Assignment 12: (continued)

Introduction to nutrient acquisition
Herbivorous and carnivorous fish
  Feeding strategies of rotifers
  Termites and symbiotic relationships
  Intracellular and extracellular digestion
  The digestive tracts of larval and adult frogs
  The digestive tracts of rabbit and cat
  The digestive tract of ruminant mammals


The digestive tracts of larval and adult frogs

One trend among related species is lengthening of the small intestine in herbivores relative to carnivores to allow more time for processing of plant material. This trend is more apparent when there is a change in diet from herbivory to carnivory within an individual animal. For example, we can compare the length of the small intestine in bullfrog tadpoles which are herbivores to adults which are carnivores. Examine the following photograph in which the small intestine that has been carefully removed from a frog tadpole.

The small intestine of a bullfrog tadpole
This tadpole has been magnified to 2x its size to allow you to measure its intestine. 

Use a piece of thread (because it is flexible) to measure the small intestine in the photograph. When your thread is the same length as the intestine, measure the thread length length with a ruler. Also measure the length of the tadpole body (snout to where body ends and tail begins) with a ruler.  Record your measurements in the table on your work sheet, and determine the ratio of small intestinal length to body length. Because the original photograph was magnified, you must divide both measurements by two. Capture an image of the completed table for question 10.

The digestive tracts of rabbit and cat

We can also compare the digestive tracts of herbivorous vs. carnivorous mammals to observe differences in intestinal morphology.

Rabbits are herbivores that digest grass by "hind gut fermentation". They have a very large blind sac called a cecum is located where the small intestine and the large intestine join together. The cecum houses microorganisms that can digest cellulose. When the food is swallowed it enters the stomach where it is mixed with stomach acid and digestive enzymes. It then moves out of the stomach into the small intestine where some nutrients are absorbed into the body. It continues into the large intestine where the food particles are sorted by size. The larger particles of indigestible fiber push the smaller fragments of digestible fiber backwards into the cecum. The indigestible particles are then passed out as dry fecal pellets. Meanwhile in the cecum, fermentation by the microbes begins the process that will produce what is commonly referred to as night feces or cecotropes. These moist pellets will be re-ingested by the rabbit as they leave the anus; they are about 56% bacteria by dry weight, largely accounting for the pellets containing about 24% protein. Thus, plant material that is difficult to digest passes through the rabbit system twice.

pellets Note the different appearance of the two types of rabbit fecal pellets.

There are many other mammalian herbivores that are considered hind gut fermenters and house bacteria in a cecum or within an expanded portion of the large intestine (for example, the horse).  However, these mammals do not produce cecotropes.     

Now that you understand the digestive process in the herbivorous rabbit, compare its digestive tract to that of the carnivorous cat in these dissected specimens. Then answer question 11 on your work sheet.

The digestive tract of ruminant mammals

Not all herbivorous mammals house cellulose-digesting bacteria in the hindgut. Ruminants (such as cattle, goats and sheep) utilize the stomach for this purpose. Ruminants are so named because they possess a greatly enlarged stomach chamber called the rumen which houses symbiotic bacteria. The stomach contains three additional chambers as well. Chewed food enters the rumen where the microbes begin digesting it. The larger particles are periodically concentrated within the reticulum and regurgitated for further chewing (cows chew their cud). The food is again swallowed and exposed to the action of the microbes, and the cycle is repeated until all large particles have been broken down. Once this has been accomplished the food is passed into the third and fourth stomach chambers (omasum and abomasum) where digestion is continued. View the following video to see the rich diversity of protists that live within the rumen contents of a cow. The numerous symbiotic bacteria are too small to be easily seen.

video of rumen contents

Digestive system of a domestic ruminant

Study the above diagram of the cow digestive tract, then search the Internet for the image of a wild animal that has a similar type of stomach. Download and label your image of the animal's digestive tract and submit it for question 12.