One of the enduring mysteries of marine biology is the nature of whale song, particularly the intricate vocalizations produced by humpback whales. These consist of short noises lasting a few seconds, usually exchanged between males and females, and extended songs that can last for hours, produced mainly by males.
The role of these songs is hotly debated but marine biologists believe they are part of a complex social system for sharing information among animals that can sometimes be separated by many thousands of kilometers.
Although biologists and activists have long recorded whale song, these sound tracks are a relatively poor records. That’s because hydrophones record sound coming from all directions at a single point. That process loses much information because sound is a rich, three-dimensional field that is hugely complex.
Human hearing exploits these fields to determine the distance and direction that sounds arrive from, to differentiate between noises received from different sources at the same time and much more. This, of course, is not possible with most recordings because microphones do not capture the three-dimensional nature of sound fields.
This loss of information is not too much of a problem for humans because most of the information we process about the world comes through vision. Whales, on the other hand, live in an environment where light does not travel far and sound plays a much more important role.
In sea water, sound travels much faster and further than in air, certainly tens and perhaps thousands of kilometers, and different frequencies travel at different speeds. Sound reflects from the ocean surface and from the cast expanse of the ocean floor, while also being bent by changes in the properties of water, like salinity and temperature. It is hard to imagine that whales are oblivious to this complexity.
And that raises an interesting question about the role that sound plays in the way whales both communicate and sense their complex environment. What’s needed, of course, is a better way to record whale song that captures its full three-dimensional complexity.
Enter James Crutchfield at the University of California, Davis, and colleagues, who have developed a hydrophone capable of recording the full three-dimensional nature of marine sound fields. They call their device a hydroambiphone and last summer, they put it through its paces in the waters off the coast of Alaska, to listen in to the acoustic activity of humpback whales in the area.
The team say that the ability to analyze the full sound fields produced by whales is revolutionary. “We believe the hydroambiphone is a new tool for marine biology that promises to greatly expand the human appreciation of the three-dimensional acoustic world of marine animals,” they say.
The device itself is relatively simple. Crutchfield and co simply attached four ordinary hydrophones pointing in different directions to a steel sphere about thirty centimeters in diameter.
Although hydrophones are omnidirectional — they record sound from all directions—the sphere blocks the sound from behind each hydrophone so that it records only the sound coming from in front of it.
In this way, the four hydrophones record the three-dimensional sound field at a specific location.
Last August, the team tested the hydroambiphone by dropping it off the back of their boat, connected by a steel cable and then recording the acoustic environment. In total, they made seventy recordings, each lasting up to an hour.
The results have been eye-opening. The hydroambiphone turned out to be highly sensitive over a wide range of frequencies down to infrasonic levels below 20 Hertz. “We were regularly surprised at the number of animals around us,” say Crutchfield and co.
But even in remote Alaskan coastal waters it was hard to escape anthropogenic sounds. The team regularly picked up the engine noises from fishing boats and cruise liners beyond the horizon, at least twelve kilometers away.
Interestingly, the team originally regarded the engine noise as highly damaging and initially stopped recording when it occurred. “The high sensitivity at first seemed a burden, obscuring and even totally masking sounds of interest,” they say.
But they later found that because this noise comes from a specific direction, they were able to remove it in their analyses, just as humans can tune out unwanted noises when necessary.
Whales, presumably, can do the same.
They also found new forms of communication between male whales during the process of bubble net feeding. In this activity, a group of whales cooperatively herd fish into a small volume of water where the shoal can be easily consumed.
Marine biologists had thought bubble net feeding was coordinated acoustically by a single male. But Crutchfield and co’s recordings revealed a second male voice singing the same song but in a different location. Towards the end of the hunt, the second male song diverges from the first. “As a vocal phenomenon these vocal coordinations are the functional equivalent of human singers harmonizing,” say the team.
They also discovered some counterintuitive features of infrasonic communication below frequencies of 20 Hertz, beyond the range of human hearing. The team were surprised to find these sounds were highly directional. “The directionality goes against conventional understanding of sound propagation since low frequencies are not associated with having a direction,” say the researchers. (Although it is not clear why biologists had thought this of infrasonic sounds that originate from a specific location.)
The team make one of their recordings available here. One feature of this recording is the extensive echoes of whale song — reflections from the ocean surface and the sea floor at a wide range of frequencies.
Crutchfield and co do not comment on this information content or how whales exploit it. However, it is thought whales and dolphins use sounds at different frequencies to locate food, to observe their surroundings and to communicate.
Crutchfield and co’s hydroambiphone will surely provide important new insight into how this sensing works.
Ref: Whales in Space: Experiencing Aquatic Animals in Their Natural Place with the Hydroambiphone : arxiv.org/abs/2312.16662