Using digital science to track whale-to-whale communications

At first glance, the HAP hydroambiphone doesn't look like a technological gem. In fact, it looks rather basic, like a large football with a lot of equipment on it... And yet, this object is now enabling Inria scientists to detect communications between whales!

Its purpose is to obtain 3D underwater recordings,” explains Alexandra Jurgens, a scientist at the Inria Centre at Bordeaux University. This gives us a huge amount of information about the underwater acoustic environment, and in particular for studying whales.”

The technology itself is not new. Recordings of this kind have already been made in military applications, for example. However, this is the first time the method has been used in the field of animal science. The aim is to obtain a spatialized recording of the whales, with the possibility of determining the origin of the various sounds heard in the underwater environment. HAP has four hydrophones (microphones that can operate underwater) placed on the sphere. Each one records only the sound that reaches it directly, and it is by assembling each recording separately that it becomes possible to recreate the surrounding environment.

The stakes are high, as modeling an underwater recording is particularly complex. Sound travels five times faster than in air, and dissipates less. As a result, it is possible to hear noises produced tens, even hundreds of kilometers away... In this context, it is difficult to differentiate whale song from other sounds around us.

The research team, composed of Alexandra Jurgens and scientists from the University of California at Davis, plunged HAP into southeast Alaska, an area where humpback whales are numerous. In August 2023 and again in May 2024, each time for two weeks, the device was placed underwater to listen to these cetaceans at the height of their hunting season. The whales search for fish together, sometimes forming groups of 20, giving rise to intense conversations between them, no doubt to coordinate their actions.

When we get the complete recording, we can localize the sounds using the device,” explains Alexandra Jurgens. This requires us to do some modelling to identify each sound, its source, its distance... That's our main goal for the moment. What's the next step? We need to understand what's going on and deduce information about the whales' behaviour.

Nevertheless, some information has already been gleaned. For example, specialists thought that the “bubble net” hunt, a technique used by whales to trap their prey, was coordinated by a single male. But in reality, HAP has identified two identical sounds produced by two different whales, offering a new insight into their strategy.

The hydrophone offers a key advantage in this research, as it eliminates the need to disentangle sounds in order to determine their origin. As each device records in only one direction, spatialization is achieved in real time. As a result, it's easier to process the sound after recording.

The software part is still complex,” says Alexandra Jurgens. We're working with an open source product until we develop our own technology, and we're still in the early stages. But these first recordings are promising and will help us to develop the rest, saving time on processing.”

What about the recorded sounds? They have been processed for 3D playback in a room dedicated to this type of experiment at SCRIME, an Inria partner laboratory. In this room, the listener sits in the middle of the room and the sounds come from the various devices placed around him or her. In other words, an ultra-immersive sound home-cinema!

While listening, the researchers realized that whales cohabit with many cruise ships in this part of Alaska. For example, in a passage of just 20 kilometers between two strips of land, these cetaceans and the boats were very close to each other, so much so that the sounds sometimes merged. At this stage, we need mathematical tools to isolate the sounds,” explains Alexandra Jurgens. It's a filtering job where we try to get rid of the noise pollution.”

In fact, at first, the team even stopped some of the recordings because of the excessive human noise. But then they realized that the sounds in question only came from one direction, and that it was therefore possible to simply remove them from the analysis, without losing information about the others. A bit like a person concentrating on a particular conversation to avoid being disturbed by the surrounding din.

Another research prospect: it seems that whales produce sounds at different frequencies to communicate and locate their food. A phenomenon that can be studied in greater detail on future missions.

For the researchers, the bulk of the analysis work therefore takes place far from the sea, and shows just how important the digital sciences have become in the study of animals. This research combines questions about animal behavior, mathematics and computer science to process and recreate sounds, and theoretical physics to better understand how sound diffuses through water.

There are specific mathematics and physics for observing animals,” summarizes Alexandra Jurgens. And today's technology makes it possible to obtain much more information, so we can look forward to major advances in the future.” All the more so as the device described here is relatively modest, even according to the scientists who developed it. The advantage? HAP is not very expensive and simple to use, requiring less work to implement in the field. What's more, it's non-invasive for the species studied.

What is the team's goal now? They intend to improve not only the simulations enabled by the software, but also the HAP device itself. So, with nine hydrophones instead of four, it will be possible to obtain an even more accurate overall view, with a much smaller margin of error. And beyond whales, these technologies have a further advantage: they can help us to better understand the underwater environment where, in the absence of light, sound plays an even greater role than on land.