• Life in the Undergrowth
• David Attenborough: A Life on Our Planet

The first episode discusses the keys to success of reptiles and amphibians, looking atthermoregulation, parental care and the time-scales on which reptiles operate. Attenborough begins in theGalápagos Islands, usingthermal imaging to demonstrate howmarine iguanas warm their bodies by basking in the sun before feeding. Meanwhile, thelizard inhabitants of aMenorcan island have a relationship with its indigenousdead horse arum plants. Attenborough visitsDassen Island to witness one of the world's greatest concentrations oftortoises – around 5,000 of them. Few reptiles are active at night, butcrocodiles can rely on water that retains much of its daytime temperature. Conversely, amphibians' moist skin would be damaged by the sun and so most are nocturnal. An exception is thewaxy monkey leaf frog, which can deal with sunlight by covering its body in a wax secretion. Apuff adder illustrates the relative inactivity of reptiles compared tomammals: one large meal can last up to a year. When it hatches at the onset of winter, the youngpainted turtle stays underground, near frozen until the spring when it can emerge. Attenborough wonders if thedinosaurs' immense size allowed them to maintain warm blood. Theleatherback turtle is able to retain body heat due to its large size and insulating body fat.

The second programme explores the world ofamphibians, of which there are some 6,000 known species. Attenborough visits Australia to illustrate how they became the first back-boned creatures to colonise land: thelungfish, which is capable of breathing air, and whose ancestors became the first amphibians. The largest of them is theJapanese giant salamander and two are shown wrestling for territory. InNorth America, themarbled salamander spends most of its life on land, yet is still able to retain the necessary moisture in its skin through the dampleaf litter. A femalecaecilian is filmed with her young, whose rapid growth is discovered to be the result of eating their mother's skin – re-grown for them every three days. The most successful amphibians arefrogs andtoads. Their calls are most active during the breeding season: females are impressed by both volume and frequency. However, gestures are sometimes needed and the poisonousPanamanian golden frog uses a conspicuous form of 'semaphore'. Most other frogs rely oncamouflage and theSouth American red-eyed tree frog is an example. AnAfrican bullfrog is shown defending its exposed tadpoles by digging a canal for them. Meanwhile, the malemarsupial frog keeps its young moist by carrying them in its skin pouches.

The third instalment takes a look at the immense diversity, social skills and displays of thelizards. While they are highly adept at camouflage, occasionally there is a need to break cover in order to ward off rivals. Attenborough holds up a mirror to ananole and causes it to extend its colourful throat flap as a warning sign. Madagascar is host to over 60 species ofchameleon but one of the largest, Meller's chameleon, is native toMalawi and two rival males are shown jousting. A femaleSouth African dwarf chameleon demonstrates its ability tochange colour when communicating to a potential mate, and the chameleon's muscular tongue is depicted lassoing its prey. In southern Australia, Attenborough uses a baited fishing rod to attract the attention of a rarepygmy bluetongueskink, thought to have been extinct for over thirty years until it was rediscovered in 1992.Shinglebacks are among the most devoted lizards and breeding pairs can reunite each year for up to two decades. Alongside South Africa'sOrange River, large groups offlat lizards feed on the swarms ofblack flies, but the males also use the occasion to indulge in social squabbling. TheMexican beaded lizard is one of the few with a poisonous bite, but males do not employ it when wrestling each other. Finally, Attenborough comes face to face with aperentie, Australia's largestmonitor lizard.

The fourth episode focuses on the most modern reptiles, thesnakes, exploring how they have managed to become successful despite their elongated body shape. Attenborough explains how they evolved from underground burrowers to surface hunters, losing their limbs in the process. With the aid of infrared cameras, atimber rattlesnake is shown lying in wait for a mouse and sensing its repeated path before despatching and eating it. A snake's constantly flickering tongue is used to gather and evaluate the molecules of its surroundings, and Attenborough visitsCarnac Island to witness a population of blindtiger snakes, which feed on the chicks of nestinggulls. He also confronts aMozambique spitting cobra, which quickly spraysvenom over the presenter's protective face visor. The similarities in colouration between the harmlesskingsnake and potentially lethalcoral snake are highlighted. An example of a snake that can tackle unusual prey is theQueen snake, which almost exclusively hunts newly-moultedcrayfish. A pair of rival maleKing cobras are seen battling and infantcobras are shown hatching: their venom is immediately as fatal as that of their parents. InArgentina, ayellow anaconda evades nearbycaimans to give birth to live young. Finally a turtle-headedsea snake feeds not on fish, but on their eggs laid on acoral reef.

The final programme covers the most ancient of the reptiles: thecrocodiles andturtles. In theGalápagos Islands, among thegiant tortoises, Attenborough explains how the creatures came to develop theirshells as a defence against predators. This is demonstrated by theeastern box turtle, whose shell includes a hinged 'drawbridge'. The aquaticpig-nosed turtle is unusual in that its eggs need to be submerged before hatching, whereas those of other species would drown; Attenborough illustrates this by dropping an egg into a jar of water: it immediately hatches. In the open ocean, malesea turtles attempt to separate a rival from its mate by attacking and overwhelming the pair, stopping them from taking in air. In northern Australia, Attenborough observes a large gathering ofcrocodiles at a flooded coastal road: they time their arrival to ambush migratingmullet. The complex communication and body language of theAmerican alligator is investigated and in Argentina, the calls of young caimans help their mother locate and lead them to a nursery pool. The mother's maternal instinct extends to releasing unhatched babies by gently crushing their eggs in its jaws. InVenezuela, a femalespectacled caiman in charge of an entire crèche leads the infants from a drying river bed on a trek to permanent water.