by NEIL SEARS and NIGEL BLUNDELL

Last updated at 23:37 10 June 2007

When Dr Brady Barr decided to dress up as a crocodile, the disguise needed to be good.

Otherwise he was in grave danger of being eaten by the real thing.

The zoologist adopted his bizarre outfit in the hope of getting closer to a colony of Nile crocodiles, which can grow up to 20ft.

His disguise was a prosthetic head attached to the front of a protective metal cage covered with canvas and a generous plastering of hippo dung to mask his human scent.

It was 13ft long - average Nile size. The head was made of glass fibre, while the frame of the body was aluminium covered with a sheet of Kevlar body armour.

Thus protected, he crawled on his hands and knees up to the fearsome reptiles, close enough to touch them.

The most dramatic moment in a documentary film about Dr Barr comes when he is approaching the largest croc on the riverbank - and mistakenly makes a sudden movement. It suddenly turns to face him head-to-head. Dr Barr whispers into his microphone: "Oh, the big one is moving towards me. He has his eye on me. I'm really nervous. I've got to back off!"

But just as he is about to retreat, another moves up behind him.

A sweating Dr Barr whispers to the camera crew: "Now I'm stuck between two giant crocs. How close is the one behind me?"

Back comes the answer: "Next to your right leg."

"So it's unsafe for me to get up and move, right?"

"I would say so!"

Dr Barr, 44, claims to be unique in having studied all 23 species of crocodilian - crocodiles, alligators and caymans - in the wild.

For his latest quest, he travelled to Tanzania, where Nile crocodiles lurk in riverbank mud holes during the dry season and crowd on top of each other in burrows. When a hippo and her calf sniffed inquisitively at him, no doubt attracted by his coating of dung, he stayed as calm as possible.

"That could have been a very dangerous situation," said Texas-born Dr Barr. "And that was before I'd even seen a croc."

Ultimately he managed to infiltrate the crocodiles' lair and attach to their tails small hi-tech 'data loggers' which monitor their activity and give scientists biological information such as the temperatures in their dens.

At least one-third of all crocodile species are endangered, and his extraordinary activities have the aim of bringing worldwide attention to their plight.

"Crawling up to the crocs wasn't easy," he added. "Worse was scrambling inside their lair.

"Any time you do that, you are asking for trouble. I am worried that I am getting too old and slow. I'm starting to understand that I can't take as many chances as I have in the past."

• Barr's documentary launches National Geographic Channel's new series, Dangerous Encounters, next Monday at 9pm.