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

Urinary and Reproductive Systems of the Beaver (Castor canadensis)

Photo and text by Jason Lavender and Wei-Chung Chen

Overview of Urinary and Reproductive Tracts:

The female beaver reproductive tract is composed of the ovaries, uterine tubes, infundibula, uterus, and the cloaca. The cloaca is a single, enlarged chamber at the end of both the urinary and reproductive tracts through which solid waste, liquid waste, and gametes exit the body. While having a cloaca is generally a primitive trait, for rodents, it is a derived trait. Possessing a cloaca gives the beaver another way to streamline its body during aquatic locomotion. Male and female beavers have very few external differences, other than females having six teats whereas males have two [1]. During non-breeding seasons, it may be impossible to distinguish visually a male beaver from a female. To distinguish between the two sexes, an internal examination of the body must be done. Even internal examination can lead to confusion, however, as some male beavers have uteri similar to those seen in female beaver. The female beaver has a single bicornuate uterus. The beaver gestation period is approximately 3.5 months, and average litter size ranges from two to five, given adequate nutrition and water availability [1].

The urinary system of the beaver is composed of two kidneys, two ureters, a bladder, and the urethra. The kidney serves to filter waste compounds from blood and it also balances the salt and water concentrations in the body. The beaver produces dilute urine that is stored in the castor gland, but most of the urine goes to the bladder for excretion. Beavers possess a pair of castor glands, for which the genus is named, that are pouch-shaped and located on either side of the cloaca. When necessary, the urine is converted into castoreum, which beavers use for territorial scent marking [1]. The function of the scent marking is to warn off transient beavers from areas that are occupied. In addition, castoreum can also provide identifying information about individuals within a family.

Dissection:

During the previous dissection of the digestive system, we were able to identity the beaver as a female. We were able to reach this conclusion because of the presence of the ovaries and the uterus. After removing the digestive system, we first identified the ovaries. Interestingly, there is a disparity in size between the two ovaries. The size of the right ovary is approximately twice the size of the left ovary. Each ovary was embedded in fat, which we removed in order to examine the uterine tubes.  The uterine tube is relatively short, no longer than a few centimeters.

A uterine horn was identified on each side of the body, meeting at midline to form the uterine body. The two horns converge at the pubic symphysis. Unfortunately, we were unable to follow the ureters’ complete course into the bladder, because the bladder itself is located deep to the pubic symphysis, which could not be separated. However, we were able to observe the cloaca, which is situated caudal to the pelvis. In addition, the castor glands are situated caudolaterally to the cloaca. Feces were visible upon inspection of the cloaca, and were similar in color and consistency (gray and grainy) to the contents of the stomach (see the beaver digestion lab). After close inspection of the urogenital system, we examined the anatomical relationships of the openings into the cloaca. It appears that the vagina opens anteriorly to urethra, which opens anteriorly to the anus. Furthermore, the castor glands open in the cloaca anterolaterally to the anus.

The beaver kidneys are relatively small and flat, and they are retroperitoneal. They are embedded in a thick fat layer attached to the dorsal body wall. Both kidneys are situated at approximately the same level, medially to the ovaries. The ureters run caudomedially, converging at the pubic symphysis where they enter the bladder.

Conclusions and Comparisons:

The most striking difference seen between the urogenital system of the beaver and other mammals (including the rabbit, cat, rat, opossum, and fetal pig) was the size and shape of the kidneys. In the other mammals listed above, the kidneys are relatively large and much rounder. The beaver kidneys, in contrast, are relatively small and very flat. The beaver is also unusual compared to the above animals (with the exception of the opossum, which as a marsupial has a very distinctive UG system) in that it has a cloaca. As a rodent, possessing a cloaca is a derived trait for the beaver. This is an adaptation seen in many aquatic and semi-aquatic mammals because it allows for body streamlining that is important to swimming.

Reference:

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