The Nature of Sex

"All nature's creatures," the British novelist Graham Swift once wrote, "join to express nature's purpose." And that purpose is illustrated in detail in this episode. This episode demonstrates that sex lives come in many varieties. Some animals mate for life, while others may spend just a few frenzied seconds with their partner. In some cases, the dad takes care of the kids, while in others the mom does all the work. And in many households, the newborns are left to fend for themselves, and will never meet their parents. Whatever the household arrangements, however, the reproductive strategy that biologists have dubbed "sex" -- in which two individuals combine their genes in an offspring, with each parent typically contributing half of the genetic material -- offers advantages.

There is nothing more romantic than a moonlit night. So goes many a love song. But for animals from bat rays to wriggling, seagoing palolo worms, the moon is more than a romantic backdrop -- it's a key signal that the time has come to mate. Such sexual signals are the focus of this episode. From the changing phases of the moon to subtle chemical surges, the program outlines the many cues that trigger sexual activity. Indeed, without these landmarks, many animals would send their newborns into the world at the wrong place and wrong time, with little chance of survival.

Every year, millions of brides and grooms promise to love and care for each other 'till death do they part. It's a profound promise. But marriage is also just one of the natural world's many mating arrangements, as this episode shows. Throughout nature, males and females negotiate a wide range of carefully made plans for conceiving and raising offspring. Indeed, monogamy -- two partners staying sexually faithful to one another -- is one of the rarest kinds of sex contracts.Overall, less than 3% of mammal and bird species practice monogamy, and in many of those species, cheating or alternative arrangements is common.

Was it his eyes or hair? Was it her nose or smile? People don't always know what attracted them to a potential mate. But it is certain that the roots of that attraction reach far back into the human past, when our ancestors were foraging across some African plain. Our sexual behaviour evolved to improve the odds that our offspring would survive to have their own children.

This fifth volume of the series continues with its illuminating look at the natural drive for sexual relations. The episode focuses on the variety of ways animals deal with mating, child bearing and birth.

In this final chapter of the series, the subject is the fruit of sexual relations: the young ones. Spectacular wildlife photography captures the birth and care of the progeny of animal species. Whether there is just one baby or a litter of twelve, parents forge strong bonds with their young.