In “Ultimate Viper,” we show how vipers have been evolving their way towards the top of the reptile heap for millions of years. Armed with an amazing arsenal of deadly tools, vipers have proven to be remarkably successful predators. Sharpened by millennia of natural selection, vipers have used these traits to penetrate almost every continent on the globe. But in an ever-changing world where skills new and old are constantly put to test, how will these reptiles find new evolutionary paths in which to continue to advance?

In “Tunder Dragons,” monitor lizards (often referred to as thunder dragons) play a crucial role in the food chain of Sri Lanka's lagoons and forests - if they make it through the first year. A mother's role ends with the laying of eggs and hatchlings are left to fend for themselves, find food and keep to the relative safety of the trees. While modern law protects the monitor lizard from being human prey, home is full of threats - sea eagles, acid-spraying ants, snakes and cannibalistic adult monitor lizards.

In “Lake of a Thousand Caiman,” deep in the heart of the Amazon, an uncharted lake emerges every dry season when the floodwaters recede. It's rumoured that this secret lair lures reptiles that have little changed since the time of the dinosaurs. Not just a few either; hundreds gather to this far off, inaccessible place. Outsiders come here at their peril; fishermen claim to have lost limbs to fearsome monsters. Are these exaggerated local legends? Or is the veil of mystery about to be lifted on one of the Earth's largest yet least studied carnivores the black caiman?

• 2 discs; 156 minutes
• © 2010