Media playback is unsupported on your device Media caption David Shukman goes inside the breeding and conservation programme

In a desperate bid to save one of the world's most endangered animals, conservationists are taking the controversial step of defacing the last survivors.

Ploughshare tortoises are highly prized for their distinctive gold and black shells and fetch exceptionally high prices on the international black market.

Efforts to steal the animals from their native Madagascar are so relentless that there may only be fewer than 500 left.

So the tortoises are now having their shells permanently engraved with a large serial number together with the initials "MG" for Madagascar.

The hope is that deliberately making the animals less attractive will reduce or even eliminate demand for them.

When the idea was first raised, it faced vigorous opposition from many in the conservation movement, the Madagascar government and also staff within the charity involved, the Durrell Wildlife Conservation Trust.

Carving into the ploughshares' shells to disfigure them is the equivalent of removing all horns from rhinos or tusks from elephants to stop those animals from being poached.

Richard Lewis, director of Durrell's operations in Madagascar, told the BBC that "we hate doing it but it's got to be done to help save the species".

"It goes against every grain and gene in our bodies to do this - everything says we shouldn't do this, what we believe in, what we stand for.

"But we think this can be a major step in stopping people wanting these animals. We believe this will be a genuine deterrent."

Image caption The hope is that by engraving the shells, the animals will become less attractive to poachers

According to Mr Lewis, campaigners fighting animal trafficking had spoken to several traders who had made clear that there would be no market for tortoises that had been engraved.

The etching is several millimetres deep and only penetrates the shell, rather than the more sensitive bone underneath, and Durrell staff are convinced that while the process may be uncomfortable for the animals, it does not hurt them.

I watched while one tortoise was engraved. It waved its legs to try to move away as an electric drill was applied but it was not in obvious distress.

The task of marking each animal is one of a range of drastic measures designed to thwart the smugglers and head off the extinction of the species.

The charity runs a captive breeding centre in the Ankarafansika National Park to encourage the tortoises to reproduce - and more than 100 young adults have since been released into the wild.

The breeding centre, itself a target for poachers, is now guarded 24 hours a day with electronic surveillance and a team of policemen.

Only in April, two Taiwanese men, posing as tourists, attempted to bribe local staff to gain access. They were later arrested leaving Madagascar with dozens of a less threatened species of tortoise in their luggage.

The only habitat where the ploughshare tortoises live in the wild - a remote and arid stretch of sand, rock and bamboo at Baly Bay in northwest Madagascar - has been turned into a national park to offer protection.

Image copyright Kate Stephens / BBC Image caption Even the youngsters have to be marked

Image caption Patrols keep watch on the animals, which are fitted with radio tags

Local people have been hired to patrol the area and to keep watch on the animals, many of which are fitted with radio tags.

But the area is too large to be guarded comprehensively and several times a year, perhaps more often, poachers slip in to steal tortoises.

The youngest of the animals - small enough to fit in the palm of your hand - are the easiest to smuggle. In 2013, officials at Bangkok airport found a suitcase that had arrived on a flight from Madagascar containing 54 young ploughshare tortoises. Wrapped in clingfilm, many of the tiny creatures were dead.

Internet searches have revealed the staggeringly high prices that can be fetched by the animals.

One site listed a ploughshare tortoise over 30 years old with a price tag of $37,900. A young adult that was 10 years old was priced at $14,200 and a baby of 8 months at $1400.

In the aftermath of a coup five years ago, there was a period with little or no control by the authorities and leading smugglers, known on the international scene, were spotted repeatedly entering Madagascar.

Mr Lewis says "they weren't coming here for a holiday" and he concedes that the effort to save the tortoise is an uphill struggle with a highly uncertain outcome.

"I'd be the first to admit at this moment that it's three steps backwards and two forwards with the poaching."

And that constant threat led to the extreme idea of defacing the animals, of wrecking the very beauty that makes them so sought after.

"It was clear that what we were doing was not enough, that animals were still going out.

"So when we first talked to the government and said 'we want to take one of your most prized animals and deface it, every one of them', you can imagine the reaction - 'are you crazy or what?'

"But we got the minister to come here [to the captive breeding centre] and after that he said 'I understand, do it'. "

So far about 70 of the tortoises living in the wild have been engraved - it's thought another 400 or so have yet to be found - along with all those bred in captivity that have reached a suitable age.

But now another potential threat has emerged. The tortoises' natural habitat in the area around Baly Bay turns out to be rich in iron and a Chinese company has plans to open an iron ore mine 30km inland.

Image copyright Kate Stephens / BBC Image caption A Chinese company wants to tap the iron deposits in Baly Bay

The proposal would see a new road running through the national park to connect the mine with a massive new port that would be built on the coast.

Although the precise zones where the tortoises live would not be directly affected, the concern is that such major new developments in the area, attracting thousands of people, would vastly increase the risk of the animals being plundered.

It would also set a precedent if industrial infrastructure is allowed inside the boundaries of a national park - at a time when many precious wildlife habitats are under pressure elsewhere in the country.

One member of the Durrell team, Angelo Ramy Mandibihasina, said he understood the balance of interests facing one of the world's poorest countries where 92% of people are estimated to live on less than $2 a day.

"You can think in two ways - if a road is built, it will bring development which is good for the economy but at the same for the environment that will create some bad things that will affect not only the species but the whole park.

"There might some part of the park that will be open for the road and people will come easily and steal the tortoises or cut wood and get some other animals.

The company involved, Wuhan Iron and Steel, told the BBC that it did not wish to comment.