Ruminating on the Digestive System

• The Herbivore Digestive System Student Worksheet
• Worksheet Answer Key
• Two Strategies of Digestion in Hoofed Mammals Teacher Reference Sheet
• overhead transparency
• dry-erase markers
• colored pencils or markers
• jumbo sidewalk chalk (at least 5 pieces)
• meter stick or measuring tape
• hula hoops or colored ribbon, rope, or towels (optional)
• 1-L bottle (optional)
• Graph Overhead (optional)

1. Ask students: If you held an apple long enough, would your hand be able to soak up all its sugars and nutrients? Nope! Your body needs to digest food, or break it down into bits that are small enough to be used by individual cells.
    
2. Have students discuss with a partner what fruits and vegetables they have eaten recently, and what they think happens to these plants while passing through their body. What plant parts can you eat, and why are they good for you? What about the parts you throw away?
    
3. Introduce the term cellulose, and explain how vertebrate animals (humans included) use bacteria in their gut to digest this complicated sugar molecule.
    
4. Ensure students are comfortable with the terms digestion and absorption.

1. Bring the data chart, chalk, and a meter stick outside for the demonstration. (Colored props to mark the location of bacteria are optional, as is a 1-L bottle to demonstrate volume).
    
2. At one end of the schoolyard, use chalk to write “RUMINANT DIGESTIVE SYSTEM: BUFFALO”. Write the word “Mouth” and draw a circle around it. The mouth is the entrance to the digestive tract, which the class will draw on the blacktop.
    
3. Lay down the meter stick and model how to take one big pace to represent one meter. Draw a thin, tube-like esophagus of this length. Likewise, students will estimate distances using wide paces while others follow at their heels to mark the ground with chalk.
    
4. Build the Buffalo’s digestive system one organ at a time, reviewing the function of each part, by assigning student teams to measure, draw, and label subsequent organs:
   • Stomach: Draw as a large circle or sac.
   • Small Intestine: Because the organ is so long, students show loop back and forth to fit it in a tighter space. A thin tube is appropriate.
   • Cecum: Draw as a circle or sac.
   • Large Intestine: Make a wider width than the small intestine, but with similar curves.
      
5. Repeat with the data for the Zebra to construct a “Non-Ruminant Digestive System”.
    
6. Ask students if they recall where bacteria are housed in each animal. (Place a colored hula hoop or other prop around the Buffalo rumen and Zebra cecum, respectively, to remind students where these important microorganisms help by digesting cellulose.)
    
7. The diagram compares length, but you can use the data chart to highlight any differences in volume, if desired. Use the 1-L bottle to reference size.
    
8. Use the drawings to review function and promote critical thinking. Discussion questions might include:
   • Why does the Buffalo’s stomach hold so much food? needs to store in between ruminating sessions; houses bacteria for digestion; four parts
   • Why does the small intestine need to be so long? absorption only occurs through the wall of the tube, so food in the center would be wasted unless it has time to float in a liquid to the edge
   • How does this contrast with the esophagus? the esophagus is just for transport, like a pipe. No digestion or absorption occurs here, so surface area of the pipe wall is not important
   • Why does the large intestine need to be so long? the animal needs to reabsorb all the water it put into the digestive tract to dissolve food, but needs to do it slowly because blood vessels are narrow
   • How is the Zebra stomach different, and what does this mean for the animal? one compartment; doesn’t store for too long since doesn’t regurgitate; small, so needs to eat many small meals each day; less bacteria here, uses own stomach juice to digest protein but can’t break down cellulose
   • Why is the Zebra cecum more full that that of the Buffalo, even though it’s the same size? this is the site of bacterial digestion for the non-ruminant
   • How might the plant matter in the large intestine of the Zebra be different that that in the Buffalo? more fibrous, since bacteria never completely broke down the sugar
   • What is role of the small intestine? Why is this organ so much longer in the Buffalo? absorption of nutrients; ruminant has more food available to absorb since bacteria finished the work upstream
      
9. Consider having two students race through the tracts, having to stop and spin around at the mouth (x2 for ruminants!), stomach, and cecum. Which route was faster? the zebra tract is faster — no need to return to the mouth; the tubes are shorter; the animal can’t absorb all the plant material, so a large amount exits as roughage

Graph Overhead

Use graphical representations of the data to further develop your students’ analytical skills. Consider giving student groups a version with a blank legend, title, and/or axes: can they guess which animal is which? How do they know? What do they remember from the exercise outside?

Communities and Competition

If zebras, rhinos, and antelopes all eat grass, why don’t they fight over food on the African savanna? Non-ruminants like zebras will eat the lower-quality tips of the grass, which is older and more fibrous. It will pass through their system quick enough. Then gazelles, as ruminants, can eat the younger, low-cellulose shoots that are left exposed. They won’t have to eat nearly as much, but will want their rumen to take advantage of all the nutrients in the plant. So each hoofed animal fills their own niche, or environmental role.

Mutualism

This lesson shows how bacteria serve important purposes outside of the ‘germ’ category. Herbivores could not digest cellulose without them, and the bacteria take advantage of a safe habitat full of constant food. Both the microbes and the animal benefit from these arrangements, so this symbiosis (or “living together”) is an example of mutualism—a relationship between two organisms in which both benefit from the association. What are other classic (or obscure) examples of mutualism? What other roles do bacteria play in an ecosystem?

Belching

As the rumen bacteria digest cellulose, gas is produced. When the bacteria in our small intestine digest vegetables, we also produce gas, and expel it by farting. However, the ruminant digestive tract is so long, that it would be uncomfortable to expel gas out the back, so instead, they belch all day! How does this byproduct contribute to global warming?

Ruminate

Can humans ruminate too? In addition to referring to the act of chewing cud, this word means “to ponder; reflect on over and over again; think long and hard about.” That’s food for thought! What other terms from science have associated meanings in everyday language?

Dung Detective

If you were wandering amid a mixed herd of zebras and buffalo, you might be able to tell apart their dung. How? The zebra, who can’t digest plants completely, will have dung with coarser, undigested grass sticking out. The dung of the buffalo, a ruminant, should be more uniform throughout. What are some other ways to track, glimpse, photograph, or document wildlife? Consider all of the five senses!

The following information is summarized in the Two Strategies of Digestion in Hoofed Mammals chart for teachers. Consider printing out a copy of this Teacher Reference Sheet for yourself to refer to as you read!

The purpose of the digestive system is to break food into particles small enough to pass through the gut to enter the bloodstream, which transports these nutrients to all the cells in the body. The basic components of the vertebrate digestive tract often serve similar purposes in diverse creatures, from fish to mammals. Yet, depending on an animal’s feeding behavior, regions used for the transport, storage, and processing of food have evolved specializations. These organs have adapted to be more efficient when digesting and absorbing nutrients from a particular diet.

This lesson compares the digestive systems of two herbivores feeding together on the fibrous, nutrient-poor grasses of the African savanna: a buffalo and a zebra. Because grass contains less usable energy than other plant parts such as fruits or nuts, these grazers need to spend a relatively large portion of their day chewing and digesting food to obtain sufficient energy. To reach the nutrients available in the body of a plant cell, the cellulose-rich cell wall must be broken. No vertebrate has the necessary chemicals to digest cellulose in order to capture the energy stored in this complex sugar. Instead, the mammalian digestive tract harbors symbiotic microbes that perform the digestive work, releasing simpler, absorbable molecules.

A buffalo, being a ruminant like antelope, sheep, and goats, chews grass only briefly before swallowing it for the first time. This material travels to the rumen (hence “ruminant”), the largest chamber in its four-part stomach. It is here where bacteria specialized to digest cellulose live and reproduce, digesting cellulose as the rumen churns. With the help of the muscle in the second stomach compartment, the ruminant periodically regurgitates wads of coarser material to be re-chewed, physically preparing the tough plant walls for subsequent chemical digestion by bacteria. Ruminants tend to chew their cud while resting rather than while grazing; a dairy cow might devote 6-8 hours of the day ruminating in sporadic 30 minute sessions. Antelopes often ruminate during the hottest parts of the day, when they would rather hang out in the shade. Rechewed cud is swallowed to return to the digestion vat in the rumen. End products move through the third and fourth stomach sections, responsible for further digestion and absorption; note that in many species, 60-75% of the plant matter has been broken down prior to touching the mammal’s own gastric juices! Because ruminants like buffalo have bacteria working upstream, they maximize the amount of nutrients absorbed later in the small intestine.

Zebras, by contrast, are non-ruminants, so plant matter passes through their system in one fell swoop. Their single, relatively small stomach necessitates several small meals a day. Much like in humans, stomach juices begin to digest protein, but the function of the stomach is mostly temporary storage. From there food is pushed to the small intestine, where the majority of starch, protein, fat, and vitamins are digested in a complex mixture of chemicals. However, recall that the zebra cannot digest cellulose. Although the small intestine is the principal organ of absorption, because bacteria have yet to digest the plant cell walls, many nutrients get passed along. This fibrous feed then enters an enlarged cecum, a pouch-like structure connecting the small and large intestines. Notice that the entrance and exit to the cecum is at the top of the organ, so that it resembles a sack rather than a tube. This equivalent to the rumen of a ruminant is the site for bacterial digestion in a non-ruminant. The nutrients from cellulose digestion are absorbed into the zebra’s blood next via the walls of the large intestine. But remember, the role of the large intestine is mostly to reabsorb the water used during digestion to recycle it in the body; this action also packs the indigestible matter into convenient balls to be eliminated.

Overall, the rougher fiber passing through the non-ruminant’s system results in a lower potential for acquiring nutrients; as such, the zebra compensates by eating more grass, and at a quicker rate. For example, a domestic horse weighing 1000 lbs must eat 2.5-3% of its bodyweight daily; that amounts to 25-30 lbs of grass each day! Although it may seem that non-ruminants (zebras) get the short end of the stick, because they are able to digest low-quality, fibrous grasses, they can survive in habitats lacking the higher-quality shoots, roots, and leaves preferred by ruminants.

• Werribee Open Range Zoo Education Service. (no date). Herbivores, Carnivores, and Constrictor Jaws: Teacher Notes. Victoria, Australia: Werribee Open Range Zoo. Retrieved December 15, 2007, fromhttp://www.zoo.org.au/Learning/Programs/VCE/Werribee/Herbivores_Carnivores
• LaShell, B.A.S. (no date). Lecture. Ruminant Digestion and Microbial Fermentation. Durango, Colorado: Fort Lewis College. Retrieved December 15, 2007, from http://faculty.fortlewis.edu/LASHELL_B/Nutr2-Rumdigestion.pdf
• Swinker, Dr. A.M. (2004). 4-H Horse Project. Fort Collins, Colorado: Colorado State University Cooperative Extension. Retrieved December 15, 2007, from http://extension.unh.edu/4H/4HPubs/COHorMan.pdf