"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.