Giraffe populations in Africa have declined an estimated 30 per cent over the past decade.

A new study has found there are at least six different species of African giraffes, and some of them are critically endangered.

Scientists in the United States and Kenya have found geographic variations in giraffes' hair coat colouracross the animals'range in sub-Saharan Africa, which suggests reproductive isolation. Giraffes, which are highlymobile and tendto roam into each others' breeding regions,have previously been thought of as one species.

One of the species identified, the reticulated giraffe, has an estimated population of just 3,000 in Kenya, Ethiopia and Somalia. That has dwindled, because of poaching and conflicts, from around 27,000 in the 1990s, the researchers said.

They also found there are only about 160 Nigerian giraffes left, while overall estimates suggesta 30 per cent decline in the past decade, withonly 100,000 of the animalsremaining.

"Some of these giraffe populations number only a few hundred individuals and need immediate protection," University of California geneticist David Brown told BBC News. "Lumping all giraffes into one species obscures the reality that some kinds of giraffe are on the very brink."

The study, which appears in the current issue of the BMC Biology journal, suggests that despite sometimes wandering into each others' regions, the distinct species of giraffe maintain an affinity for breeding with their own kind. This may have to do with sexual selection.

"The female Maasai giraffe may be looking at the male reticulated giraffe and thinking, 'I don't look like you; I don't want to mate with you,'" Brown said.