How Does Digestion Take Place in Whales?
The digestive systems of whales consists of an esophagus, a compartmentalized stomach (similar to that of ruminants like cows or hippos) and an intestine. Prey, whether ingested one at a time as in toothed whales or by the thousands as in baleen whales, are not chewed but rather swallowed whole. They then pass into the esophagus, where they are pushed toward the expandable stomach. The esophagus of the blue whale, even if it takes in 2-3 tonnes of krill a day, measures just 15 to 25 cm long when fully extended. The food then reaches the first stomach compartment, the rumen. Pre-digested food is stored there. This compartment breaks down the food by mechanical muscular movements called peristalsis. Everything is then directed toward the main stomach (or cardiac stomach), where glands produce acid and enzymes used to digest the food (hydrochloric acid, pepsin). The journey continues through a narrow channel before finally reaching the last stomach compartment, the pylorus. The latter is characterized by the presence of numerous mucus glands that facilitate intestinal transit. It is the combined actions of these different compartments that notably allow whales to digest the chitin in the exoskeletons of krill and prey swallowed whole.
The digested food continues its journey into the small intestine where nutrient absorption begins. The size of the intestine varies according to the species: it can be 5 to 6 times the length of the animal, which is equivalent to 150 m in the blue whale. As cetaceans have no gall bladder, it is the liver that provides the bile needed for digestion. Cetaceans have the largest livers of all mammals. The sperm whale produces a substance called ambergris which facilitates digestion of squid beaks that can otherwise irritate the bowel. Transit time, from the stomach to the anus, is from 15 to 18 hours.
Diagram of the Stomach of a Toothed Whale Taken from Tinker, S.W. Whales of the World. 1988.
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