrarily shifting aesthetic preferences linked to novelty and interest, akin to flared trousers, rising and falling hemlines or oscillations on suit jacket lapels. If female blues favor low-pitch mating calls year in and year out, male blues will learn what they need to do.

Skepticism toward my ‘fashion’ hypothesis is certainly warranted. It piggybacks on, but differs from, an evolutionary process called ‘sexual selection.’ Biologists understand ‘sexual selection’ as a process whereby females mysteriously find certain male displays sexy. Masters of such display mate more successfully and pass genes producing those displays (e.g., peacock feathers) on to their sons, who themselves mate successfully in turn. In The Descent of Man and Selection in Relation to Sex, Darwin himself wrote extensively about sexual selection, distinguishing it from ‘natural selection’ as described in The Origin of Species. Through natural selection, animals compete and gene pools shift on the basis of comparative success in surviving. Through sexual selection, it is comparative success in siring offspring. Darwin hints that he stumbled into his theory of sexual selection upon torturing himself as to how outlandish peacock feathers could possibly confer survival value.

Even though it may be a mating display, the blue whale pitch shift cannot be an example of genetically-propagated sexual selection. It affects entire populations—indeed the entire species—year by year: far too fast to be genetic and generational. So if we have males all changing their song display year by year in uniform ways to keep up with what females for some reason deem sexy, we have something I called ‘fashion.

Marvelous as it surely would be if female blues were selecting mates on the basis of fashion, other possible explanations require consideration. Recent studies have explored the question.

One hypothesis that quickly comes to mind is that the whales are struggling to make themselves heard over the roar of big ships and other oceanic noise pollution such as sonar and seismic surveys. This hypothesis fails on perhaps three counts. One is that the pitch shift is heard even in southern oceans, where ship traffic is slight now that whaling is over, and in other non-noisy places. Another objection is that blues pause their calling when ships are close, as if acknowledging that outscreaming them will not work. Thirdly, a struggle to outshout ship noise would likely produce an upward shift in pitch rather than a downward one. In southerly seas, pitch rises somewhat in southern-hemisphere summertime (northern hemisphere winter), quite likely so as to be heard over the far-resounding racket of cracking Antarctic sea ice. The ice booms fall in roughly the same frequency (pitch) range as blue song.

It is not that the blues go higher in pitch in order to be more audible. Rather, the upward shift in pitch results from calling more loudly, due to imperfectly-understood mechanics of sound production. (Blues have no vocal chords.) So, a clue to the pitch-shift mystery may lie in that the blues are singing less loudly year on year. Why on earth should this be?

One theory links the lower-volume singing to global warming. Rising quantities of greenhouse carbon dioxide dissolving into the world’s oceans causes them to grow more acidic. High-acid water carries sound more efficiently than does less-acidic water. Blues can hear one another at distance more easily and therefore lower their sound volume so as to conserve energy. Though this theory dovetails with climate concern, some scientists strongly question its plausibility. They doubt that the rise in ocean acidity yields a big enough difference in sound propagation to account for the downshift in whale song volume.

Another theory pivots on good news rather than bad news. Under the worldwide whaling ban over the past few decades, blue whale population numbers have rebounded nicely, from a low of perhaps four thousand to maybe 20,000 today. (Even with this pleasing rebound, blues are classified as ‘endangered.’) During the twentieth century’s first half, blues endured merciless hunting, with whalers taking an estimated three to four hundred thousand animals as steamships and explosive-tipped harpoons overcame blue whale swim speed. Meat found its way onto tables and blubber went into lamp oil, soap, perfume, candles, cosmetics, cooking oil and margarine. Baleen, with which blues strain sea food from mouthfuls of brine, became struts for corsets and parasols.

With today’s increased numbers, blues find themselves closer to one another on average as they swim the seas than they were back in the 1960s when numbers were low just as the whaling ban went into effect. This increased proximity makes it easier for blues to hear each other without shouting so loudly. Hence again, they drop their volume and thereby lower their pitch. We have been recording their pitches only since around the same time as the whaling ban came into effect. Before that, maybe their volume/pitch had been going up as they found themselves at greater average distance from potential mates.

Now the pitch is dropping back toward ‘normal’: so goes the theory. Again there are sceptics, however, who note that the downward pitch shift has been happening recently even in places where blue whale populations have not been rising significantly.

A third family of theories returns us to the theme of mate selection. What could be driving a female preference for lower-pitch calls? My earlier suggestion was that it’s purely aesthetic and ‘cultural’ so I called it ‘fashion.’ One proposed version of this hypothesis is that low-pitch singing, perhaps with longer song duration, is more challenging to perform than merely loud high-pitch singing, which is why it is more intriguing to females. This would be like dancing male birds of paradise, who earn mating privileges by displaying skills utterly devoid of survival advantage.

An alternative theory is that larger males are for whatever reason more capable of low-pitch song so the females are using low pitch as a proxy for bigness, which is a survival advantage: faster swimming, more endurance and lung capacity perhaps. Natural selection would have it that bigger males are more likely to survive until sexual maturity. But could males quickly be getting bigger year by year so as to generate the pitch drop? Maybe. Some scientists suggest that such a quick growth in size could be linked with population rebound, but the supposed mechanism for this is not exactly clear. Could it be that larger males were more heavily targeted in the days of whaling?

Still another hypothesis piggybacks on the idea that larger animals naturally produce lower pitch songs, but suggests that smaller males might also attract mates by sounding big through low-pitch serenading. Would anyone care to classify such an imitational phenomenon as ‘fashion,’ sort of like padded shoulders in men’s suit coats to mimic athletic build?

The pitch shift may stem from multiple combining causes. Conceivably, more than one mate selection theory could be true along with others and the same may go for the acid-ocean and population-rebound theories. Any mate-selection theory must seemingly confront a skeptical challenge: why now? Unless we suppose that pitch has been growing flatter ever since blue’s emergence as a species 4.5 million years ago, mate selection based on pitch (if it exists) must be more recent. Will the pitch (a thousand or a million years from now?) reach a point where it can go no lower in blue audibility, then start back upward? Oohooh, FASHION.

 

*With apologies for reminding you of Neil Diamond

A graduate of Harvard Law School, Mark Hager lives with his family in Pelawatte.

mark.hager@gmail.com

 

 

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