The click echoed through the ocean for 70 kilometers before fading. No mammal on Earth communicates louder.
Scientists studying male sperm whales have discovered that these massive creatures produce the loudest communication sounds ever measured in any mammal. The findings, published in the journal Annals of the New York Academy of Sciences, came from recording expeditions to the waters near the Seychelles, Sri Lanka, Norway, Scotland, and the Azores.
The sounds are called slow clicks, and they are nothing like the echolocation clicks whales use to hunt squid in the dark depths. Instead, slow clicks appear to be social signals sent between whales. They are produced exclusively by males, and each click can be separated from the next by five to ten seconds of silence.
Simone Videsen, a postdoctoral researcher at Aarhus University in Denmark and the study's first author, said the rhythm surprised the team. "What is extreme about slow clicks is that the whales can produce them with long time intervals between them. And they can do it very rhythmically. And over those distances," she explained. "It is really the combination of the three things that is extreme: the strength, the rhythm and the range."
The source of this incredible sound is equally astonishing. Male sperm whales possess the largest sound-producing organ of any animal on the planet: a nasal complex that can weigh more than five tonnes in a fully grown adult. This massive nose, which makes up about one-third of the whale's body length, contains structures called phonic lips. The whale snaps these car-tire-sized parts together to create each click.
Researchers measured source levels exceeding 200 decibels, confirming these as the most powerful mammalian communication signals ever recorded. The sound can travel an estimated 70 kilometers through the ocean, though the researchers still do not fully understand how other whales respond to these calls.
"A big nose sits on a big male, and a big nose will produce a lower-frequency click," said Professor Peter Teglberg Madsen, a senior author of the study from Aarhus University. He suggests the deep, loud clicks may function as what biologists call honest advertisement: a male's body itself broadcasting his size to potential mates and rivals.
But that explanation remains a hypothesis. The research team wants to find out for certain by attaching small recording computers to whales and playing slow clicks back to them in carefully designed experiments. Watching how whales react to these recordings could reveal whether the clicks are meant to attract females, warn off rival males, or serve some other purpose entirely.
For now, the mystery deepens. Somewhere beneath the waves, a male sperm whale may be rhythmically clicking his message into the darkness, waiting for a response.