Gastric Secretion in the Beaver (Castor canadensis)

The beaver (Castor canadensis) is generally conceded to be a vegetarian. This animal spends much of his life in waters which may abound in fish, but apparently nobody ever saw a beaver eat one (Warren, 1927). Willow and poplar bark are the main items in the dietary of the beaver (Warren, loc. cit.; Bradt, 1947). The summer diet may be supplemented with grass, leaves, berries, or roots, but the winter fare is probably quite limited. It is not unlikely that bark constitutes the sole winter food. It is a common experience in beaver country to see the small sticks stripped of their bark which collect on the surface of a beaver pond after the ice breaks in the spring. In any event the beaver eats and evidently utilizes large quantities of cellulose. Has the beaver, like the termite, certain protozoans in the alimentary canal which are capable of digesting cellulose, or are there some special enzymes in one or more of the digestive juices that enable the animal to gain energy from cellulose? Smith (1942) and Smith et al (1942) suggest that a special gastric gland of the beaver may be capable of secreting a cellulase. The experiments described below represent a modest attempt to add some information on this subject. Animals.-Initially three beavers were available for this investigation. One hanged himself in his cage, a second died of respiratory failure under anaesthesia (intraperitoneal pentobarbital sodium, 22 mg./kg.), and the third ( 9, 12.0 kg.) was used to obtain the results given below. Since it was not feasible to work with the unanesthetized animal, it was first lightly etherized in a box and then given an intraperitoneal injection of pentobarbital sodium (17.5 mg./kg.). The beaver's skin is very tough and puncturing it for intravenous injections is difficult. The drugs used in these experiments, therefore, were introduced by intraperitoneal injection. Others wishing to keep beavers briefly in captivity may be interested to know that the animals described here seemed to thrive on a mixture of dog biscuits, raw carrots, and a few branches of poplar. One of the beavers seemed especially fond of swimming in a bathtub that was provided for their use. Gastric secretion.-With the animal in the supine position, a tracheal cannula was inserted, and a tube passed from the mouth to the stomach. By inspection and palpation, through an abdominal incision, the tip of the stomach tube was adjusted to lie in what appeared to be the most dependent portion of the stomach. Gastric contents were aspirated at intervals by means of the stomach tube and a syringe. Volume and pH (glass electrode) were determined at once for each sample. Enzyme activity.-The presence of pepsin in the beaver's stomach contents was established by means of Mett's tubes. In order to test for cellulase activity, shredded filter paper and dried, ground poplar bark were used as substrates. Five ml. of gastric contents was mixed with 5 ml. of 0.1 N HC1 (pH approximately 1.0) and 1 ml. of toluene as preservative, and incubated with 100 mg. of cellulose substrate, or 2 Mett tubes, at 37? C. for 24 hours. Controls contained 204 Vol. 34, No. 2