Steve Leonard's Ultimate Killers

"I'm on a mission to track down the world's ultimate killers ... and put them to the test. All very different from my regular day job as a vet," says Steve Leonard.

First up are animals that use extreme speed to kill. "The fastest sprinter on Earth? It's got to be the cheetah, hasn't it?" Steve goes head to head with a fairly friendly running mate and, unsurprisingly, she easily catches Steve, who's flat out at almost 20mph. A speeding jeep proves a more testing challenge. For just a moment, the cheetah puts back its ears and really flies at over 40mph. But after 40 seconds, she is exhausted - a cheetah is fast, but fragile.

A cheetah goes from 0-60mph in three seconds, but a barracuda does it six times faster. Steve sees what a barracuda can do when a fellow diver gets his hand ripped open as an unseen barracuda attacks - an attack so fast that he hasn't a hope of escape. But in terms of outright top speed, the black marlin is the fastest fish in the ocean. To target its prey, 60mph yellow fin tuna, a marlin can accelerate to an unbelievable 80mph.

Bass Rock is home to 40,000 living guided missiles. Dive-bombing at speeds of over 90-mph, gannets stun their prey. Steve gets out on the ocean to watch these living missiles smash into the water. "It's got more built-in safety features than a Volvo ... a skull like a crash helmet, and even a driver's air sac."

But when it comes to extreme speed, none of the other contenders gets near the peregrine falcon. At 90mph, it is just warming up. Steve decides to put it to the test and climbs to 10,000ft, perched on a piece of plywood on the outside of a hot-air balloon. Inside the balloon is Lady, a remarkable peregrine falcon trained to dive with Steve. At last the conditions are perfect and, after a heart-stopping countdown, Steve leaps and free-falls. Within seconds, his onboard speedo hits 158mph and, a few moments later, Lady catches him up, effortlessly streaking to 180mph. Steve says: "She was playing around with us effortlessly. No style, but shed-loads of speed."

In the second edition Steve is bobbing around in shark-infested waters off Seal Island, South Africa. "We're going to play a little trick ... on a great white shark," he says. With the help of shark researcher Rob Lawrence, Steve drags a lightweight, sealshaped decoy - a cut-down boogie board -from the back of the boat. In an explosion of power, a two-tonne great white launches itself 13 feet out of the water, thinking that it has found a tasty morsel. "Do you get much water-skiing out here?" laughs Steve. But the great white is nothing but a big bully when it turns on a seal - it's like Mike Tyson picking a fight with an eight-year-old. So which is the world's strongest animal?

Deep in a rainforest in Panama, Steve and expert Alberto Palleroni go in search of a harpy eagle, the strongest bird of prey in the world. Alberto regularly climbs up to nests to check on the chicks' health. "Trouble is, the nest is around 120 feet up, in a very wobbly tree," says Steve. They reach the nest and discover a recently-hatched chick - already the size of a turkey. But it's not long before mum turns up. "That is a serious budgie!" says Steve. She has talons the size of a grizzly bear's, three times more powerful than a Rottweiller's jaws, capable of popping monkey skulls in a single crunch. Steve and Alberto don't hang around for long.

The Cave of Death in Sulawesi, Indonesia, is home to some of the biggest, most powerful snakes in the world - giant, reticulated pythons. Steve experiences first-hand the enormous strength of a 15-foot python as it wraps itself, vice-like, around him. Back at base, in fear for their livestock and children, the locals have recently killed a python. It's just had a heavy meal and the massive lump in its body looks big enough to he a human. Steve performs an autopsy and there's a collective sigh of relief when he discovers that it's a huge, partly-digested, wild boar.

An average polar bear weighs the same as seven fullygrown men and, with a single swipe of its huge paws, can take out prey three times its size. Finding these bears in the wild is tricky, but Doug Allen has been filming them for over 10 years and is also an expert in Arctic survival. He and Steve team up, and Steve sees his first polar bear up close ... almost too close: "It may not look that scary on the TV, but my heart was pounding," he admits.

But there is no question that the clear-cut winner this week is the killer whale. It combines brain power and strength to take on the largest animas in the world - blue whales, which are 10 times its own size. Steve is desperate to meet these animals on their own terms. In British Columbia, he paddles his kayak within feet of them. A 6-foot fin glides right alongside Steve, but this is just the tip of a living iceberg - underneath, there's a four-tonne killer. But they seem more playful than threatening, breaching out of the water right beside a stunned Steve. In the wild, there hasn't been a single recorded attack on a human being. :Mission accomplished. A breathless Steve says: "That was probably one of the most amazing things I've done in my life."

In "Chemical Killers", "I'm going to check out the most venomous animals on the planet. I want to find out which could kill me, or you the fastest," says Steve

It takes Steve three days, by plane and boat, to get to the tiny Indonesian islands where the first contenders live. Resembling scaly, prehistoric monsters, komodo dragons are confirmed man-eaters. Their mouths are packed with up to 70 different types of toxic bacteria, and one slashing bite from their razor-sharp teeth delivers a deadly dose of bacteria straight into the wound. After the attack, the komodo waits for its unfortunate victim to die of bloodpoisoning, about 72 hours later. What does Steve think of them? "Big teeth, big claws, bad breath."

Komodos are found only on a handful of islands in Indonesia but Steve's next contenders, spiders, are everywhere. Nearly all spiders have fangs to inject their victims with venom. The biggest spider in the world, the Goliath bird-eating spider, turns out not to be particularly venomous, but in Texas Steve tracks down the small and deadly brown recluse spider. Talking to Leanne and Robin, two unfortunate brown recluse victims, Steve says: "Their bites were genuinely shocking ... I really wasn't prepared for what I was about to discover." A single bite can have appalling results, causing horrible ulcers, terrible pain for years as the venom continues to digest flesh, and sometimes amputation.

Next, Steve encounters some highly venomous scorpions which specialise in extreme, agonising pain. Steve escapes without a scratch but expert Major Scott Stockwell is stung numerous times during the interview, including by the world's most deadly scorpion, the "death stalker". Scott knows that, as a big healthy adult, he's not going to die and has learnt to overcome the agonising pain by the power of his mind alone. But though scorpions won't kill healthy adults, they can kill the young and old from massive overstimulation of the nervous system, in about seven hours.

In the Australian outback Steve meets Graeme Gow, who has been bitten twice by the world's deadliest snake - and survived. The snake in question is the inland tiapan, and one bite contains enough venom to kill dozens of healthy adult humans. Graeme is also the most bitten man on Earth, having sustained 183 venomous snake bites to date. The tiapan that got Graeme injected only a microscopic amount of venom, but it was enough to turn him into "one of the living dead" for two years. Steve and Graeme find the snake way out in the outback where it lives and, as Steve gets close, he finds out why this snake has to be so breathtakingly deadly to survive there isn't much food out here and every attack matters.

In "Pack Hunters" as ultimate killlers go, wolves rate is surprisingly low. Recent studies have shown that this legendary pack hunter doesn't have such a high kill rate - on average just 10 per cent but they do take on massive and formidable prey, such as bison. "Working together massively increases their chances of catching and killing big prey," explains Steve.

Steve sets off up the Amazon to check out the piranhas that hunt in packs as big as 20,000 individuals. Pausing at the local market to buy a duck as bait, Steve heads upstream to shallow water where the packs of piranha get squeezed together. Steve splashes the duck around as though it's wounded and soon feels the tell-tale thumps as the small piranhas bite. Within moments, the water starts to boil as the piranha pack moves in for a full feeding frenzy. Suddenly, it's all over, and Steve pulls up a bare skeleton.

How about lions - how good are they? Steve takes a flight to Africa but, on the way, events take a dramatic and unexpected turn for the worse. He hadn't felt well the night before, but on board the plane to Nairobi, Steve's fever kicks in really seriously. By the time it lands in Kenya, he has to be wheelchaired off the aeroplane, shivering uncontrollably, and is rushed to hospital. He is soon diagnosed as having tick bite fever, a serious illness which, if untreated, kills around 10 per cent of its victims.

After a short stay in hospital, still not feeling too clever, Steve is off for his rendezvous with the lions. He hooks up with the BBC Big Cat Diary team and, for three days, becomes part of the lion crew. Despite appearances, the lion pride doesn't really co-operate at all - there are no real tactics - but they have a 30 per cent kill rate which, with the sorts of massive, powerful prey they take on, is very impressive.

But this week's outright winner is Africa's wild dog. Steve's in his element racing across the scrubland in Botswana, desperately trying to keep up with a pack of hunting wild dogs. He says: "As pack hunters, the wild dogs have it all the numbers, the teamwork, strength, endurance." But there's more to it than just being good hunters. Steve notices quite a few of the dogs are carrying some very nasty injuries but discovers that healthy dogs keep coming back to check that the injured dogs are still on the trail of the hunt. He concludes: "On the outside, they look like a bunch of scruffy street urchins but, underneath, there's a close-knit supportive team which takes hunting to the level of genius - with a 90 per cent strike rate."