By Richard Black
Environment correspondent, BBC News website, the Canary Islands
A Cuvier's beaked whale makes a rare foray to the surface
Most articles about scientific subjects start by telling you what people have discovered about something.
This one is mainly about what people have not discovered about something.
"There's not much known about these creatures - where they live, their lifestyle," says Ted Cranford from San Diego State University in California, US.
"In fact, they might be the least understood group of large mammals on Earth."
The creatures in question are beaked whales - rarely seen, elusive, private, yet the subject of some attention in recent years because of the damage that military sonar systems appear to inflict on them.
This summer, the research vessel Song of the Whale is attempting to fill in some of the gaps in our understanding as it sails in and around the Canary Islands, home to several beaked whale species.
Operated by the International Fund for Animal Welfare (Ifaw), the yacht's main research tools are hydrophones - underwater microphones - that listen for and track the whales' characteristic high-frequency clicks.
And there is much to discover.
Scientist Oliver Boisseau explains how to listen out for beaked whales
"Some species have never been seen alive, and these are animals as big as an elephant," says Vassili Papastavrou, a whale biologist with Ifaw who is on board Song of the Whale.
Just about the most obvious question you could ask about any kind of animal is "how many of them are there?".
Well, we have no idea - for any of the beaked whale species.
So let us take another obvious question: how many species are there?
Nobody knows how many species of beaked whales exist
Once again, the answer is unclear.
The 2002 reference work Sea Mammals of the World book, written by leading lights in the cetacean academic world, lists 21.
But, it says, Arnoux's beaked whale may actually be the same as Baird's, while DNA analysis suggests the apparently single species of southern bottlenose whale could be two.
The International Union for the Conservation of Nature (IUCN), in its Red List of Threatened Species, lists 17.
And, to drive the point home, the list says that for 15 of those species, there is simply not enough data to know whether they are threatened with extinction to any degree or not.
The kind of data gathered by Song of the Whale and other missions around the world gives snapshots of how the animals behave.
But for most beaked whale species, more data has come down the years from examination of dead animals - for some species, corpses are the only source, as they have never been seen alive.
Sea Mammals of the World notes dryly that the northern bottlenose whale is the best understood of all the species because it used to be commercially hunted; and some hunters kept good records of what they caught and what they found.
From a well-preserved corpse - even better, from many corpses - scientists can determine an animal's size, gain clues to its diet and longevity and take DNA samples for analysis.
They can also look at how species have adapted to the environment they live in.
And perhaps the best understood and most fascinating adaptation of beaked whales is in their heads.
Ted Cranford used a military scanner to look at the whale's head
Over millennia of evolution, this family has developed an acoustic echo-location system that uses windpipes to generate sound, fat to focus it and bone structures that help channel reflections back to the ears.
The basic structure has been known for decades; but last year Ted Cranford had a unique opportunity to get a closer view.
A Cuvier's beaked whale washed up on the shores of Oregon. The people who found it managed to get the head quickly frozen; and Dr Cranford persuaded those in charge of Hill Air Force Base nearby to let him put it through a scanner routinely used for examining rocket motors.
The scan, which took several days, showed that the conventional view of sound production was about right.
Clicks are generated when a structure just below the blowhole known as "monkey's lips" smacks together.
That generates a wave radiating spherically outwards, which is transformed into a directionally forward-focussed plane wave as it passes through a fat "melon" - an acoustic lens.
So the sound - too high-pitched for a human to hear - shoots out in front of the whale, and if it hits food, such as a squid, a portion is reflected.
The reflection was thought to travel to the whale's ears, via its lower jawbone. But the scan suggested a more important route is under the bone.
Air sacs are perfect acoustic mirrors
Top of head for transmitting, bottom of head for receiving; a neat system.
The sound reception part appears to be very complex, involving fat bodies that focus sound and air sacs that reflect it.
"Air sacs are perfect acoustic mirrors," says Ted Cranford.
"The whales need to be able to isolate their ears from each other in order to maintain their directional sense, and one of the best ways to do that is through air sacs."
The use of air sacs is almost incredible when you think that the whales are diving as deep as 2km (1.2 miles), where the pressure equates to 20 megapascals (about 200 atmospheres).
The whales' lungs collapse as they dive, a defensive mechanism against damage from the huge disparity of water pressure outside and air pressure inside; yet somehow the air sacs channelling sound, which appears to be key to the whales' hunting success, stay operational.
What we know about beaked whales may be fascinating; but it is dwarfed by our ignorance.
Ifaw believes its line of research, concentrating on observing animals in the wild, can answer questions about their behaviour that autopsies of dead whales cannot address.
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