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Revealing marsupials: a scientist discovers diversity in a group of South American animals.

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Marsupials are famous for their unusual baby buggies: Mom frequently totes her infants around in a built-in pouch on her belly called a marsupium. In this group of animals, young are born extremely underdeveloped and finish growing outside the mother's body.

When people think of marsupials, they usually envision kangaroos or koalas--animals that inhabit Australia. But many marsupials live in rain forests in South America. Scientists now think that marsupials originated there, and over time spread to Australia (see map, p. 15).

Despite marsupials' long history in South America, relatively little is known about the species there. Robert Voss, a biologist at the American Museum of Natural History in New York City, is searching the rain forests for clues about these unusual animals.

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BABY ON BOARD

Like humans, marsupials are mammals that have four limbs and hair, and nurse their young with milk. However, marsupials have a very short gestation period, or length of time offspring spend developing inside the mother's body. As a result, marsupial babies are tiny at birth. "A baby opossum is about the size of a grain of rice," says Voss.

A newborn marsupial is hairless, blind, and helpless. Only its hands and forelimbs are well-developed. That's important because right after it is born, a baby marsupial will use them to climb up its mother's body to the safe spot on her belly where it will nurse and grow until it can live on its own. For marsupials like kangaroos, this spot is inside a pouch. But not all marsupials have a pouch. "In some cases, the babies just hang from the mother," says Voss.

DIFFERENT VARIETIES

There is a lot of diversity among the world's roughly 300 species of marsupials. Much of this variation is found among Australian species. That continent is home not only to koalas and human-size kangaroos, but also to marsupial anteaters and squirrel-size sugar gliders that soar through the air on "wings" of skin supported by their fore--and hind-limbs.

In comparison, South America's marsupial species seem much less diverse to the untrained eye. Many look similar to the North American opossum, with prehensile tails that are adapted to grasp branches. Most of them are semi-arboreal, or live partly in trees and partly on the ground, and eat a diet of fruit and small animals. But a closer look at these species shows they have more variation than was first thought. "People have underestimated the diversity in a lot of different groups of South American marsupials," Voss says.

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SOUTHERN DISCOVERIES

South America is home to some ecologically specialized species, including the water opossum (Chironectes minimus)--the world's only semiaquatic marsupial. It has webbed hind feet, water-repellent fur, and a pouch that closes when it goes underwater.

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Equally unique is a group of opossums in the family Didelphidae that has a unique ability to eat poisonous snakes without harming itself. Voss and his colleagues found that a protein that's important for blood clotting in these animals has undergone unusually rapid evolution. As a result, they might be more resistant to the venom of the snakes they eat than other animals would be.

Voss's work is helping to identify the diversity among South American opossum species. "We are finding they each have special things they can do," he says. But there is yet a lot to learn: "We are still at the basic stage of biologic exploration."

MARSUPIAL ORIGINS

Scientific evidence suggests that marsupials spread from South America to Australia 40 to 80 million years ago, when South America, Australia, and Antarctica were part of a single landmass known as Gondwana (see map, below). The continents slowly moved to their current locations. Now marsupials thrive in Australia, are gone from Antarctica, and still survive their in South America.

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check it out

The ancestors of the only North American marsupial--the Virginia opossum--hailed from South America, moving north about 3 million years ago after the Panama land bridge formed. Look for possums in your backyard, perhaps hanging by their hairless tails or "playing possum" (playing dead). And among the coyote and bison dioramas of the Hall of North American Mammals at the American Museum of Natural History, find the diorama where a gray fox and an opossum are eating persimmons. Learn more about opossums and the mammals of our continent by asking your teacher or visiting www.amnh.org.

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For more Science Explorations content, visit: www.scholastic.com/scienceworld

PRE-READING PROMPTS:

* What makes marsupials different from other mammals?

* Are any marsupials living in your area?

* What is diversity? Can there be diversity within a species?

DID YOU KNOW?

* You may think that the word "joey" refers only to baby kangaroos. But this word is used to describe all infant marsupials--from koalas to opossums.

* The only marsupial species found in North America is the Virginia opossum. Its gestational period is 13 days. When born, the Virginia opossum is only about 3 millimeters (0.112 inches) long. The opossum is the only nonprimate with opposable thumbs, which are located on their hind feet.

* Most marsupials are herbivores or onmivores. A few are carnivores, including the Tasmanian devil. These creatures don't really spin around in a tornadolike rage like the famous cartoon character. Instead, an angry Tasmanian devil will bare its teeth and growl.

CRITICAL THINKING:

The red kangaroo in Australia has a gestational period of 30 days. The joey then stays in its mother's pouch for another 235 days! What life skills does an animal need to be self-sufficient? What skills would you need to be self-sufficient? Could you survive alone at your current age?

CROSS-CURRICULAR CONNECTIONS:

GEOGRAPHY: Marsupials can be found in countries throughout North America, South America, Central America, and Australia. Have students select a family of marsupials to investigate. They should include in their report any varieties within the family and species, where the species are located and why they are well-suited to their ecosystem.

RESOURCES

You can access these Web links at www.scholastic.com/scienceworld.

* For fact sheets on a variety of marsupials visit: www.marsupialsociety.org.

* Learn more about North America's only marsupial, the Virginia opossum, here: http://opossum.org.

* Still crave more marsupials? Check out Ronald Nowak's book Walker's Marsupials of the World, Johns Hopkins University Press, 2005. It's full of marsupial information and images.

DIRECTIONS: Circle the incorrect word or phrase and write the correct word or phrase above it.

1. Marsupials have four limbs and hair on their bodies, and they nurse their young and have a long gestation period.

2. The only parts of a baby marsupial that are well developed are its eyes so it can make its way to its mother's pouch or safe spot on her belly where it will nurse and grow.

3. Most of the diversity within the 300 species of marsupials can be found in South America.

4. Many marsupials are semi-arboreal, which means they live exclusively in trees.

5. A group of opossums in the Didelphidae family have developed a special blood protein that makes the opossum especially susceptible to the venom of poisonous snakes.

Answer

1. long/short

2. eyes/hands and forelimbs

3. South America/Australia

4. exclusively/partly in trees and partly on the ground

5. especially susceptible/more resistant

CRITICAL-THINKING SKILLS

What Do You Want to Know?

Before you read "Revealing Marsupials" (p. 14), answer the questions below. Put your answers in the column labeled "Pre-Reading Answers." Once you have finished reading the article, answer the questions again. This time, write your answers in the column labeled "Post-Reading Answers" to see how much you learned.
Question                    Pre-Reading Answers    Post-Reading Answers

1. What is a marsupial?

2. Name three marsupials.

3. Where in the world can
you find marsupials?

4. Roughly how many
species of marsupials
are there?

5. Do you think there is
great diversity among
marsupials?

6. Do you think any
marsupials are
semiaquatic, meaning
they spend their lives
in water as well as on
land?


Conclusions

1. Were your answers different after you read the article? How?

2. What were some things that you learned about marsupials?

3. List three more things that you would like to learn about marsupials. Research them on the Internet or in the library to find the answers.

Answers

Pre-reading: Answers will vary.

Post-reading:

1. A marsupial is a mammal whose young is born extremely underdeveloped and finishes growing outside the mother's body. Many, but not all, marsupials have marsupium, or pouches,

2. Answers will vary, but may include kangaroos, koalas, Virginia opossum, water opossum, and arboreal woolly opossum.

3. You can find marsupials in Australia, South America, and North America. (As mentioned in the lesson plan on TE 3, marsupials are also found in Central America.)

4. There are roughly 300 species of marsupials.

5. There is great diversity among marsupials. They range from the large, hopping kangaroos to the opossums in the family Didelphidae, which are resistant to snake venom.

6. The water opossum is the only marsupial that is semiaquatic.

Conclusions: Answers will vary.
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Article Details
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Title Annotation:LIFE: MARSUPIALS
Author:Norlander, Britt
Publication:Science World
Geographic Code:30SOU
Date:Dec 7, 2009
Words:1511
Previous Article:Hands-on science (no lab required).
Next Article:You asked: what is plaque?
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