Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Digestive system of the Beaver (Castor canadensis)

Photos and text by Jason Lavender and Wei-Chung Chen

Overview of Digestive System:

The main component of the beaver’s diet is wood. Wood is not only difficult to chew, it is difficult to digest, and the beaver’s digestive system reflects this fact. Digestion begins in the mouth, where the first mechanical and chemical breakdown occurs through chewing and enzyme breakdown, respectively. Saliva contains many of the enzymes that begin the digestion process. The beaver salivary gland is very large, occupying an expansive region in the upper thorax, neck, and up to the caudal border of the mandible. By chewing, the beaver breaks down wood into small splinters and particles that travel down the esophagus and enter into the stomach through the cardiac sphincter. The stomach serves as the main storage chamber for ingested food, and is the first major site of digestion. Different portions of the stomach secrete different enzymes in order to maintain the correct pH. The fundus region, closest to the cardiac sphincter, secretes mucus to protect the stomach from acidic and corrosive enzymes. The antrum portion of the stomach secretes enzymes and acid that help break down hard particles such as wood. The lesser curvature of the stomach has a peculiar gland patch, the cardiac gland, which secretes enzymes into the stomach. This gland also assists in digestion of high fiber compounds [1].After larger, more solid particles are somewhat digested, they enter the small intestine via the pyloric sphincter. The small intestine is divided into three portions: the duodenum, jejunum, and the ileum. The beaver’s small intestine is very long, and is the major site for nutrient absorption. At the ileocaecal junction, the site where the small intestine, caecum, and large intestine meet, much of the undigested material enters the caecum, where fermentation by symbiotic bacteria occurs. Beavers are hindgut fermenters that rely heavily upon bacteria in the caecum to digest and obtain nutrients from tough foods like wood. The beaver caecum is very large and is bigger than the stomach. After the symbiotic bacteria break down the indigestible material (cellulose) in the caecum, what remains then enters the large intestine, where the water and minerals remaining after digestion in the caecum are absorbed. The beaver large intestine is much shorter than the small intestine, although it is slightly larger in cross section.

Dissection:

We began the dissection by making a cranial-to-caudal incision through the linea alba. The beaver has an extremely large, multi-lobed liver. After exposing all of the abdominal viscera, we removed the digestive system carefully, making sure not to remove any portions of the urogenital system accidentally. The kidneys were especially difficult to locate, and though they are embedded in fat in the body wall, they are attached by fascia to the digestive tract, thus special care was taken to avoid damaging or accidentally removing the kidneys. We first cut the esophagus just above the liver (the stomach is dorsal to the liver, and is not visible in situ). We next cut the sigmoid colon, separating it from the rectum. We had to make multiple small incisions in order to separate the digestive system organ from the internal body wall.

After removing the abdominal viscera, we found that nearly all of the organs were tightly coiled and bound together by thin layers of mesentery. In order to measure the length of the intestines, we had to carefully separate the mesenteries from the surface of the small intestine, large intestine, and stomach.

We next made a cut around the lateral surface of stomach. The stomach wall is very thin (approximately 0.25cm) and has an elastic quality. The stomach was completely full, and the contents were grayish in color. Upon closer inspection, small particles and splinters of wood were clearly visible.

Conclusions and Comparisons:

There are multiple adaptations in mammalian digestive systems related to diet. The beaver is a hindgut fermenter, meaning that it relies upon a symbiotic bacteria-containing caecum in order to fully digest and absorb nutrients from its food. The rabbit is another example of a hindgut fermenter. However, compared the beaver, which has a very large digestive system, the rabbit is more limited by its body size. The rabbit likewise has a large caecum, but the rabbit also engages in a behavioral adaptation in order to complete its digestion process. Rabbits are coprophagous, meaning that they ingest small pellets of feces that contain still undigested plant material. Rabbits must digest their food twice in order to obtain the necessary nutrients, whereas beavers are able to survive while digesting their food once due to the complexity, and length of their long intestines.

In comparison to the rabbit and the beaver, which are both hindgut fermenters, foregut fermenters such as cows have highly developed stomachs. Cows, for example, have multi-chambered stomach which allows food to remain in the stomach much longer to be digested. Goats, which are also hindgut fermenters, have a very large and complex stomach as well. The goat also has a very long small intestine, ever longer in relative terms than the beaver’s small intestine.

Benefits of being a foregut fermenter are that the material entering the small intestine and large intestine for absorption has already been broken down, for the most part. In hindgut fermenters, some nutrients that can only be absorbed in the small intestine will be lost. This is true for the rabbit, which must re-ingest its already digested food in order to absorb nutrients that could not be absorbed during the first digestion.

References:

1. Müller-Schwarze, D., & Sun, L. 2003. The Beaver: Natural History of a Wetlands Engineer. Comstock Publishing Associates, Ithaca.

2. Morgan, L.H. 1868. The American Beaver and His Works. J. B. Lippincott & Company, Philadelphia.