Background:
Cattle originally evolved over millions of years through a process of natural selection-also known as “survival of the fittest”-which made them adaptable to a wide variety of environments, including most of those inhabited by another highly adaptable species: humans. Once humans discovered how to domesticate cattle about 4,000 years ago, they began to selectively, or “artificially,” breed them for specific desired traits like meat and milk production. This resulted in animals fit less for survival in the wild than the satisfaction of human needs, but in purely genetic terms, the arrangement has proven highly successful for cattle. Cattle now thrive throughout the world in over 800 different breeds, each more or less successfully adapted to their environment and the needs of their human caretakers.

The excerpted clips from the NATURE episode “Holy Cow!” illustrate some of the naturally evolved and artificially selected attributes of cattle.

Suggested Focus Questions:

Clip 1: A Cow’s Digestive System

1. What role do microbes serve in a cow’s rumen?
2. Does this clip illustrate naturally or artificially selected attributes?
3. Why would a cow’s ability to eat grass make it ideal for human domestication?

Clip 2: Desirable Breeding Traits in Cattle

1. How many different breeds of cattle have humans created?
2. Would the qualities that make good beef cattle help them survive in the wild?
3. Besides milk and meat production, what other traits might humans wish to breed in cattle?

Clip 3: Different Breeds of Cattle

1. What might indicate that a cattle breed is adapted to cold climates like Scotland?
2. What do the different breed names refer to?
3. Why might “adaptability” be a desirable trait in a specific breed?

Downloadable QuickTime versions of the video segments:
(Note: To downoad a video, right-click on the video title and click “Save Link As…” or “Save Target As…” On a Mac, press the CTRL key and simultaneously click the mouse, then save the link.)

Clip 1, “A Cow’s Digestive System”

Clip 2, “Desirable Breeding Traits in Cattle”

Clip 3, “Different Breeds of Cattle”

GRADE LEVEL: Grades 9-12

TIME ALLOTMENT: Two to three 45-minute class periods

OVERVIEW: Cattle evolved through the slow process of natural selection until human domestication, which rapidly accelerated their development as an artificially selected species fit less for survival than to satisfy human needs. This lesson focuses on how and why humans have been so successful in selectively breeding cattle to suit these needs, while also exploring the limitations and consequences of this success.

Natural selection describes the process by which organisms best adapted to their environments are the ones that survive and reproduce. The Introductory Activity helps students understand that some traits, such as the ability to digest grass, made certain species more desirable for domestication by humans. The Learning Activities explore how, by nurturing and protecting animals that might not have survived in the wild, human domestication interrupted the process of natural selection. Reproductive success was no longer primarily determined by an animal’s most naturally adaptive survival traits, but rather by its artificially selected traits desirable to humans (primarily milk and meat production). The culminating activity presents a case study in which students consider the consequences of losing the naturally selected attributes of breeds less adapted to domestication.

Students should already be familiar with the concepts of evolutionary adaptation, natural selection, and DNA/genetic engineering.

SUBJECT MATTER: Biology/ Living Environment

LEARNING OBJECTIVES:

Students will be able to:

• Describe the traits of cows that make them suitable for domestication.
• Explain how the cow’s digestive system has adapted to its environment.
• Compare and contrast natural and artificial selection.
• Describe various breeds of selectively bred cattle and their desirable traits.
• Discuss some limitations and negative consequences of selective breeding.

STANDARDS AND CURRICULUM ALIGNMENT:

From the National Science Standards for Science Content, Grades 9-12.

CONTENT STANDARDS C: Life Science
As a result of activities in grades 9-12 students should develop understandings of:

Biological Evolution

• Species evolve over time. Evolution is the consequence of the interactions of (1) the potential for a species to increase its numbers, (2) the genetic variability of offspring due to mutation and recombination of genes, (3) a finite supply of the resources required for life, and (4) the ensuing selection by the environment of those offspring better able to survive and leave offspring.
• The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms.
• Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life forms, as well as for the striking molecular similarities observed among the diverse species of living organisms.

Behavior of Organisms

• Like other aspects of an organism’s biology, behaviors have evolved through natural selection. Behaviors often have an adaptive logic when viewed in terms of evolutionary principles.

New York State Core Curriculum Alignments
From the Living Environment Core Curriculum.

STANDARD 4: Students will understand and apply scientific principles and theories pertaining to the physical setting and living environment and recognize the historical development of ideas in science.

Key Idea 2: Organisms inherit genetic information in a variety of ways that result in continuity of structure and function between parents and offspring.

Performance Indicator 2.2: Explain how technology of genetic engineering allows for human to alter genetic makeup of organisms.

2.2a: For thousands of years new varieties of cultivated plants and domestic animals have resulted from selective breeding for particular traits.

2.2b: In recent years new varieties of farm plant and animals have been engineered by manipulating their genetic instructions to produce new characteristics.

Key Idea 3: Individuals and species change over time.

Performance Indictor 3.1: Explain the mechanisms and patterns of evolution.

3.1e: Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life-forms as well as for the molecular and structural similarities observed among the diverse species of living organisms.

3.1f: Species evolve over time. Evolution is the consequence of the Interaction of (1) the potential for a species to increase its numbers… (4) the ensuring selection by the environment of those better able to survive…

3.1j: Behaviors have evolved through natural selection. The broad patterns of behaviors exhibited by organisms are those that have resulted in greater reproductive success.

3.1k: Evolution does not necessitate a long-term progress in some set direction. Evolutionary changes appear to be like the growth of a bush: Some branches survive from the beginning with little or no change, many die out altogether, and others branch repeatedly, sometimes giving rise to more complex organisms.

MEDIA COMPONENTS

Video

NATURE, Holy Cow, selected segments:

Clip 1, “A Cow’s Digestive System”

Clip 2, “Desirable Breeding Traits in Cattle”

Clip 3, “Different Breeds of Cattle”

Access the streaming and downloadable video segments for this lesson at the Video Segments Page.

Web sites

Breeds of Livestock
An Oklahoma State University Web site featuring photos and descriptions of various breeds of cattle from around the world.

World Climate Map
A map of the world showing different climate zones.

Genetic Engineering
A Regents’ preparatory Web site featuring a description of how the meaty English Shorthorn cow was selectively bred with the heat-resistant Brahman cow to produce the Santa Gertrudis, a hybrid which possesses the positive characteristics of both parent breeds.

Punnett Squares
An interactive Web-site which explains how Punnett squares can be used to determine the likelihood that certain traits will be passed on to future generations.

A Dying Breed
A New York Times article which discusses the pros and cons of increasing hybridization by Bahiman cattle ranchers in Uganda of their native Ankole cattle with Holstein cattle from the United States.

MATERIALS

For each student:

• “Traits of Ankole and Holstein Cattle” Student Organizer (PDF)(RTF)

For each group:

• “Man’s Best Friend?” Student Organizer (PDF)(RTF)
• “Cattle Breeding” Student Organizer (PDF)(RTF)
• “Finding the Balance” Student Organizer (PDF)(RTF)

For the class:

• “Traits of Ankole and Holstein Cattle” Student Organizer Answer Key (PDF)(RTF)
• “Man’s Best Friend?” Student Organizer Answer Key (PDF)(RTF)
• “Cattle Breeding” Student Organizer Answer Key (PDF)(RTF)
• “Finding the Balance” Student Organizer Answer Key (PDF)(RTF)
• Computer with Internet access, projector, and screen

PREP FOR TEACHERS

Prior to teaching this lesson, you will need to:

Preview all of the video clips and Web sites used in the lesson.

Download the video clips used in the lesson to your classroom computer, or prepare to watch them using your classroom’s Internet connection.

Bookmark the Web sites used in the lesson on each computer in your classroom. Using a social bookmarking tool such as del.icio.us or diigo (or an online bookmarking utility such as portaportal) will allow you to organize all the links in a central location.

Next: Proceed to Activities.

The polar bear rules the north. To the hardy native people who settled the harsh lands of the Arctic, the powerful hunter with the ghostly white coat is known as the “lonely roamer.” But most of us know the huge mammal as the polar bear. And the story of how the world’s largest land predator prospers in one of Earth’s harshest environments is the subject of the NATURE program Great White Bear.

In extraordinary scenes collected across the top of the world, Great White Bear shows that polar bears are prodigious roamers indeed. Single bears have been known to trek and swim as far as 3,000 miles across icy seas and mossy, treeless tundra in search of food. Typically, however, studies suggest the bears stay within a home range that is just a few hundred square miles. Still, polar bears, by far, range over the largest territories of any bear.

The reason for the huge territories, scientists believe, is the unpredictable availability of their favorite food: ringed seals. As Great White Bear shows, populations of this common, four-foot long arctic seal can build up and melt away mysteriously, much like the ice sheets the sleek swimmers often inhabit. In good years, the bears may not have to travel far to find a seal meal. But in bad years the dark, cream-spotted animals are few and far between.

Though polar bears are excellent swimmers — their scientific name, Ursus maritimus, means “sea bear” — they usually aren’t fast enough to catch a seal in open water. Instead, in winter, the bears creep within striking distance of one of the breathing holes the seals have made in the ice. When a seal pops its head out of the hole to catch a breath, as it must do every ten minutes or so, the bear leaps and yanks the unsuspecting bather out of the water.

Sometimes, however, bears take a more direct route: as Great White Bear shows, they will crash through the top layer of the ice itself in an effort to trap the seals that may be resting in the hollow space below.

Ringed seals are a staple of the polar bear diet. In early summer, when the seals like to bask in the warming sun, hunting bears must use a different strategy. They wait until the seals are asleep and then creep close, freezing in place when the snoozers periodically open an eye to check for danger. Eventually, if the seals don’t spot the stalker, the bear gets close enough to make a powerful pounce.

More often than not, however, the seals escape: some studies have shown that bears outwit their prey less than five percent of the time.

But when the hunting is good, bears can be finicky eaters. They will easily polish off a 100-pound meal consisting of the seal’s energy-packed skin and blubber, or fat layer. But they commonly will leave much of the less nutritious muscle behind. Young bears will sometimes devour the leftovers, but it is often another Arctic inhabitant — an Arctic fox or gull — that rushes in to claim a free meal.

It can take a lot of seals to satisfy a polar bear. Males can be ten feet tall and weigh 1,500 pounds, while females are smaller, topping the scales at about 550 pounds. Remarkably, however, the huge animals can withstand lengthy periods without food. Male bears, for instance, are routinely forced to go without a major meal for three or four months each summer, when melting ice prevents them from hunting seals. And pregnant females apparently go without food for eight months — a record among mammals. Mothers even keep fasting for some weeks after their one-pound cubs, usually twins, are born between late November and January. By the time the cubs have left her care one to three years later, however, the mother has rebuilt her energy stores and is ready for another litter.

To survive their forced diets, polar bears burn a thick layer of reserve fat. In some cases, this layer can be up to five inches thick. But the blubber doesn’t just store energy: in winter, it also provides an excellent insulating blanket in temperatures that can plunge to 40 degrees below zero. Other adaptations, such as a small tail and ears and two layers of specialized heat-trapping fur, also help the bears conserve heat. In addition, each hair of a polar bear’s coat is hollow and transparent, helping to draw the sun’s rays toward the bear’s black skin. But when things get too cold, even polar bears must seek shelter, digging out snow caves in which they curl up like giant furry balls.

In summer, however, that same fat can present a problem. Polar bears can easily overheat when they run — which probably explains why they spend much of their time loping across the landscape at such a leisurely pace. In the southern part of their range, where summer temperatures can rise to levels downright blistering for bears, they may even take to the water simply to stay cool. Such strategies have allowed polar bears, which can live for 30 years, to prosper for at least 200,000 years in a challenging Arctic landscape that is by winter enveloped in icy darkness and by summer bathed by a never-setting sun.

But hunting and environmental changes, such as signs that a warming climate could be melting pack ice, may be taking their toll on the bears. And concern is growing about pollutants, such as mercury and other toxic chemicals, that are making their way into the once pristine Arctic food chain. As a result, scientists are keeping an increasingly close eye on the up to 40,000 bears that live in Canada, Russia, Alaska, Greenland, and Norway. “If a polar bear population is healthy, then one can probably safely assume that the rest of the components of that food chain are doing well,” explains a spokesman for the Canadian Wildlife Service, which has sponsored extensive monitoring studies. “However, if problems develop with polar bear populations, it may indicate problems elsewhere in the ecosystem.”

The goal, scientists involved in such studies say, is to make sure that the Arctic’s great white bear continues to roam across the top of the world.

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 . . .