Only 90,000 giraffes still roam the African plains, far fewer than the endangered African elephant, according to Sir David Attenborough. Just 15 years ago there were believed to be about 150,000 giraffes in the wild. Since then, these numbers have fallen by 40%.

"These gentle giants have been overlooked," Sir David Attenborough says in Africa, the BBC documentary. "It's well known that African elephants are in trouble, and there are perhaps just over half a million left," said the wildlife expert. "But what no one realises is there are far fewer giraffes. They are killed for their meat, and their habitats are being destroyed. Time is running out." He followed a conservation team as they relocated 20 animals across the Nile in Uganda, to be safe from oil prospectors.

Despite the warning, the species official conservation status as judged by the International Union for Conservation of Nature is still "of least concern".

Dr Julian Fennessy, executive director of Giraffe Conservation Foundation, aims to mitigate threats to giraffes across Africa and raise funds for conservation projects.

"I am absolutely amazed that no one has a clue. This is a silent extinction. Some populations number less than 400," he said.

"That is more endangered than any gorilla or almost any large mammal in the world. Giraffes have gone extinct in seven countries in Africa. It's not going to happen again. There is no giraffe going to go extinct on my watch."

Fennessy set up the Giraffe Conservation Foundation to move animals away from the drilling and worked with the Ugandan Wildlife Authority to move 20 of the Murchison Falls park giraffes. Experts believe growing human populations, fuel wood collections, hunting and drought have all contributed to the decline of giraffes.

Giraffe extinct
Sir David Attenborough: giraffes are facing 'silent extinction' STEPHANIE PILICK/GETTY

One of the most endangered populations is a group of more than 1000 Rothschild's giraffes in Murchison Falls National Park, Uganda, beside the Nile. The land has at least 75% of Uganda's discovered oil, and drilling plans are under way.

Tom Okello, the park manager, said: "We keep some stock outside of the oil area so in the event that some impacts may come out of the oil, we have a separate population somewhere else."

However, as the BBC documentary illustrates, giraffes cannot simply be tranquilised, as they might suffer fatal injuries as they fall. Once shot with a tranquiliser, they must be chased on foot and pulled to the ground using ropes. The teams have a 20-minute window to administer the antidote or the animal will die.

It is dangerous work, as the giraffes weigh more than a tonne and have a ferocious kick. They must be blindfolded and led into a trailer to be ferried over the river.

In the film, the animals are released, wearing specially designed satellite collars that fit around their ossicones - the hornlike structures on top of their heads. The collars are used to show the animals exploring their new home, some covering up to over 11,000km sq.