The ocean harbours some of the strangest, biggest and most elusive creatures, but due to the difficulties of ocean exploration, many of the creatures that reside in its depths are still poorly understood. Such is the case with the sperm whale (Physeter macrocephalus). These whales can dive down over 2000 m and have the largest brain on Earth. While males are solitary creatures, females live in family groups or ‘units’ and use a repertoire of calls which they also share with their clan – a sort of cultural group that also share similar behaviours. Other toothed whales such as dolphins, orcas and pilot whales communicate using whistles, but sperm whales communicate using patterned sequences of clicks, called codas. These codas are similar to their echolocation clicks, but lower in frequency. But is the target audience for these clicks other members of their clan, or just their family unit? Ellen Jacobs, currently of Georgetown University, USA, and a group of researchers from Aarhus University, Denmark, Calvin University, USA and the Scottish Oceans Institute, UK, set sail to find out.
Aboard a sailboat off the coast of Dominica, an island country in the Caribbean, Jacobs and colleagues began the difficult task of using an array of five underwater microphones to record the whales’ clicking calls between April and May 2018. The team was able to record 444 codas from the whales during their 42 days at sea. After calculating how deep the whales were when making their calls, the researchers concluded that sperm whales only make coda calls until they are ∼250 m deep before switching to the echolocation calls they use to hunt. Jacobs and colleagues also determined that the whales’ codas were rather loud. The volume of the clicks within each coda was 161 dB, the equivalent to being underwater, 100 m away from a large ship. This means that the codas are some of the loudest calls used by toothed whales. However, the team also discovered that the codas are not the loudest calls that the sperm whales can make.
To determine the intended audience for the codas, the researchers needed to find out just how far these calls would travel underwater. Based on the volume of the call, the ambient noise in the water and how deep the whales were when calling, Jacobs and colleagues calculated that the calls only travel about 4 km on average before they are too quiet for the whales to hear. This distance just happens to coincide with the maximum distance from one side of the family unit to the other.
So, the sperm whales are not shouting their calls, they are probably using the codas to communicate with other members of their family unit rather than their far-off larger social clan. However, the true meaning of the calls is still a mystery. ‘Whether the codas are used to reaffirm group membership or just how they make calls in that cultural group, remains to be understood,’ says Jacobs. More of these studies are needed if we are to understand the mysterious animals that inhabit the world's oceans and the challenges they might face in our ever-changing world.
