The rabbit, as an herbivore, is uniquely designed to consume large amounts of plant material. The plants that rabbits eat are high in fiber, which is indigestible to mammalian digestive enzymes. This means that humans and many other animals cannot utilize the nutrients found in these plants. However, the rabbit’s digestive system makes it able to consume these plants and make the most of their nutrients.
Understanding your rabbit’s digestive, or gastrointestinal, system will help to choose the best diet for your pet. Because rabbits consume fibrous plant materials, they must eat a lot to meet their nutrient needs. However, their gastrointestinal tract is small compared to other herbivores, such as horses and cattle. So to accommodate large amounts of plants, food moves through the tract relatively quick. Their digestive system is designed to make the most efficient use of the nutrients found in their diet.
Functions of the digestive system:
It is important to consider the roles of the digestive system before looking into its parts in more detail. The primary function of the digestive system is to digest food and absorb nutrients. Digestion is the process of breaking food components into small enough units that they can be absorbed across the digestive tract wall. This provides nutrients for the animal to use for daily needs or for production, such as during growth, pregnancy, and lactation.
Another function of the digestive tract is protection from toxins, or bacteria, in the food. The environment in the stomach kills many bacteria and enzymes (proteins that break down large molecules in foods to smaller particles) and acid may destroy many toxins. In addition, the immune system is closely linked to the digestive system to prevent infections. The digestive tract plays an important role in waste removal from the body, as well.
Parts of the digestive system:
The first part of a rabbit's digestive system is the mouth. The rabbit uses its lips to grab food and pass it back to the teeth to cut and grind the plant material. Because this plant material can be coarse and abrasive, the rabbit’s teeth grow continuously to account for tooth wear. If fed a diet with too little fiber or plant material, the rabbit’s teeth will continue to grow without being worn and may become overgrown or develop malocclusion. This is a serious health concern in rabbits and may require veterinary treatment. For more information, see Dental Problems in Rabbits.
A rabbit has 16 deciduous (baby) teeth and 28 permanent teeth. Its adult teeth consist of four upper and two lower incisors (front teeth) as well as 22 total premolars and molars (back teeth). You may notice only two upper incisors when looking at your rabbit’s teeth and this is because two of the upper incisors are smaller and behind the other pair. These incisors are called the peg teeth. Incisors function to tear and grab the food.
Premolars and molars are the back teeth, sometimes called the cheek teeth. The rabbit has three upper premolars and two lower, along with three upper and three lower molars. Their primary function is to grind the food to a smaller particle size to allow for the food to be swallowed.
In addition, rabbits utilize saliva secreted into the mouth to moisten the food to help with lubrication and movement through the gastrointestinal tract.
Once food is swallowed, it passes through the esophagus. The esophagus is essentially a tube that transfers food from the mouth to the stomach.
Rabbits have a relatively large stomach to allow for holding of large meals because they are crepuscular, meaning they eat primarily at dawn and dusk. Once food is in the stomach, it begins to be broken down through hydrolytic and enzymatic digestion, which means acid and enzymes are used to break down the compounds to a smaller size.
The primary secretions of the stomach include mucus, hydrochloric acid, and pepsin. Mucus is important to protect the stomach lining from the acid and enzymes and to help moisten the food. Hydrochloric acid is important in decreasing the pH of the stomach to allow enzymes to work, and it kills or inhibits bacteria found in the food. Pepsin is a proteolytic enzyme, which breaks down proteins.
The muscles of the stomach churn or mix the food with stomach secretions. At this point the mixture is called chyme, or digesta.
The chyme passes into the small intestine from the stomach. Flow into the small intestine is regulated by the pyloric sphincter. The small intestine is the place where the majority of digestion and absorption of nutrients occurs. The small intestine can be divided into three sections – duodenum, jejunum, and ileum.
The duodenum is the first section and is the site of most digestion. Buffers are secreted from the pancreas into the small intestine to increase the pH to a more neutral level, as the stomach acids would make the digested food have a low pH level. In addition, a variety of enzymes are secreted by and into the small intestine to break down the food. These include proteolytic, lipolytic (fat digesting), and amylolytic (starch digesting) enzymes.
The jejunum is the middle section of the small intestine. Many nutrients, such as amino acids, fatty acids, and glucose, are absorbed here. The last section of the small intestine is the ileum. In the ileum, the remaining digested nutrients are absorbed, as well as the B vitamins.
Large intestine and cecum
The final sections of the digestive system are the large intestine and cecum. The cecum is a blind sac, or pouch, that comes off the junction of the small and large intestines. It is located where the appendix is in humans. The large intestine, or colon, is the section that continues from the small intestine to the anus of the animal. Any undigested food and all the fiber from the rabbit's diet will pass from the small intestine to the large intestine. At this point, the rabbit’s digestive system is able to sort the material into two portions – that which can be further broken down and used and that which cannot.
The material that cannot be broken down any further, such as indigestible fiber, passes directly into the large intestine. Here water is reabsorbed and the material is passed, making up the round droppings you see in your rabbit’s cage. This indigestible fiber is important in the diet of the rabbit as it helps to stimulate intestinal contractions, which keeps the chyme moving through the gastrointestinal tract. Rabbits that do not consume enough fiber from hay are more likely to develop gastrointestinal blockages, or stasis.
The material that can be broken down further, primarily soluble fiber and proteins, moves into the cecum, a large blind sac. The cecum may be the most important part of the digestive system of the rabbit. The cecum has 10 times the capacity of the stomach of the rabbit. Here massive quantities of bacteria and microorganisms reside. These bacteria ferment, or digest, the material that passes into the cecum and use it to produce their own cells, proteins, and vitamins. The bacteria turn the indigestible fiber into digestible nutrients - some of which are directly absorbed across the wall of the cecum for use by the animal, while others are excreted.
About eight hours after a meal, the material from the cecum is packaged into a small moist pellet called a cecotrope. When the rabbit is ready to pass the cecotrope, a signal is sent to the rabbit’s brain causing the cecotrope to be consumed by the rabbit as it is being expelled. This allows the material to go through the entire digestive system again and let the rabbit get additional nutrients from the plant material. This process, called cecotrophy, allows rabbits to utilize high-fiber plant material that other animals may not be able to.
For proper digestive health, a rabbit’s diet should be designed with its unique gastrointestinal system in mind. The diet should be high in fibrous materials to provide for proper dental health, as well as to ensure movement through the digestive system and for fermentation in the cecum to occur to produce cecotropes. This fiber should come from high-quality plant materials to allow for sufficient nutrient utilization as the passage rate in the rabbit is relatively rapid. Choosing a high-quality pelleted diet and hay will be optimal for proper gastrointestinal health in the rabbit.
Lisa Karr-Lilienthal, Ph.D.- University of Nebraska-Lincoln