Sperm whales communicate using short sequences of clicks known as codas, which they exchange while coordinating within their groups. Scientists have long classified these sequences using the number of clicks and the timing between them.A study published on April 15 in Proceedings of the Royal Society B has however reported that there’s more to it than meets the ears: the codas also vary in their acoustic structure in ways that resemble patterns found in human speech.“Before this work, sperm whale vocalisations were often treated as a kind of Morse code — simply patterns defined mainly by timing,” Gasper Beguš, associate professor of linguistics at University of California, Berkeley, and senior author of the study, said. “What we’re showing here is that there’s another layer of structure within the clicks themselves.”A learnt structureUntil now, classification relied on two measurable features: the number of clicks in a sequence and the spacing between them, known as inter-click intervals. These patterns produce distinct coda types. For example, a 1+1+3 coda consists of two clicks separated by pauses, followed by three clicks in rapid succession, while a 5R coda contains five evenly spaced clicks. Because different whale groups use different sets of these patterns, the system is unlikely to be entirely inborn. Instead, scientists think at least part of it is learnt within groups.When the researchers broke each click down into its frequency components, they found that the sounds fall into two distinct categories: some have a single dominant frequency peak, while others have two. In human linguistics, such peaks are known as formants—the resonant frequencies that allow us to distinguish an “ah” sound from an “ee.” Consequently, the authors label these whale categories “a” and “i.” These two types are clearly distinct, and the same pattern of clicks—for example, a 1+1+3 coda—can be produced using either the “a” click or “i” click.