There are actually four species of giraffe, not one as previously believed, researchers said Thursday in a discovery that could change conservation efforts for the world's tallest mammal.

The study in the journal Current Biology is based on DNA evidence from skin biopsies of 190 giraffes across Africa.

Giraffe populations have dropped dramatically in Africa over the past few decades, going from about 150,000 to less than 100,000.

But giraffes have been little studied, compared to other large animals like elephants, rhinoceroses, gorillas and lions.

Until now, researchers believed there was one species of giraffe, and as many as nine subspecies.

The latest data shows there are four distinct species of giraffe which apparently do not mate with each other in the wild.

"Those four species include (the) southern giraffe (Giraffa giraffa), Masai giraffe (G. tippelskirchi), reticulated giraffe (G. reticulata), and northern giraffe (G. camelopardalis), which includes the Nubian giraffe (G. c. camelopardalis) as a distinct subspecies," said the study.

It also said the genetic differences among giraffe species "are at least as great as those between polar and brown bears."

"We were extremely surprised, because the morphological and coat pattern differences between giraffe are limited," says Axel Janke, a geneticist at the Senckenberg Biodiversity and Climate Research Centre and Goethe University in Germany.

"Consequently, giraffes should be recognized as four distinct species despite their similar appearance."

Now, researchers say some giraffes could be considered for listing as a vulnerable or endangered species on the Red List maintained by the International Union for Conservation of Nature (IUCN).

For example, the northern giraffe numbers less than 4,750 individuals in the wild. There are fewer than 8,700 reticulated giraffes, making each species among the most endangered large mammals in the world, the research team said.