Sunday 14 November 2021

Arctic Sonar Beams

Dolphins and porpoises are well-known for using biosonar to navigate their way around and detect the fish on which they feed. There are a large number of different dolphin species, with well over 30 in the Oceanic dolphin family alone, which includes not only the regular-sized species but also much larger "dolphins" such as the killer whale. Given this physical variety, and the variation in habitats between the different species, it's unsurprising that there are also many variations in the exact details of the sounds they emit and how they use them.

Much of this variation seems to date from the Middle to Late Miocene when both dolphins and porpoises underwent a rapid increase in the number of known species. This may be related to the loss of the connection between the Mediterranean and the Indian Ocean, creating a change in the worldwide circulation of water and corresponding changes in the habitats available to these animals, allowing them to exploit new opportunities.

Of course, dolphins and porpoises are not the only cetaceans to use biosonar, something that evolved long before they did. But their biosonar is often more sophisticated than that of their relatives and part of the reason that they share this feature is that they also share a relatively recent common ancestor, living something like 18 million years ago during the Early Miocene, well after the split from most of their larger relatives.

However, it turns out that porpoises and Oceanic dolphins are not each other's closest relatives. That's because, somewhere around 11 to 15 million years ago, the line that led to the porpoises split into two. This created what we now regard as a third family of "delphinoid" cetacean in addition to the two more obvious ones. It contains just two living species and both of those also have unusually sophisticated sonar.

While the origins of the family lay in much warmer waters, the two living species are both inhabitants of the Arctic Ocean. Indeed, they are the only species of cetacean that live only in the Arctic, and nowhere else, something that might naturally lead to specific adaptations to that environment. 

The more widespread of the two is the beluga (Delphinapterus leucas). These are highly vocal animals, producing a great variety of calls, whistles, and clicks that can be heard by humans, and spend so much of their time "singing" that they are often nicknamed 'the canaries of the sea'. Most of these sounds are communication, much as is the case with other forms of whale song and, due to its complexity and frequency, most studies about the sounds that belugas make have been focused on this.

But, like all other toothed whales, even those less closely related to dolphins (such as sperm whales)  belugas do also echolocate. Here, at least if we want to know what they do in natural settings, rather than in captivity, we know rather less. 

Still, captive studies do give us the basic parameters. We know, for instance, that the peak energy of the clicks is at a frequency between 30 and 50 kHz - about two to three octaves above the highest note on a piano, and at least half an octave beyond what the human ear can detect. We also know that they are capable of changing the characteristics of the sonar ping depending on the situation (background noise, clutter in the water, etc.) being able to hit much higher notes if they need to.

But there are some reasons to suppose that some of the other characteristics of beluga sonar are different to those seen in most dolphins. For instance, we know that dolphins can not only change the pitch and volume of their pings, but how tightly focused the resulting sonar beam is. They can use this, for instance, to widen the beam as they approach their target, presumably so that it has less chance to move beyond the beam's edge. 

In most circumstances, however, a narrow beam is helpful, since it makes it easier to locate exactly where the target object is before you try and approach it. This, it is thought, may be part of the reason why smaller dolphins and porpoises produce higher-pitched sonar than larger whales - they are unable to produce pings as loud as those of whales, and, in order to keep the beam width relatively constant, have to increase the pitch to compensate

Belugas are much larger than most dolphins, although not quite all of them; at around 4.5 metres (15 feet), the average male beluga is about the size of the smallest female killer whale. But the Arctic environment that they are adapted to may affect the details of the sonar that they need to use. Significantly, we know that the closest living relative of the beluga - the one other member of its family I mentioned above - can produce highly focused and directional sonar beams. 

Indeed, this animal, the narwhal (Monodon monoceros) produces the narrowest sonar beams of any cetacean, with a beam width of just . In comparison, most sonar-using cetaceans that have been studied have beam widths of around 8 or 9°, reaching 10° in the harbour porpoise. The widest such beam appears to be that of the Ganges river dolphin (which is not closely related to the 'true' or Oceanic dolphins) at nearly 15° - although, it has to be said, even that isn't exactly unfocused.

Might the beluga, a close relative living in a similar habitat, be capable of similar fears to the narwhal?

Studies on captive animals conducted in the 1980s suggest that they can certainly get close, but it's much harder to know what they're doing in the wild. But not, as it happens, impossible.

In 2013, a scientific research team out looking for narwhals off the coast of Greenland as part of another study twice came across pods of belugas instead. Landing their helicopters on the sea ice, they deployed their listening equipment to monitor the animals, placing a vertical array of sixteen hydrophones on a weighted cable in the water nearby. Despite this being essentially opportunistic, the results do represent the most detailed study of how belugas use their sonar in the wild.

For one thing, while the recordings showed the same broad spectrum clicks as earlier studies have done, the peak intensity of the pitch was much higher than seen in captivity - with a low-frequency peak at 97 kHz (so, two octaves higher than the earlier studies) and a higher one at 147 kHz. This is a good deal higher than the corresponding peaks in the sonar output of killer whales, and higher than that of narwhals, too.

Because the hydrophones were strung out on a weighted cable hanging in the water, it was also possible to determine how wide those beams were when they hit the array. It's also possible, by comparing the time of arrival of the sound at different channels down to a 200th of a second, to come up with a reasonable estimate of how far away the animal was when it emitted the click. And then trigonometry tells us how focused the beam must have been to produce those results.

This turns out to be narrower than the studies on captive animals had implied, possibly due to the different circumstances. The figure this study arrived at was 5.4°, barely wider than the narwhal, and thus the second tightest known sonar beam of any cetacean. They are also quite loud, at over 200 underwater decibels (which, for complicated reasons, are not the same thing as air decibels). From what we know of a beluga's hearing abilities, that should be loud enough for it to locate a codfish at 300 metres (1,000 feet). This may be useful in colder environments where fish are less abundant than in the tropics.

It seems likely that the tightly focused beams used by both belugas and narwhals are also related to their Arctic environment. Given their ability to vary the focus and pitch of their clicks, and the fact they are different in captivity, they are probably also different in different parts of the beluga's range - they can, after all, reach the southern waters of Hudson's Bay and, unlike narwhals, often venture into the northern Pacific. But, in the locality of this study, in the waters between Greenland and Baffin Island in Canada, there is extensive sea ice, especially in winter. A narrow beam would reduce the chance of sound bouncing back off the underside of the ice, as well as making it easier to find the holes in that ice that they need to surface and breathe. 

Such narrow beams do produce problems of their own, but the study also showed that belugas, like some other cetaceans, regularly scan the water by scanning their beam up and down to find as much as possible. (Presumably, they scan side-to-side as well, but there's no way to know that from the single vertical array used in this particular study). This sort of behaviour is known to be common in bats, and has been seen in some cetaceans before, but never before in belugas likely because the studies had simply never been done.

In the longer term, the results of this study do have some practical benefits. By telling us what beluga clicks sound like in the wild, in terms of pitch and so on, it becomes possible to distinguish them from other species, such as narwhals and killer whales, that inhabit the same waters. The volume of the sound that the belugas produce means that they should be audible to standard monitoring arrays at a distance of 800 metres (half a mile) and such arrays are often used to assess the abundance of different animals in waters across the world, determine migration routes, and so on. 

As the Arctic warms, directly endangering local animals and also bringing more shipping to ice-free waters, that may become more necessary than ever.

[Photo by Ansgar Walk, from Wikimedia Commons.]


  1. Why the quote marks on "delphinoids"? The Delphinoidea is a recognize superfamily, AFAIK?

    1. Certainly recognised, yes. I was just highlighting it as an unusual word.