Diving with wild crocodiles
As prehistoric predators, crocodiles have a fearsome man-eating reputation, but little is known of their behaviour underwater in the wild. Now scientists are risking their lives to study the reptiles in their natural habitat. But can it ever be safe to dive with a crocodile?
They have been around for more than 100 million years, but much crocodile behaviour remains a mystery, as they spend around 80% of their lives underwater.
Nile crocodiles can grow up to 6m long, live up to 100 years old, and weigh up to half a tonne. Fully grown crocs have the strongest bite of any animal, and they are indiscriminate about what they eat - wildebeest, fish or humans.
End Quote Dr Adam Britton Crocodile expert
Once on the riverbed, these man-eaters didn't appear to see us as prey”
The Nile croc is a protected species in Botswana under wildlife law, and the crocodile population is slowly growing as water levels rise and more habitat becomes available.
But, as the human population also increases, crocodile attacks are on the rise. Figures show 55% of croc attacks are fatal and it is hoped that, by understanding croc behaviour, lives may be saved.
So how can we find out more about wild crocs, when it is so dangerous to get into the water with them?
Zoologist Dr Adam Britton has been studying crocodiles for nearly 18 years, and headed to Botswana's Okavango Delta to run a week-long pilot research project investigating where they hide before attacking, and how they detect prey.
How to survive a dive:
The team was led by experienced croc dive experts Andrea Crawford and Brad Bestelink, who developed safety protocols to ensure the divers were not hurt.
• Only dive in winter months: Crocodiles are less active and lose their appetite in the cooler water
• Get in and out of the "kill zone" quickly: Crocodiles attack at the water's surface
• Stay close to the riverbed: Crocodiles attack objects silhouetted against the light
• Always keep out of striking range of a crocodile's head
• Look for smaller-sized crocs: They may be less likely to attack anything bigger than themselves
• Learn to read a crocodile's attitude: Only get close to crocs that appear to be comfortable with divers
"As soon as a croc disappears underwater we don't know what happens... and although they have been studied in captivity we can never be sure whether the animal is behaving naturally or not.
"Normally if you want to do anything with a croc, you have to catch it and drag it in to a boat - and one of the problems with this is crocodiles get incredibly stressed and can take several days to recover - if you mishandle a croc when you catch it, it does nothing for up to a week and you can lose a week's worth of data," he says.
Diving with the crocodiles to observe them and and carry out initial experiments was the only way to gather data - so diving experts Brad Bestelink and Andrea Crawford had to set strict guidelines to reduce the chances of an attack.
Adam Britton sayd: "Crocodiles target prey on the surface - so the main 'danger zone' is getting in and out of the water.
"The first thing is to get to the bottom. Once on the riverbed, these man-eaters didn't appear to see us as prey, and we could study their senses, their movements, and discover where they choose to hide, all without being attacked.
"We established that as long as the circumstances are right, the croc is not bothered about us observing it underwater."
The team observed crocs "walking" on the river bottom - which helps them conserve energy, and found they could even touch the crocodiles' tails, as long as they made no sudden movements and kept away from their heads.
Taking extra precautions though, the dives were held in the winter months when there were fewer crocs in the water, and only when there was better underwater visibility.'Croc caves'
Crocodiles breathe air, but can slow their heartbeats down to just a few beats an hour, enabling them to stay underwater for more than an hour at a time. So where do they hide?
"We wanted to find out where they rest... so we had to go inside 'croc caves'," says Adam Britton.
"These are areas under banks, where the banks are overhanging, where the water is fractionally warmer and the water is very still, and if you look up, the croc has a better view of what's on the surface - silhouetted against the light."
Adam Britton says he has a hypothesis on why crocs gravitate to these spots.
"Comparing these to areas in the main channel showed, as you might expect, that those areas were preferably not only for shelter but also for hunting.
"It's likely that crocodiles are using some of these areas to listen, taste and watch for signs of activity (from prey, and from other crocs) before moving out to investigate."
By uncovering these resting places, the team are able to advise divers, swimmers and other river-users to stay in fast-flowing areas of the river, if they want to avoid a croc attack.
Find out more:
Watch the first episode of Swimming With Crocodiles: Botswana on Sunday 19 February, on BBC Two at 21:00 GMT.
Adam Britton also wants to know how well crocodiles can see underwater. Crocodiles have a third eyelid - a transparent membrane that they close when underwater. His hypothesis was that, at the very least, they would be able to see shadows and shapes through it.
"We tested the croc's vision by using a light-coloured object (a polystyrene ball) and it reacted as soon as it got to within a metre of its head," he says.
"We have shown quite plainly that crocodiles have reasonable vision underwater, enough to detect relatively small objects underwater within striking range of their head, and enough to detect larger objects underwater outside of striking range.
"They can also see silhouettes, such as objects above them, with ease. Obviously the water has to be clear enough to permit this, and this isn't always the case for crocodiles in the wild."
- Crocodiles (Crocodylus) can be found in rivers and swamps in Africa, Asia, Australia and the Americas
- There are 13 living species in this genus, some of which are now endangered
- Crocodiles are believed to have changed very little in 20 million years
- Nile crocodiles are extremely protective parents. Both males and females have been reported to help their young hatch by gently rolling the eggs in their mouths
- The saltwater crocodile (also known as the Estuarine Crocodile) is the largest reptile in the world, and adult males can be aggressively territorial
- Watch a crocodile strike in ultra slow-motion
He also noted that they found that crocodiles have excellent underwater hearing, and are able to detect low frequencies inaudible to the human ear and vibrations for many metres. But it is an area which also needs further study.
"We know from the way they catch fish underwater [that] they are very sensitive to vibrations... and they have receptors on their jaws which fire off," says Dr Britton.
"We found they were able to detect the sound we were transmitting to talk to the boat. It was only when we stopped using the transmitters [that] we could get closer to them."
No-one has yet determined the range of frequencies that crocodiles are sensitive to underwater, or established the role of their ear flaps for modulating sounds, Adam says.
He hopes his research will improve human safety in crocodile-infested areas and is heading back to the Okavango Delta later this year.
"Next time I hope to be able to gather tissue samples from crocodile tails which would open a whole Pandora's box of opportunities for study.
"There's a big push to make the Okavango Delta a crocodile conservation area - and this will help us establish the genetic movement of crocs in different areas and help find where the breeding areas may be."
He says protecting the reptiles is important, as they're an integral part of the Okavango ecosystem, but there is conflict between local people and the species.
"Improving safety for people who live and work in the Okavango and surrounding areas is going to be a critical factor in convincing them to tolerate crocodiles.
"We can only do that once we have a better understanding of the potential risk crocodiles pose to people through their behaviour."
Swimming With Crocodiles: Botswana is on Sunday 19 February on BBC Two at 21:00 GMT and episode two Swimming with Crocodiles: Australia is on Sunday 26 February.