For comparison, the genetic differences among giraffe species are at least as great as those between polar and brown bears.

The unexpected findings highlight the urgent need for further study of the four genetically isolated species and for greater conservation efforts for the world's tallest mammal, the researchers said.

"We were extremely surprised, because the morphological and coat pattern differences between giraffe are limited," said researcher Axel Janke.

Giraffes are also assumed to have similar ecological requirements across their range, he added, "but no one really knows, because this megafauna has been largely overlooked by science."

Giraffes are in dramatic decline across their range in Africa. Their numbers have dropped substantially over the last three decades, from more than 1,50,000 individuals to fewer than 1,00,000.

Despite that, the researchers said that there has been relatively little research done on giraffes in comparison to other large animals, such as elephants, rhinoceroses, gorillas, and lions.

About five years ago, Julian Fennessy of Giraffe Conservation Foundation in Namibia approached Janke to ask for help with genetic testing of the giraffe. Fennessy wanted to know how similar (or not) giraffes living in different parts of Africa were to each other, whether past translocations of giraffe individuals had inadvertently "mixed" different species or subspecies, and, if so, what should be done in future translocations of giraffes into parks or other protected areas.

In the new study, Janke and his research group examined the DNA evidence taken from skin biopsies of 190 giraffes collected by Fennessy and team all across Africa, including regions of civil unrest. The extensive sampling includes populations from all nine previously recognized giraffe subspecies.

The genetic analysis shows that there are four highly distinct groups of giraffe, which apparently do not mate with each other in the wild.

As a result, they say, giraffes should be recognized as four distinct species. Those four species include (1) southern giraffe (Giraffagiraffa), (2) Masai giraffe (G. tippelskirchi), (3) reticulated giraffe (G. reticulata), and (4) northern giraffe (G. camelopardalis), which includes the Nubian giraffe (G. c. camelopardalis) as a distinct subspecies.

The elusive Nubian giraffe from Ethiopia and the South Sudan region was the first described some 300 years ago, Fennessy says, and is now shown to be part of the northern giraffe.

The discovery has significant conservation implications, the researchers say, noting that the International Union for Conservation of Nature and Natural Resources (IUCN) Species Survival Commission Giraffe and Okapi Specialist Group recently submitted an updated proposed assessment of the giraffe on the IUCN Red List taking into consideration their rapid decline over the last 30 years.

"With now four distinct species, the conservation status of each of these can be better defined and in turn added to the IUCN Red List," Fennessy said.

"As an example," he added, "northern giraffe number less than 4,750 individuals in the wild, and reticulated giraffe number less than 8,700 individuals--as distinct species, it makes them some of the most endangered large mammals in the world."

Janke and Fennessy said that they are now analyzing the amount of gene flow between the giraffe species in greater detail. In addition to expanding the ecological and species distribution data, they want to better understand the factors that limit gene flow and the giraffes' differentiation into four species and several subspecies.