Humpback whales (Megaptera novaeangliae) predominantly dine on krill and small fish that travel in schools. They are able to catch their prey in a variety of ways, including stunning them with slaps from their powerful fins or blowing bubbles to concentrate them into a small area. While it has been well-known that whales can cooperate for hunting purposes, it was not well-understood how each individual whale was able to contribute to the process.
Another aspect of humpback hunting that has not been well-understood by scientists is how they hunt in deep waters as a group, where the light is minimal. A new study has revealed that groups of humpback whales hunt in dark waters using vocalizations and other acoustic clues to locate their prey. The research was led by Susan Parks of Syracuse University, and the paper was published in an open access format in Scientific Reports.
"Humpback whales are known to cooperate with others to corral prey near the surface," Parks explained in a press release. "Recent studies suggest they may cooperate [with each other], when feeding on bottom prey, as well.”
Parks’ team studied humpacks living in the Gerry E. Studds Stellwagen Bank National Marine Sanctuary, near the mouth of Massachusetts Bay. The researchers attached a total of 56 recording devices to the whales, which allowed them to listen to sounds made by the whales while hunting in deep water. The team was able to collect instances of the whales hunting cooperatively and individually, contrasting the differences in the two approaches.
The most common prey during these deep-sea excursions were sand lances. Though they are commonly called “sand eels,” they are actually small fish that remain buried in the sand on the sea floor, with just their heads poking out.
The data revealed that when hunting by themselves, the whales were very quiet. However, when working together, the humpbacks would make “paired burst” noises that sounded like a tick-tock. These paired bursts only occurred when hunting along the sea floor, and did not occur when the whales were working cooperatively closer to the surface.
There was also some evidence that these vocalizations cause other whales to join in the hunt, by essentially announcing where food exists along the seafloor.
"Hints of behavior suggest that other whales who overhear the sounds are attracted to them and may eavesdrop on other whales hunting for food," Parks continued.
Now that researchers know that the whales do make the noises, they need to clarify why. The authors suggest that these noises could be used for communication purposes among the whales. Another reason might be that the paired bursts are able to disrupt the sand, exposing the sand lances and making them easier to collect. It could also be a combination of these factors. Further research may provide more clues on this activity.