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Behavior

Whistle While you Work (for Lunch): Dolphin Communication Techniques During Foraging

Paper: Nikolina, Rako-Gospić, Marta Picciulin, and Ceccherelli Giulia. “Influence of Foraging Context on the Whistle Structure of the Common Bottlenose Dolphin.” Behavioural Processes (2020): 104281. https://doi.org/10.1016/j.beproc.2020.104281

How do dolphins communicate?

Dolphins are widely regarded as some of the most intelligent mammals in the animal kingdom. A main piece of evidence for this large brain-power is how dolphins are able to communicate with each other and receive information from the world around them via clicking and whistling. Clicks are used to identify objects in a dolphin’s environment – such as a tasty meal – much in the way bats use echolocation: a sound is produced by the animal, bounces off of objects in the immediate environment, and then is returned to the animal for sensory processing. Dolphin whistles however, are primarily used for social aspects of dolphin life, from identifying particular individuals, to broadcasting one’s location, and even coordinating group behavior. Dolphins are typically found in groups, called pods, so whistling is a frequent vocalization and is of paramount importance for these creatures.

A bottlenose dolphin. Bottlenose dolphins typically travel in groups called pods, and eat a wide variety of foods, from softer squid and fish to harder crabs and shrimp. They are commonly found close to shore across large areas of the Atlantic and Pacific Oceans. Image by NASA via Wikimedia Commons.

Since dolphin whistles are useful for many different tasks, this begs the question: Do dolphin whistles differ depending on the task a group of dolphins is performing, and if so, how? Additionally, are these sounds affected by the presence of human fishing vessels? To get at these questions, researchers based at the Blue World Institute of Marine Research and Conservation in Croatia examined two different bottlenose dolphin populations – one off the coast of Sardinia, Italy, and one near Cres Lošinj, Croatia.

Examining dolphin sounds and activity

The dolphin populations in Italy and Croatia have slight differences in habitat. Off the Sardinian coast, waters are a bit deeper and there are not as many fishing boats as in the shallower, more crowded waters of Cres Lošinj. Researchers analyzed two aspects of the dolphins in each of these habitats: their behavior and vocalizations during said behaviors. Behaviors were only monitored during foraging events when dolphins are looking for or actively acquiring food. During these behaviors, dolphin pods need to be tightly coordinated with other group members to ensure that food is successfully trapped, everyone is accounted for, and all are able to eat. Three behaviors were noted: 1) Foraging in the presence of motor boats; 2) Foraging in the absence of motor boats; 3) Foraging while interacting with trawling vessels. With this last behavior, dolphins follow trawling vessels (which drag large nets through the water, often also scraping the sea floor) and pick off food that escapes from or is trapped in the nets.

When examining the vocalizations the dolphins performed, researchers mainly noted the length and frequency of whistles. Whistles can increase or decrease in frequency, show no change in frequency, or have some combination of patterns. The whistles were only recorded when the dolphins were clearly foraging, so that the whistles associated with other behaviors would not confuse the results of the experiment.

A map showing the sites where the two bottlenose dolphin populations were studied. The red dots on the map indicate where researchers recorded dolphins, both off the coast of Sardinia, Italy, and just offshore of Cres Lošinj, Croatia. Despite their apparent closeness, the study sites differ in the habitat they provide for the dolphins and the amount of fishing activity. Image by Francesca Giammona.

What does the dolphin say?

In Sardinia, dolphins were mainly seen engaging in foraging behind trawling vessels. In contrast, Croatian dolphins mostly foraged in the absence of motor boats. At both locations the dolphin whistles were distinct for each different foraging behavior. Between the populations, the dolphins also had a very different repertoire of whistles. In fact, researchers could place whistles into the correct population over 70% of the time just by hearing the whistles for each behavior.

In both populations, though whistles with a combination of frequency patterns were the most common, whistles differed in the number of times the pattern changed, the range of frequencies used, and the length of the whistle. Croatian dolphins always whistled for longer than their Sardinian counterparts for every behavior analyzed, and their whistles always had a higher maximum frequency. Sardinian dolphins foraging behind trawling vessels would use all frequency patterns, while Croatian dolphins never used unchanging or completely decreasing frequency whistles near trawlers. Croatian dolphins on the whole had many more whistles with increasing frequency, particularly when no boats were around.

An example of a trawling vessel, this one from France. Trawling vessels drag a large net, seen on the back of the boat here, along the seafloor. As the net collects fish and invertebrates, dolphins may pick off the trapped organisms. In addition, as the net is moved or brought up to the surface, dolphins are able to forage on the organisms that fall out of or escape the net. Image by Philippe Alès via Wikimedia Commons.

Human effects on dolphin communication

In the end, researchers showed that these dolphin populations are clearly impacted by human activity. There is the obvious behavior dolphins have adapted where they use human trawling vessels to get an easy meal, but the fact that dolphins whistle differently when motor boats are present or absent reveals a deeper story. When dolphins are near boats, they are likely experiencing a lot of noise pollution from the boats’ propellers and engines, which could make their normal whistling behavior unable to be heard by their fellow pod members. This would result in distinct whistles for different foraging behaviors, as was seen in this study. The fact that most Croatian dolphins would only forage in the absence of motor boats is further evidence to this point, as noise pollution from crowded waters may make creating new whistle patterns, and foraging as a whole, very difficult.

Different dolphin populations tend to have different whistles to begin with, usually based on habitat, pod size, and the average age of pod members. The number and type of boats present in an area is an emerging variable that scientists should take into account when examining dolphin foraging and communication. For those of us who are not dolphin researchers, this work will hopefully allow us to appreciate how these intelligent animals are able to adapt to, and even take advantage of, human impacts in nature.

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