Killer whales produce calls in specific combinations

In a recent study, we found that killer whales do not produce calls in a random order but that they produce specific combinations of different call types and also often repeat the same call types.

Examples of a call combination observed in 2009.

In some of the first acoustic recordings the Icelandic Orca Project collected in 2009, we already noticed a pattern of 2-3 call types that were always given together and seemed to follow a specific pattern. These call combinations are quite apparent in the recordings and are often remarked on by new students or people listening to the recordings for the first time. Now, we finally had the chance to look at these patterns in more detail.

Surprisingly, call combinations were very common in recordings of killer whales. They made up 50% of the transitions between call types. Another 30% of the transitions were repetitions of the same call type or same subtype. Therefore, 80% of the transitions in Icelandic killer whale calls were either part of a specific call combination or were repetitions. The remainder did not follow any clear patterns.

The three call combination clusters that emerged from the analysis. Each call category is shown as a coloured rectangle and the arrows with the percentages indicate the probability of one call category being followed by another.

The call combinations could be divided into three groups or clusters. Cluster A included the call combinations that were so obvious to us in the early recordings. It contained a total of six different call categories that followed a clear pattern in the ordering of the sounds. These combinations are given as two or three calls and either start with call type I38.1 or I38.2. The second call was either I11.4, I39 or I77 and if a third call was added, it was always I69. We also found two other call combination clusters that had not been so obvious when listening to the recordings. Cluster B contained two call types, I63 and I72.3, which were often given in this order and the combination was often repeated. Cluster C included three call types that were all closely linked to I45.

https://video.wixstatic.com/video/129255_a9fb764df53041c7880c52e291f60029/720p/mp4/file.mp4

Examples of 2- and 3-call combinations from cluster A: I38.1 + I11.4 and I38.1 + I11.4 + I69.

https://video.wixstatic.com/video/129255_949315d6d8364cb3890125e8daa1d8d7/720p/mp4/file.mp4

Examples of 2- and 3-call combinations from cluster A: I38.1 + I39 and I38.1 + I39 + I69.

The call combinations were recorded on tags deployed over several years, on different individuals from various social clusters and from all age- and sex-classes, indicating that they are widely spread and might play an important role in the communication system of Icelandic killer whales. However, what this role might be, remains unclear. One possibility is that they are related to the behavioural context of the animals. Most of the call combinations were not produced close in time to feeding events and could potentially indicate socialising. Alternatively, call combinations could play a role in long-distance communication with other group members, as suggested by previous studies in killer whales and narwhals. This is supported by recent research that shows that buzz-like sounds (like e.g. I69), can transmit over long distances. A third option is that the call combinations could be individual- or group-specific. However, we will need more detailed studies to investigate these possibilities further.

A group of killer whales of different age- and sex-classes in Vestmannaeyjar. Photo: Sara Tavares.

Many baleen whales are known to produce complex song displays, such as the famous songs of the humpback whales. However, such complex combinations of sounds have not been reported for toothed whales. Instead, several toothed whale species are reported to produce repetitions of the same vocalisation and some shorter vocal sequences. While there is clear patterning in the call production in killer whales, we have so far not found anything that is as complex in structure as the songs of baleen whales. Some of the earliest studies on killer whale calls from the North Pacific showed that certain calls were highly repetitive and noted combinations of specific call types. For example, N8 in the resident killer whales is never produced without being preceded by N7, but N7 is not always followed by N8. With our new study, we provide a detailed look at call combinations and also mostly find combinations of two or three calls. To learn more about these combinations we hope to look more closely at the behaviour during which they are produced in the future. Furthermore, studies across different killer whale populations could also be very informative on how different call types and calling patterns may evolve.

This study was led by our PhD student Anna Selbmann under the supervision of Dr. Filipa Samarra, Dr. Paul Wensveen and Prof. Jörundur Svavarsson at the University of Iceland and in collaboration with Prof. Patrick Miller from the University of St. Andrews. The full paper is freely available here: https://www.nature.com/articles/s41598-023-48349-1