What Can Dolphin Whistles Tell Us About Their Health? 

Most of us can probably remember a time when we came down with a slight cold, yet trudged into work. Perhaps we took care to mask the signs of our less-than-healthy state, greeting others in what we thought was a normal tone. But then, to our surprise, a concerned co-worker asked: “Hey, are you not feeling well today?” 

What gave you away? Maybe your voice was the telltale sign.

Just as humans use vocal cues to pick up on a person’s state of being, a similar approach can help scientists assess the health of animals. Specifically, this approach entails looking at vocal biomarkers. Vocal biomarkers are changes or features that can be measured in the voice (or call) that offer clues to the health status of the speaker. Scientists use acoustic analysis methods to identify changes in the health of animals such as livestock, pigs, and chickens. Now, a new study suggests that features of dolphin whistles may be indicators of their health too; a significant finding since dolphins typically hide signs of illness.

Whistling as a Form of Communication

Bottlenose dolphins produce multiple types of whistles to communicate, often relying on their own signature whistle. This unique call identifies the dolphin, much like a name or voice does for a human.

Since 1965, when signature whistles were discovered by Melba Caldwell and David Caldwell, the practice of studying dolphin whistles has continued to evolve. Researchers have learned more about dolphin anatomy and ecology, how they navigate their ocean environments, and their communication behavior.

For example, dolphins can change the features of their whistles depending on changes in their environment. Features include things like frequency (pitch), amplitude (loudness), and duration (time). Relating to frequency, one study showed that mother dolphins whistle to their calves at a higher frequency, like we might do when talking to our babies. Another study revealed that when dolphins are alone, they produce their signature whistle more often, but when they are in a group, they more frequently use shared whistles amongst their pod members. They can even copy another dolphin’s signature whistle. Evidence also exists that changes in whistle rate can signal distress.

Equipped with all of this knowledge, it’s no stretch to think that dolphin whistles may change when the dolphin’s health changes. If that’s true, how can we use that information to monitor the health of dolphin populations?

The Connection Between Whistle Characteristics and a Dolphin’s Health

Of course, dolphins cannot directly tell their veterinarians how they’re feeling or describe their symptoms. They are known for being stoic animals, often showing signs of illness only after a condition has progressed. But a recent five-year study set out to explore if a dolphin’s health could be classified based on the acoustic features of their whistles.

Fifteen bottlenose dolphins at the U.S. Navy Marine Mammal Program (MMP) served as the focal population for this study. Researchers conducted weekly “acoustic physical” exams where dolphins produced their signature whistle in response to a hand signal. These whistles were recorded, creating a consistent snapshot of the dolphin’s “voice” over time.

To figure out how changes in their whistles might reflect health status, researchers collaborated with NMMF and Navy veterinarians and professional care experts. They compared extensive medical records maintained on the dolphins to the whistle recordings. In total, over 36,000 whistles were collected for the dataset. Each whistle recording was paired with detailed health information and labeled as either “normal” or “abnormal”. “Abnormal” classification included conditions such as gastrointestinal issues, infections, or critical illness.

Figure 1 from the Dolphin Health Classifications from Whistle Features study (Jones et al., 2024) shows just how similar dolphin whistles can visually look, even when produced in different health states. These spectrograms are how researchers can see the sounds they record, with frequency (in kilohertz) on the y-axis and time (in seconds) on the x-axis. The left-hand columns show two signature whistle examples for three of the participating dolphins who had a normal health state at the time of recording. On the right, whistles from the same dolphins recorded during various abnormal health states.

Analyzing the Data Set

To analyze this vast amount of data, the researchers turned to machine learning. Machine learning is a type of artificial intelligence that allows computers to find patterns in large datasets. In recent years, it has become especially powerful in bioacoustics, the study of biological sounds. For this study, researchers used a gradient boosting classifier machine, a type of machine learning program that sorts data into categories.

First, the program was given a set of whistles along with the correct answers, such as who the dolphin was and whether the dolphin had a “normal” or “abnormal” health diagnosis. This trained the model to learn any subtle acoustic patterns associated with different health states. Then, the model was given a smaller set of whistles it had never seen before and was asked to classify them. Such testing was repeated many times, with new recordings added each week.

In the end, the model was able to distinguish a dolphin’s health status from their whistles with 72.3% accuracy. The model was especially good at identifying dolphins experiencing critical illness, suggesting serious health problems may produce detectable changes in vocal patterns.

An Interesting Finding

One interesting finding from this study was the discovery that low-frequency components of dolphin whistles were more relevant indicators of health. This parallels observations in humans, whose voices can drop in pitch during certain illnesses. The study, however, was careful to point out that more research is needed in this area.

In the end, results like these are promising, and new methods such as machine learning provide exciting avenues for monitoring animal health.

For rescued and rehabilitated wild dolphins released back into the ocean, this would be helpful, as scientists could continue tracking their recovery remotely.

Understanding the Definition of “Abnormal” 

It’s also important to take into account what “abnormal” means. In a controlled environment, like the U.S. Navy Marine Mammal Program, abnormal health may look very different from “abnormal” in the wild.

Dolphins in professional care receive continuous medical attention, including regular checkups, balanced diets, and immediate treatment at the first sign of a change in health. Attentive veterinary teams give dolphins around-the-clock care. While dolphins in these environments may still get sick, it is often caught very quickly before anything more serious develops.

By contrast, wild dolphins face more complex challenges, exposure to natural disasters being one of them. For example, since 2010, the National Marine Mammal Foundation (NMMF) has been monitoring wild dolphins in Barataria Bay, LA, assessing how oil exposure from the Deepwater Horizon oil spill disaster impacted them. Dolphins at sea also encounter other environmental stressors such as changing food availability or boat traffic and noise.

It would be interesting to examine how environmental pressures, such as these, might produce different kinds of vocal changes. Future studies are needed to test whether the same acoustic markers identified in the Navy’s study apply to wild populations, or whether new models can be trained to interpret the whistles of dolphins living in less predictable conditions.

Listening to More Than Just Dolphin Whistles

The hope is that analyzing bottlenose dolphin whistles will extend beyond diagnosing health problems and become another tool for understanding and protecting marine ecosystems. If their whistles change, it may signal that something in their environment has changed as well. 

Dolphins are indicator species, meaning their health can reflect the state of their environment. Take, for instance, the health of river dolphins in the Amazon and Orinoco River basins, where dolphins serve as “early warning indicators for potential public health risks, such as mercury contamination in rivers.” Laboratory tests have shown high mercury levels in these dolphins, along with evidence of bacteria.

So the next time you ponder dolphins whistling beneath the waves, consider that these sounds might hold clues not just to who they are, but how they are doing.

If we listen closely enough, we might also hear when the ocean itself needs care.

This research was made possible with support from the Office of Naval Research.