Producer: Ronnie Godeanu

Art Director: Sabina Daley

Designer: Lenny Drozner

Writer: David Malakoff

Production Artist: Meiza Fleitas

Flash Programmer: David Hirmes

Technical Director: Brian Lee

Scientific Consultant: Susane Lee

Graphics Intern: Bill Cavaliere

Thirteen Online is a production of Thirteen/WNET New York’s Kravis Multimedia Education Center in New York City. Anthony Chapman, Director of Interactive & Broadband.

© 2000 Thirteen/WNET New York

All Rights Reserved

Television Credits

A Production of Genesis Film Productions in association with Thirteen/WNET New York and Channel 4 (UK)

We recommend these Web sites for those interested in the subjects shown on the program. All links are valid as of June 9, 2000.

The Kinsey Institute for Research in Sex, Gender, and Reproduction
http://www.indiana.edu/~kinsey/
The Kinsey Institute Web site supports interdisciplinary research and the study of human sexuality.

The Naked Mole-Rat Burrow
http://nationalzoo.si.edu/animals/webcams/
Check out pictures from a live Web cam in a mole rat burrow and other detailed, interesting information from the National Zoo.

Naked Mole-Rat
http://www-personal.umich.edu/~cberger/syllabusfolder/animaldiversity/Heterocephalus_glaber.html
A species profile from the University of Michigan’s Animal Diversity Web.

Patagonian Mara
http://www.pbs.org/edens/patagonia/steppanm.htmm
Information on this rabbit-like creature from “The Living Edens: Patagonia.”

Introduction to the Bacteria
http://www.ucmp.berkeley.edu/bacteria/bacteria.html
Find out about the most common organisms on earth from the University of California. Bird

Bacteria Cam: Growth of Streptococcus Pneumoniae
http://www.cellsalive.com/cam0.htm
Watch them multiply at this site from Cells Alive!

The Australian Brush Turkey
http://www.amonline.net.au/factsheets/brush_turkey.htm
Get the lowdown on these mound builders from Australia Museum Online.

Unisexual Whiptailed Lizards
http://www.amnh.org/exhibitions/expeditions/treasure_fossil/Treasures/Unisexual_Whiptail_Lizards/lizards.html?50
Information from American Museum of Natural History.

The Redback Spider Insect
http://www.qmuseum.qld.gov.au/nature/arachnids/theredback.html
Pictures, links, and more from Australia’s Queensland museum.

Of Course Size Matters: Pheasant Hens Prefer Cocks with Bigger Spurs
http://www.teorekol.lu.se/ekol_inst/mol_ekol/faswww/faseng.htm
A study of pheasant sexual selection from Lund University.

Sex and Reproduction
http://www.mhhe.com/biosci/genbio/biolink/student/olc2/chapterindex56.htm
A textbook overview from McGraw-Hill Higher Education.

Animal Behavior Society
http://www.animalbehavior.org/ABS/
Links and more from this scientific society.

Chimpanzees and Bonobos
http://www.panda.org/resources/factsheets/species/fct_chimp.htm
Fact sheets from the World Wildlife Fund.

Print Resources

Alcock, John. ANIMAL BEHAVIOR: AN EVOLUTIONARY APPROACH. New York: Sinauer, 1997.

Bagemihl, Bruce. BIOLOGICAL EXUBERANCE: ANIMAL HOMOSEXUALITY AND NATURAL DIVERSITY. New York: St. Martins Press, 1999.

Bennet, Nigel C. AFRICAN MOLE-RATS: ECOLOGY AND EUSOCIALITY. London: Cambridge University Press, 2000.

Daly, Martin. SEX, EVOLUTION AND BEHAVIOR: ADAPTATIONS FOR REPRODUCTION. Washington: Prindle Weber, 1983.

De Waal, Frans. CHIMPANZEE POLITICS: POWER AND SEX AMONG APES. Baltimore: Johns Hopkins University Press, 1989.

Fleisher, Paul. LIFE CYCLES OF A DOZEN DIVERSE CREATURES. New York: Millbrook Press, 1996.

Solomon, Nancy G. COOPERATIVE BREEDING IN MAMMALS. London: Cambridge University Press, 1996.

Spomer, Ron. THE RUT: THE SPECTACULAR FALL RITUAL OF NORTH AMERICAN HORNED AND ANTLERED ANIMALS. San Francisco: Willow Creek Press, 1996.

Stockley, Corinne. ANIMAL BEHAVIOR. New York: EDC Publications, 1992.

Nobody knows exactly what these proto-human societies were like. But we can gather some hints from still-living relatives, such as chimpanzees. Like humans, they live in family groups. And, like humans, they can spend years nurturing a newborn to maturity. But when it comes to sex, there are some key differences. Most importantly, female chimps mate just every few years, when they are fertile, a fact they widely advertise to males with bright red genitalia. Attracted and aroused by the sight, male chimps will crowd around the female, competing to copulate as often as possible. In contrast, it isn’t obvious to men when women are fertile.

This change may have helped forge closer bonds between women and their male partners — cooperation that may have been key to successfully raising kids. The general idea is that since men didn’t know exactly when women were ready to conceive, they hung around in a bid to improve their odds of becoming fathers. And to maintain a male’s interest, human females may have evolved other attractions, such as curvaceous breasts. Human females, in fact, are the only primate to have permanently swollen breasts — a significant attraction for males evolved to equate swollen breasts with fertility. That ancient biological history may help explain today’s continued fascination with cleavage among both men and women.

Men have also evolved assets designed to attract a mate. Large muscles, for instance, may have once signaled a man’s prowess as a hunter and defender. But strength alone probably wasn’t enough to attract and keep a mate. To be successful, men also had to show that they had the smarts to be creative and dependable providers, clever enough to find food and shelter for their families in an often hostile environment.

Today, physical attributes such as muscles and breasts may have little meaning in modern societies where most people work in offices and limit their foraging to the local grocery store. But they still hold powerful sex appeal, a fact not lost on advertisers who use bikini-clad supermodels and ripple-chested jocks to sell everything from cars to dish detergent. Sex, it seems, has long been a best-seller.

Overall, less than 3% of mammal and bird species practice monogamy, and in many of those species, cheating or alternative arrangements is common. Men in some cultures, for instance, have many wives. In others, many men share a single wife. A bird called the Jacana, or Lily Trotter, has a similar sex contract. A single female will have four or five male partners, each carefully caring for eggs laid by the female.

Still other creatures carry this idea to an extreme. African mole rats and termites, for instance, live in underground colonies with a single queen that produces all of the offspring in the colony.

As a result, almost all of the colony’s inhabitants are brother and sister. Male elephant seals, in contrast, play king, ruling over a harem that includes every female on a long stretch of beach.

These arrangements, however different, have the same goal: increasing the number of offspring carrying the parents’ genes — or, in the case of termites, the common genes of the entire colony. And in each case, the contracting partners work together for the common goal: the survival of a new generation.

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 A TIME AND A PLACE, Part 2 of NATURE’s THE NATURE OF SEX. 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.

Consider the Pacific grunion, for instance. Each spring, millions of these little fish fling themselves up onto Pacific beaches to deposit their eggs in the wet sand. But they don’t choose just any night for sex. They ride the spring tide, one of the highest tides of the year, when the moon’s pull is the strongest. The choice ensures that there will be enough big waves to get back to the ocean — and that their eggs will be high enough on the beach to be out of reach of scavenging fish. Arrive a few days early or late, and both fish and eggs will perish. The moon, however, isn’t the only sexual timepiece. The sun is also an important trigger. Many mammals become amorous during the lengthening days of spring — a sign that warm weather is coming, and with it adequate food supplies for their offspring. But for other animals, rain is the most important romantic event. Darkening skies and heavy drops, particularly in the arid plains of Africa, mean that there will soon be enough grass to feed young gazelle and other browsing animals. Not surprisingly, predators such as jackals time their births to arrive at the same time, using the easily captured baby browsers as food for their own young.

Other signals are produced by the animals themselves. Many insects and mammals, for instance, produce chemicals called pheromones, subtle perfumes used to attract the opposite sex. Others change color, shape, or grow special body parts, such as sharp, curled horns, to signal that they are ready to reproduce. Indeed, few sights are more impressive than a bird in colorful breeding plumage, or a fish that has turned as bright as a neon sign to attract a mate. Now, all they need is a nice, romantic, moonlit night . . .

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.

From the sea horse that mates in an hypnotic underwater ballet to the rodent who copulates until he literally drops dead, THE NATURE OF SEX spans the globe to illustrate how an astonishing diversity of life forms find their mates and conceive, raise, and protect their offspring.

This Web companion to the four-part series takes a close look at the primal instinct that causes animals to come together in order to pass along their genes to the next generation. We also examine how timing can be key in the mating process, and how varied sex contracts create not only new life, but diversity, in both animals and humans.

Online content for The Nature of Sex was originally posted June 2000.