Blainville's beaked whales enter stealth mode
Blainville's beaked whales, which are among the world's most enigmatic cetacea, go silent in shallow waters.
Researchers have discovered that the whales refuse to communicate with each other near the surface.
By becoming silent, the whales enter a stealth mode that prevents them being detected by predatory killer whales.
The study, one of the first to record how beaked whales communicate, also recorded sounds made at the deepest recorded depth by any mammal.
Beaked whales are deep-diving, toothed whales.
Little is known about them, in part because they spend so much of their time in the ocean depths.
Some species have been barely sighted, and scientists suspect there may be more species of beaked whale awaiting discovery.
Also, very little is known about how beaked whales communicate or avoid predators.
So Natacha Aguilar of La Laguna University in Tenerife, Spain and colleagues at the Woods Hole Oceanographic Institution in Massachusetts, US and Aarhus University, Denmark, conducted the first study into how beaked whales communicate when diving.
Using suction cups, the researchers attached electronic listening devices to eight Blainville's beaked whales, recording them for 102 hours in total.
They recorded the sounds made by the whales though the water column, as they came up to breathe and swim near the water surface, and also as they dived to depths of 900m.
The results, published in the journal Marine Mammal Science, revealed that Blainville's beaked whales fell silent once they entered waters that are shallower than 170m.
Above this depth, the whales did not communicate with one another at all, while they were also silent when ascending from dives: a climb through the water that could take an average of 19 minutes.
That is despite the fact that these whales spend 60% of the lives swimming in waters shallower than 170m, and would be expected to communicate with one another to maintain social ties, particularly as they swim and dive in close knit groups.
When the whales swam in deeper waters, they did sound off.
At depths below 450m, the whales made a series of echolocation clicks, interspersed with so-called buzzes, tonal whistles and short bouts of repeated clicks.
The whales likely make the echolocation clicks to navigate and hunt prey.
But the whistles and repeated clicks, which the researchers dubbed "rasps", had never been recorded before.
These could serve to help the whales co-ordinate their movements as they disperse at the bottom of a dive to hunt.
The researchers believe that entering a stealth mode when swimming in shallow waters is an anti-predator strategy.
Killer whales, more appropriately known as orcas, are shallow divers that prey on many whale species in shallower waters.
By swimming in stealth mode, the beaked whales avoid broadcasting their location to orcas.
Hiding in the oceans this way may be an effective avoidance strategy, as beaked whales cannot out-swim orcas and have few other defences against them.
"For Blainville's beaked whales that live in cohesive associations and co-ordinate their activities, keeping silent near the surface is an unexpected behaviour and strikingly in contrast with that of other toothed whales," the researchers write in the journal.