Multiple GI Tracts? Jeez!

Instructional Guide

Lesson Objective

Students will be able to learn the key differences between monogastric, ruminant, and avian digestive tracts.

Intended Grade Level

This lesson is intended to introduce college-level Animal Physiology students to three different digestive systems present among animal species. Before beginning the lesson, students should be familiar with the cellular processes occurring during digestion. Students should know about the organ systems and their general functions. Students should also know which species are categorized under ruminant and monogastric and how diets differ between monogastric, ruminant, and avian species.

Learning objectives

• Students will compare and contrast monogastric, ruminant, and avian digestive tracts.
• Students will  identify why different digestive systems are advantageous for different species.
• Students will explain which digestive systems correspond to different diets and why.

Lesson Format

This lesson will take approximately 45-60 minutes for students and is intended to be completed in groups of 3 or 4. Begin the class by giving a brief overview of the lesson instructions. An overview slide is provided to project in front of the class.

Overview Slide

Overview:

• Students will break into groups of three. If the number of students is not divisible by three, groups of four are permissible. Each student will oversee learning one of the following digestive tracts: monogastric, ruminant, or avian. In groups of four, two students should collaborate on the avian digestive system. Students should read and take notes on their designated section. (15 minutes)
• Students will share in their groups about their learnings from the reading and research on their respective Gastrointestinal systems. (15 minutes)
• Once the information has been shared amongst the group, students should work with their group to answer the quiz questions at the end of the chapter. (20 minutes)
• For students that want supplemental learning material, links to YouTube videos on each digestive tract can be found at the end of the lesson.
• Optional: Students can label and color different GI tracts to improve their understanding of monogastric, ruminant, and avian digestive systems.

After explaining the structure of the class, students can break into groups and begin working. Using an online timer or alarm on your phone to monitor when students should be switching sections is recommended.

Materials needed:

• Computer or iPad to access the online chapter
• Note taking materials – pencil and paper or an ipad
• Timer on computer or phone
• Printer to print out coloring pages
• Color pens, crayons, or markers

Lesson Background

Instructors should understand the differences in the anatomy and physiology of monogastric, ruminant, and avian digestive systems.

For monogastrics, the instructor should understand where and how carbohydrates, lipids, and fats are digested. They should have a working knowledge of the variations in the anatomy of the GI tract in different animals and why those differences exist. They should understand the process of breaking down protein in the single-chambered stomach. This includes knowledge of chief and parietal cells and the conversion of pepsinogen into its active form, pepsin. They should be able to explain that this is done as a safety mechanism in the stomach, as it stops pepsin from digesting the stomach itself when there is no protein to be digested. In terms of the large intestine, a deep understanding of hindgut fermentation is necessary. The presence and function of the cecum needs to be understood as well as why it is an essential feature for hindgut fermenters (horses, rabbits, etc.) and why it is reduced or absent in non-hindgut fermenters (cats, dogs, pigs, etc.).

For ruminants, the instructor should know the basic flow of feed through the digestive tract and how rumination works. They should understand the process of breaking down feed and forages in each chamber of the stomach and why the anatomy of these stomachs is best suited for each process. The instructor should know the differences between a pseudo ruminant and a true ruminant and be able to describe the differences. The instructor should know the difference in GI tracts of a young ruminant to a grown ruminant. Understanding why ruminants are designed the way they relate to feed availability, predation, and the use of these animals related to production is needed to help students differentiate them from other animals.

For the avian section, instructors should have a basic understanding of how and why digestive systems are different depending on the bird’s diet. They should understand the flow of food throughout the digestive tract and the functions of each organ, particularly the ones that differ from monogastric and ruminant species including the crop, ventriculus, proventriculus, and cloaca. The instructor should feel comfortable explaining why birds’s digestive systems are designed the way that they are. Many differences in the avian digestive system, such as the beak, crop, shorter digestive tract, and cloaca are linked to flying. Supplemental videos have been provided at the bottom of the instructor and student page to provide further understanding if needed.

Activities

Student Page

Common misconceptions and challenge points

For monogastrics, the reasoning and function of the cecum in hindgut fermenters may present a challenge to some students. Not all monogastrics have a functioning cecum, so they must understand the reasoning for a cecum in some monogastrics.

It may also be challenging for students to understand the differences between hindgut fermenters and ruminants. Fermentation takes place in both, but the difference between where and how this happens should be understood.

The differences in diets may also be difficult to grasp, as monogastrics span across carnivores, omnivores, and herbivores. Based on these diets, the monogastric digestive tract will differ with anatomy, which may also be confusing for students. There is not one defined monogastric digestive tract; most monogastrics differ in one way or another. Therefore students may have a hard time grasping the nuanced differences between monogastric anatomies.

A misconception that pseudo ruminants are the same as ruminants may appear. While both have similar digestive tracts, pseudo ruminants have one less chamber and a larger cecum used for greater absorption and digestion.

The main challenge point for the avian digestive system is that it is highly variable depending on the species of bird. Birds can lack certain digestive structures or have structurally different organs. Examples include the paired ceca, proventriculus, and ventriculus. Please remind students that this is an introductory lesson to the differences between different digestive systems, and the goal of the avian section is to understand the general digestive system of birds rather than specifics.

Another challenge students may encounter is determining where the “true” stomach is for monogastric, ruminant, and avian species. For ruminants, the “true stomach” is at the last chamber (abomasum) of the stomach, whereas in avian species, it is at the first chamber (ventriculus) of the stomach. Monogastric species only have one chamber in their stomach, which is the “true stomach” for them.

Assessment

Monogastric Key

True or False: All monogastrics have salivary amylase in their saliva.

True

What is prehension?

The acquiring of food into the mouth via the lips, teeth, and tongue.

Out of dogs, cats, pigs, and horses, which would be most likely to consume mostly hays and grasses as its primary food source? Why?

Horses, because they are hindgut fermenters, meaning they have an enlarged cecum. The cecum houses millions of microbes that can break down the cellulose of the structural fibers consumed by hindgut fermenters. Other monogastrics have reduced or absent cecums and do not have the microbes needed to break down cellulose into energy.

Which monogastric has a spiral colon? Why?

Pigs have a spiral colon. This allows them to digest some of the cellulose they consume.

What must fats be mixed with in order to be broken down and digested?

Bile from the gallbladder.

What is the function of villi?

To increase the surface area of absorption in the small intestine.

Describe the enzymatic activity that occurs in the stomach.

Pepsinogen is secreted by chief cells. This is the inactive form of the enzyme. Parietal cells secrete hydrochloric acid. This then cleaves pepsinogen into its active form, pepsin. Pepsin is the enzyme that can then break down proteins in the stomach.

Match the correct cell to what it produces in the stomach:

• Chief cells – hydrochloric acid
• Parietal cells – pepsinogen

• Parietal cells – pepsin

• Chief cells – pepsinogen

Ruminants Key

True or False: Ruminants have a hard pallet and lack bottom incisors.

False. Ruminants have a hard pallet and lack TOP incisors.

Explain the process of chewing cud.

Chewing cud starts with the ruminant chewing forage to around 1-1.5” and saliva is mixed to help digestion in the rumen. Then it is swallowed and goes to the rumen. When a ruminant is comfortable and not activity eating or drinking, the esophagus then brings chewed food – cud – back to the mouth. The ruminant grinds down the feed into small pieces and is mixed with saliva again. The cud is then swallowed again, and the small pieces go into the reticulum for further digestions while the larger pieces go back into the rumen to follow the cycle again.

What enzymes are found in the saliva of a ruminant that helps aid digestion?

Lipase and Amylase

Name the order of the chambers of the ruminant’s stomach.

Rumen » Reticulum » Omasum » Abomasum

Which chamber is also known as the true stomach of a ruminant as it is most similar to a monogastric?

Abomasum

Why does milk bypass the rumen in young ruminants?

The microbes in the rumen would ferment the milk causing it to curdle and can cause colic.

Explain the differences and similarities between a ruminant and a pseudo-ruminant.

Pseudo-ruminants have a three chamber stomach unlike ruminants which have a four chamber stomach. They both regurgitate their feed and chew their cud when relaxed and not actively eating or drinking.

Name three species that are ruminants.

Cattle, sheep, goats

Avian Key

What avian stomach chamber facilitates secretory digestion?

The proventriculus.

What is the expansion of the esophagus that acts as storage for birds?

The crop.

What is a mute and why do birds have this?

A mute is the combination of fecal, urine, uric, and reproductive waste of the bird. Birds excrete the mute because they only have one opening (the cloaca) to excrete waste products.

What is egestion/casting?

Egestion/casting is the process of expelling nondigestible material, including fur and bones, through the oral cavity.

Which chamber of the avian stomach is non-digestible material (bones, fur, etc) held in before the initial process of egestion/casting begins? 

The ventriculus/gizzard.

Are avian small intestines typically longer or shorter than mammals? Why?

The small intestine of avians are typically shorter since it allows them to be lighter for flight. 

What two features of the esophagus allow birds to swallow food that isn’t chewed more easily? 

Longitudinal folds in the esophagus and mucus from mucosal glands.

What type of digestion occurs in the ventriculus/gizzard?

Mechanical digestion.

Where is the majority of dietary fiber broken down in avian species?

The ceca.

Coloring Pages (Answers)

Avian Digestive System Coloring Page (Answers)

Llama Ruminant Coloring Page (Answers)

Cow Ruminant Coloring Page (Answers)

Dog Monogastric Coloring Page (Answers)

Pig Monogastric Coloring Page (Answers)

Further exploration 

Videos provided on student page

• monogastric vs ruminant digestion
• Digestive system (humans)– with references to the platypus
• Virtual chicken
• Bird digestion- adaptations for flight

Image Use Disclaimer 

Diagrams created by chapter authors and are CC for NC.