Learning objectives

1. Understand the movement of food through the digestive systems of birds, horses, and cats.
2. Understand the basic functions of each component of the digestive system
3. Compare and contrast the digestive systems of birds, horses, and cats. Explain why these differences and similarities exist.

Birds

Birds, class Aves, are a group of mostly flighted, feathered, beaked animals that come in a wide range of sizes, dietary requirements, and ecological purposes. Their digestive systems reflect the incredible variety of their diets. Birds’ diets range from omnivorous (meat and plants), herbivorous (plants), granivorous (grains and seeds), carnivorous (meat only), frugivorous (fruits), piscivorous (fish), and insectivorous (insects). Birds also have a unique constraint on their physiology that we do not see in other animals- they need to fly! This leads to an overall shortened digestive system with an overall emphasis on efficiency and speed. If you can imagine, it is difficult to fly with the added weight of a big meal! Although beyond the scope of this lesson, we see similar consequences of flight in their urinary system as well.

Let’s explore the movement of food through a bird’s very unique and specialized digestive tract!

1. Beak or Bill (Depends on the species)

Bird beaks come in a wide variety of shapes and sizes, having evolved to each species’ specific diets. Birds do not have teeth, but some species may have serrations, especially those with herbivorous diets such as Canada Geese. The serrations do not function as teeth to break down food mechanically, but rather, they help the bird grip and tear at vegetation.

2. Esophagus

The food moves from the beak to the crop through the esophagus.

3. Crop

The crop is unique to birds and serves as a site primarily for food storage. The crop is located at the base of a bird’s long, S-shaped neck. Food from this point can either be regurgitated (sent back up to the beak) or moved to the proventriculus. The crop is absent in some species.

4. Proventriculus

The proventriculus is where chemical digestion takes place. Acid and enzymes break down the food particles before moving to the ventriculus.

5. Ventriculus (Gizzard)

The ventriculus, or gizzard, is the site of mechanical digestion. Strong muscles create crushing pressures that help further break down food. Some species, such as pigeons, may ingest “grit” to aid in mechanical digestion. Grit is generally small, hard objects, such as rocks, that sit in the ventriculus to provide a bit extra punch!

6. Small Intestine

The small intestine continues to work on the enzymatic digestion of food while also absorbing important nutrients, such as protein, sugar, calcium, and phosphorous. Enzymes are sent to the small intestine from the pancreas, and bile acids are sent from the gall bladder (absent in some species). These enzymes and bile acids play an important role in continuing to break down food!

7. Ceca

Laying between the small intestine and large intestine are the very developed ceca! These vary greatly in shape and size among species. In birds, the ceca function in water absorption and bacterial fermentation.

8. Large Intestine

The large intestine in birds includes the colon and the cloaca and opens to the exterior vent. The colon is short and functions in water absorption. The cloaca also serves as the exit point for the urinary and reproductive systems!

Knowledge Check – Birds

1. Explain what impact flight has on the development of bird digestive tracts.
2. Name which parts of the digestive tract vary by species in birds and which parts can be entirely absent.
3. Group the components of the bird digestive system by function, such as water/nutrient absorption and chemical/enzymatic/mechanical digestion.

Horses

Horses, part of the Equidae family, are hooved mammals of various sizes. Horses are herbivores with a uniquely complex digestive system that allows them to ferment plant materials similar to a ruminant. Horses, however, are classified as monogastrics meaning they have one stomach. This sets them apart from ruminants such as cows who have a chambered stomach for the process of fermentation. Instead, horses consume forages such as hay and grasses throughout the day, eating small meals, to push the digesta through their foregut, and into their hind gut where the plant material is fermented by microbes in the cecum. This key distinguishing feature of the equine digestive system plays a critical role in their food digestion as it breaks down their heavily fibrous diet that would otherwise be indigestible. Through this fermentation process, the horse is able to extract crucial energy and nutrients from the forage that cannot be obtained from previous steps in the digestive tract. This leads to a shortened foregut, with an elongated hindgut to maximize nutrient absorption for the horse.

1. Oral Cavity

Horses grasp their food with their lips and cut grass with their incisors. Once the food is in the horse’s mouth, they grind their food on their molars with a sweeping side to side motion. The equine has hypsodont teeth, meaning that they continuously erupt. They appear as long pillars on x-ray, but as they are worn down through the grinding process, they appear shorter and shorter throughout the horse’s life. This process mechanically breaks down the feedstuff before it enters the stomach.

2. Esophagus

When the horse swallows, strong muscles force the feedstuff down the esophagus to the stomach. This process of the muscles contracting is called peristalsis. Horses are unique in that their peristalsis function prohibits feedstuff from traveling up the esophagus. This, in tandem with a tight valve at the stomach, prevent horses from being able to vomit. Once the food travels down, it will not come back up. Therefore, if a horse eats something that does not sit well in the stomach, they can colic as it cannot come back up, but must continue through the system. Colic is a term referring to extreme abdominal pain. In horses this can become deadly as the digestive tract can twist causing a blockage.

3. Stomach

The horse’s stomach is small in comparison to their size. Because horses are grazers, constantly consuming small amounts of forages, the feedstuff moves through the stomach quickly so they can spend the majority of their time breaking down in the hindgut. The feedstuff is enzymatically broken down in the stomach while making its way to the small intestine. The horse’s stomach is designed to have a continuous flow of small amounts of ingesta to it for the stomach acid to begin breaking down the food, therefore, if a horse’s stomach is empty for too long, the acid within the stomach can cause irritation to the stomach lining and eat away at it causing ulcers. Ulcers are painful blemishes to the stomach lining.

4. Small intestine

The feedstuff enters the small intestine after the stomach. The small intestine is composed of three sections. The duodenum, jejunum, and the ilium. Unlike most other mammals, the horse lacks a gall bladder. The function of the gall bladder is to store bile, a fluid like compound that assists in the digestion of fats. Due to the lack of gall bladder, the bile produced by the liver is constantly secreted into the duodenum of the small intestine a little at a time. In the small intestine, simple sugars, minerals, fat, fat soluble vitamins, and amino acids are broken down enyzmatically and absorpbed. The muscles of the small intestine contract to move food along, just as in the esophagus this movement is called peristalsis.

5. Large intestine (Cecum, large colon, small colon, rectum):

The large intestine is the star of the show when it comes to the equine digestive system. Now that the food has exited the foregut, it enters the hindgut into the cecum. The cecum is the site of fermentation for horses. The feedstuff moves into the cecum from the ilium of the small intestine to be broken down by microorganisms through the chemical fermentation process. The microbes take care of the fibrous plant stuff the horse consumes to create byproducts essential for the horse such as volatile fatty acids which provide a large amount of energy to the horse.

Once the feedtstuffs has been fermented in the cecum it continues it’s journey through the large colon and small colon. The large colon continues the fermentation process to maximize nutrient absorption. In the small colon, the horse absorbs much of the water content from the digestive process and forms the undigested feedstuff into fecal balls. The fecal matter then moves to the rectum and is expelled through the anus.

Knowledge Check – Horses

1. Explain the function of horse’s teeth and how they are different from many mammals.
2. List the flow of ingesta through the digestive tract of the horse and what roles each part plays such as water and nutrient absorption, and mechanical, enzymatic, and chemical digestion.
3. Identify how the horse is able to ferment the food they consume despite their monogastic stomach.

Cats

Cats, part of the family Felidae, are fur covered mammals with sharp teeth and an uncanny ability to destroy furniture. Cats are obligate carnivores, meaning their diet consists primarily of meat and they are unable to properly digest vegetation. Cats only have one stomach because they do not need to ferment any of their food, unlike horses and other herbivores. The digestive system consists of shorter intestines in cats due to their carnivorous diet. Meat is digested more rapidly through the gastrointestinal tract than vegetation. Another defining feature of the cat is that they have a small cecum, which is a part of the large intestine. They have no use for fermentation and as a result the cecum has become a small blind pouch on the ascending portion of the colon.

1. Oral Cavity:

Cats have brachydont teeth, meaning short teeth with long roots, like humans. They catch prey with their canines and incisors and chew their food with their molars.

Cats also have spikes on their tongues to help break up their food and move it to the back of their mouths so they can swallow it.

2. Esophagus:

The esophagus contracts and move the food down into the stomach after swallowing, also known as peristalsis. It can also move material up and out of the oral cavity when vomiting, known as reverse peristalsis, which is helpful when dealing with hairballs.

2. Stomach:

Cats have a gastric stomach that digests food by secreting enzymes and stomach acid to break down the food.

3. Small Intestine:

Consists of: Duodenum, Jejunum, Ileum

The small intestines are very short in length

The small intestines absorb nutrients from the digested material after it leaves the stomach. They also play a role in the absorption of water.

4. Large intestine:

Consists of: Colon, Cecum*, Rectum

The cecum is a small blind pouch and it is not actually a part of the flow of digested material through the gastrointestinal tract.*

The large intestines are also very short in length. The primary function of the large intestine is to absorb water and excretes feces.

Knowledge Check – Cats

1. List the flow of ingesta (food) in order through the gastrointestinal tract.
2. How does the oral cavity break down food?
3. What are the roles of the small and large intestines?

Assessment

1. What is a function of ceca in birds?
   1. Water absorption
   2. Mechanical digestion
   3. Hormone production
   4. Bile acid excretion
2. Identify the correct order of digestive system components in birds
   1. Esophagus, Small intestine, Ventriculus
   2. Crop, Proventriculus, Ventriculus
   3. Small intestine, Cloaca, Large intestine
   4. Ventriculus, Proventriculus, Large intestine
3. (T/F) Birds have long gastrointestinal tracts to ensure thorough digestion. This helps them fly.
4. “Gizzard” is another word for ____ and its function is _____.
   1. Proventriculus, chemical digestion
   2. Large intestine, water absorption
   3. Ceca, bacterial fermentation
   4. Ventriculus, mechanical digestion
5. Horses ferment their food in which part of the digestive tract?
   1. Stomach
   2. Duodenum
   3. Small intestine
   4. Cecum
6. What can happen if a horse ingests something that doesn’t sit well in the digestive tract?
   1. Eat more to help it pass
   2. Colic
   3. They use their special hind gut fermentation
   4. Vomit
7. (T/F) The forage horses consume, spends most of it’s time in the small intestine fermenting for vital energy and nutrient absorption.
8. Cats are ______.
   1. Obligate carnivores
   2. Hind-gut fermenters
   3. Herbivores
   4. Omnivores
9. Cats have short intestines because…?
   1. They are small animals
   2. They don’t eat large amounts of food
   3. They eat primarily meat
   4. They ferment their food
10. (T/F) Cats cannot vomit.