It may be a curlew or a kakapo. It may be the baiji, or a bat. But no one knows for sure now what is the rarest creature on the planet.
The title is going begging this week after the death of Lonesome George, a giant tortoise from the Galapagos Islands who was the last remaining member of his sub-species.
George was thought to be about 100 years old, which makes his death tragically premature as giant tortoises are thought to live to about 200.
There are other species hovering around the top of the endangered species list. But none has quite George's distinctive and unchallengeable claim to uniqueness, as the sole remaining member of his type on the planet.
George was from Pinta Island in the Galapagos, and there were no more members of the Pinta sub-species of the giant tortoise to be found anywhere.
Indeed, scientists believed that Chelonoidis nigra abingdoni had become extinct until George was discovered on Pinta in 1972. Subsequently, he became part of the Galapagos National Park tortoise breeding programme and was encouraged to mate with females of a closely related sub-species but never succeeded in reproducing.
However, he did become a tourist symbol of the Galapagos, famed as the Pacific archipelago visited by the young Charles Darwin in 1835 and now receiving 180,000 visitors a year. (Darwin's observations of how similar species differed slightly from island to island - including the tortoises - started his thinking on the origin of species and evolution.)
A dozen giant tortoise sub-species remain on the islands, with about 20,000 animals in total. When Darwin visited, they were plentiful but later in the 19th century they were hunted for their meat by fishermen and sailors until their population dropped alarmingly. Their population has now recovered - except, of course, for the Pinta sub-species.
National Park officials said George was found dead in his corral by his keeper Fausto Llerena, who had looked after him for 40 years.
So who is to replace him at the pinnacle of the endangered species list? The best candidates are those creatures which may have officially gone extinct, but might be surviving in tiny numbers. In mammal terms that means the baiji - or Yangtze river dolphin of China - which has been driven to the brink, or possibly over it, by the pollution the Yangtse has had to accommodate during China's industrialisation.
In 2006, an expedition failed to find any of the animals, and it was declared extinct, but the following year a baiji-like creature was filmed in the river - so perhaps a tiny number remain.
A similar situation concerns what may be the world's rarest bird, which is probably one of two curlew species - either the eskimo curlew of North and South America, or the slender-billed curlew of Europe, Asia and North Africa.
The eskimo curlew has not been recorded with certainty since 1963 although there have been possible sightings as late as 2006 in Nova Scotia; and while a slender-billed curlew was seen in Northumberland in 1998, there has been no photographic proof.
There are many other birds that are down to small numbers, such as the kakapo, the flightless nocturnal parrot of New Zealand, or perhaps the rarest duck, the Madagascar pochard, of which only 60 individuals remain.
The rarest mammals that are definitely still with us include the northern hairy-nosed wombat of Australia (just over 100 remain), the Seychelles sheath-tailed bat (fewer than 100), and the Javan rhinoceros (fewer than 60).
But none exists in such definite and splendid isolation as Lonesome George did.