This is a classic 'fingertip' species, clinging to survival above an abyss of extinction. About 560 live in zoos and even fewer remain in the wild. Most of those live in a precarious nature reserve called Poco das Antas, 70 miles north-east of Rio de Janeiro.
Yet the tamarin may pull through, largely because its cute appearance makes it a drawcard for wildlife conservation. Conservation institutes, charities and zoos have spent more than pounds 1million trying to preserve the tiny wild population in Brazil and reintroduce zoo-born animals to supplement it. More than 120 zoos around the world, including Jersey and London, are collaborating in the captive breeding programme. Over the past 20 years the programme has been so successful that zoos are running out of space and many adults are being given contraceptives.
The ecotourists had come from Rio de Janeiro in an air-conditioned coach. Of the dollars 90 ( pounds 47) they each paid, dollars 20 went towards the conservation effort.
But ecotourism can bring problems as well as money. Within a few minutes of the humans arriving, the tamarins, a former zoo family flown in from Los Angeles two years ago, came down from the low canopy to head height and were moving along the low boughs, peering curiously at us and squeaking. They remained unafraid of people. That meant a charming, though unnaturally close, encounter for their whispering, camera-clicking visitors. However, their tameness also makes the tamarins easy to kill or capture for illegal trade that can bring in thousands of dollars.
'You could say this particular family is being sacrificed for ecotourism,' says Ariane Janer, the Dutch tour operator who began the first coach trips in June.
Our visit was to a patch of forest outside the Poco das Antas reserve, where young Brazilian guides can locate the family of tamarins (two parents, twin infants and an adolescent) by the radio transmitter and antenna on the collar of the patriarch. The wild population is being saved from the stress of being stared at by visitors.
The daily excursions started as a two-week trial during the United Nations Earth Summit in Rio de Janeiro in June. The National Zoological Park in Washington DC, the Smithsonian Institution and the World Wide Fund for Nature, the prime movers in the reintroduction project, are now considering whether the tours should become permanent.
The reserve was established in 1974 by the Brazilian environmental agency, Ibama. It covers 20 square miles, but only three- fifths of that is wooded and suitable for tamarins. A few wild animals are thought to linger in other scraps of woodland in Rio de Janeiro state.
The tamarin is unique to South America's other great tropical forest - the one that, unlike the Amazon, has almost disappeared. Until the Portuguese arrived, Brazil's long, mountainous Atlantic coastline was a broad strip of trees covering 400,000 square miles. Now less than 5 per cent of the forest remains and much of this has been disturbed or is secondary regrowth - like that in Poco das Antas.
Poco das Antas supports about 300 wild, but intensively studied, tamarins and is thought to be at maximum 'carrying capacity'. The tamarin is a territorial creature, with each family of four to eight needing about 40ha (100 acres). Those at Poco das Antas spend much of their time in border disputes.
Nor is the reserve secure. A dam was built on its edge during the Eighties to irrigate nearby farmland. The lake it created has silted up and the dam has cracks, but it has succeeded in drying out parts of reserve and increasing the risk of forest fires.
The introduction of zoo animals began in 1984. After health checks and a spell in quarantine, they are released in family groups headed by an adult male and female that pair for life (probably about a dozen years in the wild). One or both of the adults has a radio collar attached. The mother normally has twins, which, when adolescent, remain with the parents helping to raise the next set of offspring and learning parentcraft before eventually leaving.
The first few tamarins were released into the reserve, but the scientists soon decided to switch to forested areas nearby - although zoo-born animals have bred with wild ones. 'We didn't want to disrupt the original population,' says Dr Devra Kleiman, the programme co-ordinator based at Washington DC's National Zoological Park.
The landowners providing forest outside the reserve are big farmers and businessmen based in Rio who have agreed not to clear trees and to try to prevent hunting. In any case, the land tenure laws and subsidies that encouraged them to burn forest have been changed. Ines Castro, a Brazilian research assistant, says: 'They're starting to take a pride in the tamarins, and there is now a waiting list.'
The zoo animals tended to scatter and lose each other at first - bad news for such a social animal. They also lacked the ability to move swiftly and surely through the canopy because they were used to stiff climbing structures in zoo cages rather than real, springy boughs and branches. They crashed around and ended up spending too much time on the forest floor, exposed to predators.
The newcomers have fairly high death rates - of just over 100 introduced, about 30 survive. But they have bred, and their 77 wild-born young are faring better - 52 are still alive. They have begun to raise their own families and may have taught their parents a few survival skills.
Each family is provided with a nest box in the trees. For the first year they are given food and water once a day, then every other day for the next six months and then they are on their own. They have to relearn a natural diet of fruit, insects, eggs, nectar and the occasional snake and frog.
Natural enemies are large snakes, hawks and owls and small cats such as the ocelot. Once the radio trackers traced a tamarin signal emerging from a boa constrictor enjoying a post-prandial doze.
A few local people still hunt in the forest with dogs for sport and food, despite the efforts of the project's Brazilian 'educational outreach' staff in schools and communities. Others take them alive for sale; the zoo tamarins can be lured into traps fairly easily. Eight are thought to have been captured. Four were traced and returned to the wild - only for two of them to be taken again.
Three days before it was due to be shipped to Brazil last year, a zoo tamarin was found to have been exposed to a lethal virus that could have been passed on to the wild population. Until a few years ago the virus had been found only in African and Asian monkeys, but it spread to South American species in several US zoos and has killed tamarins.
This potential catastrophe was averted, and overall the project has to be regarded as a great conservation success story . . . so far.
The project's backers plan to continue the reintroductions; this month a new batch of families from Germany was released. More trees will be grown inside and around Poco das Antas to create extra habitat, and landowners will be encouraged to donate more patches of remaining forest. Dr Kleiman hopes that over the next 30 years the area of protected habitat will be quadrupled and numbers in the wild raised to 2,000. 'Frankly, we still have an enormous way to go,' she says.
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