The love-struck red titi monkeys sat in their branch with their two prehensile tails romantically entwined. The emperor tamarins with their outlandish white moustaches played mischievously in the canopy, while the noisy trumpeter birds stalked the dank undergrowth of the forest floor below.
It was a steamy 27C, but feeling much hotter because of the 75 per cent humidity, and as the watery spring sunlight of a March morning filtered through the plastic domed roof, a little bit of tropical rainforest emerged through the artificial mist pumped out from the aerial nozzles of the humidifier.
A chunk of South American rainforest has been transported to central London as part of a new open gallery of plants and animals, where a variety of small mammals and birds are allowed to roam freely among species of trees and shrubs that are normally only found in equatorial jungles.
Rainforest Life, the latest exhibit at London Zoo, will open to the public tomorrow and is expected to draw crowds eager to experience one of the world's most vibrant ecosystems – which for many will only ever be seen through the televisual narrative of a David Attenborough documentary – first hand.
Stepping in from the chill of a British spring day, the suffocating heat and humidity of the rainforest gallery hits you like a warm, damp pillow. The screeches, squawks and chattering of monkeys among the verdant leaves and dark tree trunks indicate that this is no ordinary hothouse.
There are no barriers or bars separating the walkways from the wildlife and the animals can come up to the visitors, something the golden-headed lion tamarins are likely to do. It is part of the ethos of trying to create an authentic rainforest experience, according to the zoologists behind the scheme.
"The rainforests, alongside the oceans, are the most precious natural treasure we have. They are the lungs of the world. They provide our food, our medicines and our oxygen," said David Field, zoological director of London Zoo. "Rainforest Life will give people a real taste of the rainforest. These forests are living ecosystems which are home to thousands of endangered animals and they are disappearing fast. Hopefully by seeing how breathtakingly amazing rainforests are, we can encourage visitors to protect the world's last remaining few.
"Using knowledge we've got from being in Brazil we've created this environment because we've also got to make the other animal here – the humans – feel they are in a rainforest. They need to be able to step into this building and be transported," he said.
The exhibition, created in the zoo's 1960s-built Clore gallery, places living plants alongside about a dozen species of small mammals and birds. The grey-winged trumpeter bird and sun bittern forage together – in a bickering sort of way – on the forest floor, while monkeys, tamarins and sloths patrol the canopy above.
Red-cowled cardinals, a bird from north-eastern Brazil, dart from branch to branch and pose a particular danger to any human visitor careless enough to stand too long below their perches – this writer suffered a direct hit on his left shoulder.
One of the prize exhibits is the southern tamandua, a tree-climbing anteater with an improbably long proboscis, rich golden fur and a prehensile tail. A shy creature, it climbs through the branches using its sharp claws and tail, occasionally stopping to stand on its hind legs or flick out its tongue, which can stretch to 40cm.
"We had a particular desire to show a range of biodiversity. It is particularly dynamic and we may swap species as we go along. It really is a living, breathing ecosystem," Dr Field said.
"We want people almost to be able to see them giving birth, watch them rearing young, and to see how they react with other babies and other animals. We give the animals the choice: if they want they can hide away completely. They've got enough nestboxes and shelters. If they want to they can come to the walkway, but it's up to them," he added.
Tony Dobbs, a senior keeper, said that trying to get the right balance of rainforest wildlife was crucial. Letting predators loose in the enclosure was obviously not going to work, but it was also important not to introduce strongly territorial animals.
"We didn't know how the emperor tamarins would react because they are new to this. But we've even seen the young emperors play with the golden-headed tamarins. They've started to interact in a positive way," Mr Dobbs said. "Each has their own particular sleeping area that they tend to favour. But there doesn't seem to be any competition over them because it's quite a big area. Everyone has their spot, nobody's really fighting. It's worked out really well," he said.
One problem that has emerged, however, is the intrusion into the gallery by the odd British-born mouse. But even this difficulty has found its own solution, said Dr Field. "We might get the odd little rodent in here but we use a natural rodent control. Our trumpeter birds beat any cat in terms of catching mice – and its natural," he said.