The Southern Ocean is vast and harsh. Its storms, steep ice and boundless distances hide entire worlds from human view. Research vessels come here rarely and only briefly. Stationary hydrophones hear only a small patch around themselves. Satellites hovering above the waves are powerless: a voice hidden beneath the water's thickness is invisible to them.
But these deserted regions are constantly traversed by those for whom the Southern Ocean is home and hunting ground.
Nine female southern elephant seals set off on their usual months-long journeys in search of food. They crossed major oceanic fronts, dived to depths of more than one and a half kilometres and drifted slowly, regaining their strength. With them were tiny electronic helpers — bioacoustic recorders that captured every sound around them.
Thus the animals' routes turned into a moving network of underwater observation, and the elephant seals helped scientists hear the voices of whales in places where a human vessel never goes.
Nine wanderers of the Southern Ocean
The study was conducted by an international team of specialists: Anatole Gros-Martial, Maëlle Tortrot, Baptiste Picard, Mathilde Michel, Dorian Cazau, Sarah Bazin and Christophe Guinet. They all work at leading French research centres dealing with marine ecology, bioacoustics and ocean observation: CEBC (Centre for Biological Studies in Chizé), ENSTA Bretagne (Brittany National Graduate School of Engineering) and Geo-Ocean (geooceanography laboratory).
The team analysed a three-year array of recordings from 2017 to 2020, collected by nine female southern elephant seals:
- three animals set off from the coast of the Valdés Peninsula in Argentina — the only continental colony of this species;
- six — from the Kerguelen archipelago in the southern part of the Indian Ocean, home to the world's second-largest population of elephant seals.
Each animal was fitted with a DTAG4 acoustic recorder — a miniature device that continuously recorded underwater sound at a sampling rate of 32–38,4 kHz. Besides sound, it recorded pressure (depth), the magnetic field and the animal's body acceleration, allowing scientists to understand what it was doing at every moment.
The researchers studied with particular attention the drift periods — moments critical for sound analysis, when the elephant seal almost stops moving actively and slowly descends or rises through the water column, like an invisible lift.
At such moments the noise from the animal's own movement decreases, and the recording becomes a clear window into the acoustic world of the Southern Ocean.
Why elephant seals in particular?
Southern elephant seals are natural extreme divers. They are able to remain in the open ocean for months, making dives deeper than is accessible to the overwhelming majority of traditional research methods.
But the main thing is that their path does not coincide with the routes of research vessels. Their direction is determined by the primary need to survive, the search for food, invisible currents, the relief of the seabed and their own individual strategy. That is precisely why the animals' data can lead scientists to remote regions that rarely or never appear in the programmes of shipborne expeditions.
Of course, the elephant seals did not set off on a mission to "listen to whales". They continued to live by their own laws, cared about food, and the device attached to their head simply recorded the sound environment through which they swam.
The value of the method lies precisely in this paradox: the research platform moves not over the ocean but within it — together with a living being that naturally crosses different depths, pressures and water masses. Science rides on nature without disturbing it.
Who was heard in the depths
The recordings contained signals from several species of baleen and toothed whales — from giants like Antarctic blue whales to the more modest beaked whales. In some datasets, the researchers discovered unusually high bioacoustic activity.
The geographical distribution of the voices revealed a pattern linked to large-scale ocean structures:
- Antarctic blue whales were most often recorded near the polar front — an invisible boundary where cold polar waters meet warmer northern ones;
- humpback whales — near the subantarctic front in the Indian sector of the Southern Ocean;
- sperm whales — along significant changes in the seabed relief in the Atlantic sector.
Fronts are not just lines on a map, but dynamic boundaries where water masses of different temperatures and salinities meet. Here, nutrients rise from the depths, and dense aggregations of tiny krill and small fish form. Predators follow them — both whales and other marine mammals. Acoustic recordings have, for the first time, allowed us to hear how living creatures distribute themselves within this invisible architecture created by ocean physics.
However, it is important to remember the limitation of the method. A detected voice speaks only of the acoustic presence of a species in a given location. It does not show the exact number of animals that produced the sounds, and it does not always allow for determining the exact distance to the source. One actively vocalizing whale can leave a dozen signals, while several silent hunters will remain unnoticed. Therefore, high acoustic activity is not always an indicator of a large aggregation of animals.
Animal, sound, and environment — the trinity of knowledge
A standard underwater hydrophone answers a simple question: what is sounding near me, at this moment, in this place?
A bio-logger on an elephant seal adds much more information to each sound: the animal's full route, its depth, movement speed, water temperature, and salinity. Researchers gain the ability to link a whale's voice with precise coordinates, depth, water mass characteristics, and the behavior of the host animal at practically a single point in space-time.
This opens up new questions:
- near which oceanic fronts whales usually sing and hunt;
- which areas of the seabed they prefer;
- how their presence is linked to specific water properties — temperature, salinity, pressure;
- how these patterns might change as the climate steadily transforms ocean conditions and reduces sea ice.
Thus, the elephant seal becomes not just a platform for an instrument. It is a connecting link, a bridge between several layers of knowledge: the movement of a living creature, planetary-scale physics, and the voices of its neighbors.
What has the ocean reminded us of?
To draw closer to the mystery of a vast expanse, a person need not chart a new route through it or build permanent stations. Sometimes it is enough to respectfully follow the life that already knows this path and has walked it for thousands of years.
An elephant seal seeks food, rowing its flippers through the cold black water. A whale sends its voice through the deep. Currents bind distant corners of the planet with invisible threads. A scientist learns to hear and distinguish the connections that arise between them.
Each remains true to its own nature, but together they create an integral system of knowledge — a living dialogue with the ocean.
