"All nature's creatures," the British novelist Graham Swift once
wrote, "join to express nature's purpose." And that purpose is
illustrated in delightful and sometimes dizzying detail in NATURE's
THE NATURE OF SEX. As Part 1: THE PRIMAL  INSTINCT
shows, birds, bees, and even barnacles and naked mole rats are
driven to join forces to reproduce and pass along their genes
to the next generation.
As THE PRIMAL INSTINCT illustrates, sex lives comes 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. Most of all, sex assures variety. Every newborn
produced by sex carries a unique set  of
genes that may give them a survival advantage in a changing environment.
Slightly longer legs could prove the difference between outrunning
a predator and becoming just another meal. And a slightly heavier
coat of fur, or a thicker layer of fat, may enable an animal to
survive a winter that brings a cold end to others. Variety not
only brings spice to life, it seems, but is also a key to survival.
Not all organisms need sex to multiply, however. Bacteria, for instance,
survive just fine by dividing into two genetically identical copies.
Similarly, some creatures -- such as the whiptail lizards and aphids
featured on THE NATURE OF SEX -- can produce genetically identical
clones without the benefit of a partner. But these identical offspring
can be much more vulnerable to disease and changing conditions than
more mixed breeds. That's why sex, in all its shapes and forms,
has proved so successful a reproductive strategy.
Despite its central place in all of our lives, sex remains an often
mysterious and poorly understood primal impulse. Researchers are
still trying to understand how mates choose each other. Sometimes,
the signs of a winner are obvious, such as the sleek coat or colorful
plumage that signal good health. But other attractions are more
subtle. Only female fiddler crabs, for instance, may ever know if
it's the size of a male's waving, outsized claw -- or something
else -- that proves so seductive.
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