The numbers and movements of an obscure sub-species of blue whale are intriguing conservationists.
By Alex Kirby
BBC News Online environment correspondent in Berlin
The pygmy blue whale, known to science for only 40 years, is certainly more numerous than its bigger relative. But scientists say they can do little more than guess at its real numbers worldwide.
The pygmy whale has a distinctive tadpole shape
Image courtesy Kato et al 2002
They think confusion between the two whales may be misleading observers and hampering conservation efforts.
News of the quest for more data emerged at the annual meeting here of the International Whaling Commission (IWC).
There are two sub-species of what cetacean scientists call "true blues", the Northern and Southern Hemisphere whales.
Blue whales may well be the largest creatures ever to have lived on Earth.
The northern variety reaches 29 metres in maturity, and the southern blue can measure 30 metres or more.
The oddly-named pygmy blue whale reaches an impressive 24 metres, as big as a fin whale.
The first pygmy blues were caught near the Antarctic island of Kerguelen by Soviet and Japanese whalers in 1959.
Dr Hidehiro Kato, of Japan's Fisheries Research Agency, told BBC News Online: "The true blue is torpedo-shaped, but the pygmy looks more like a tadpole, with a relatively big head and short tail.
"We know we're not talking about juvenile true blues because of the vertebrae. They contain two discs, which in an adult fuse together, as they had in the first pygmies found.
"We're also confident the pygmies aren't an entirely different species. If you compare blue and fin whales, there's a lot of biological and morphological difference between the two - more than there is between true and pygmy blues.
"Their behaviour is different, too. The pygmies we've studied breed in the Indian and South Atlantic oceans and go south to the Antarctic to feed. But they don't often go much further than 55 degrees South, while the true blues go right up to the edge of the pack ice.
"We think there may be as many as 10,000 pygmies globally."
They are also found off California, Peru, Sri Lanka and Madagascar.
Scientists believe the total number of true blues may be no more than 5,000, and say even that could be an over-estimate.
They say reports of blue whales may in fact describe pygmies. One said: "If you find a blue whale in tropical waters, it's quite likely to be a pygmy."
The main difference between the two whales, apart from their body shape, is in the shape of the blowhole, which is hard to see from a ship.
Dr Kato said: "We are working to develop a shipboard identification system. If we can get a photo of the blowhole, then we can easily tell the difference between them."
Both true and pygmy blue whales would have been among the species benefiting from the proposed South Atlantic whale sanctuary, which Brazil and Argentina want to see established.
But the IWC meeting voted for both that and a similar proposal for a South Pacific sanctuary by too small a margin.
Neither obtained the three-quarters majority it needed, so both will have to wait at least another year.