Howler monkeys back in Rio's Tijuca park after 100 years

Just after their cage is opened, four hesitant howler monkeys take their first steps inside Tijuca National Park, in the heart of Rio de Janeiro.

Soon they are feeling confident enough to climb trees and eat fruit.

This species had been absent from Tijuca National Park for about 100 years, according to estimates from researchers.

This forest is famous for being the home of Rio's famous Christ the Redeemer statue and also as one of the world's largest urban forests.

However, due to poaching and deforestation, many of the animals from the park have disappeared, making it an "empty forest".

Now scientists want to change that.

"Tijuca is a starting point for techniques in reintroducing fauna to forests," says Fernando Fernandez, a researcher from the Federal University of Rio de Janeiro (UFRJ), which runs the project with funding from the private foundation Grupo Boticario.

Some of these animals are important for spreading seeds, helping big trees reproduce.

But howler monkeys particularly help the forest through their faeces, which are a source of nutrients to the soil.

The monkeys were taken to the forests on 3 September, after spending eight months being prepared in a UFRJ lab.

There they were medically tested, given ankle bands (for monitoring purposes), and learned to live together as a group.

"That's very important, as howler monkeys need social groups to live in the forest," says Mr Fernandez.

There were initially five monkeys, but one of them did not get along with the dominant male - and was therefore expelled from the group.

Mr Fernandez hopes to bring three more groups - comprising five monkeys each - to Tijuca in the next few years.

The goal is that the population becomes self sustaining.

The project began in 2010, when the scientists started to repopulate Tijuca with local rodents (called cutias) that are also cherished for their seed-spreading abilities.

Now there are 35 of them inhabiting the park.

In the future, the plan is to populate Tijuca also with native boars, kinkajous, native sloths and the famous - and endangered - golden lion tamarins.

"Our goal is to reconstruct the fauna in the long term," says Mr Fernandez.

Millions of annual visitors to Tijuca National Park now have a chance to spot the howler monkeys in the middle of the jungle.

Soon enough visitors might be able to hear them as well: howler-monkeys' screams can be heard from kilometres away, but they only shout with such enthusiasm during territorial disputes - which might take a while, since there are too few monkeys in the forest for the time being.

Mr Fernandez is both excited and concerned about how the monkeys will respond to humans.

"I hope people react well, because these animals are really charismatic," he says.

"But I do recommend that people keep their distance - avoid interacting with them and please do not feed them, as it can be harmful. Watch and enjoy, because they are really cool, but from a distance.

"Many people visit Tijuca National Park to visit the Christ the Redeemer but there will be more and more animals there. It is an opportunity to appreciate the fauna and learn how to preserve it."