Digestive system of humpback whales

The digestive system of humpback whales (Megaptera novaeangliae) is adapted to digest tiny crustaceans such as krill, plankton, and small fish (including herring and mackerel) which the whale filters from the ocean using its baleen, a sieve-like structure in the upper jaw. Humpback whales belong to the parvorder baleen whales, which are carnivorous, seasonal feeders.

Mouth

The baleen, a sieve-like structure in the upper jaw made of keratin, is used to filter plankton, among other food sources, from the water. Ventral grooves are expandable concave furrows that line a whale's throat and have the peculiarity of stretching like an accordion. They allow whales to open their mandible to 90 degrees to gulp water and prey. They expel saltwater through the baleen plates, leaving behind krill and plankton. The whale's tongue plays a vital role in filter feeding, as it is used to push the water out of its mouth while keeping the prey trapped in its baleen bristles. As humpback whales do not have teeth, their food is swallowed whole and travels down the esophagus into the preliminary stomach (known as the forestomach) to be ground.

Compartmentalized stomach

Baleen whales have a multi-chambered stomach, divided into three distinct compartments with separate functions – the first being the forestomach. The role of the forestomach is a disputed topic. The forestomach of humpback whales is highly distensible and functions as a holding chamber for quantities of ingested food, which is of importance in animals that feed opportunistically. An extensive muscularis externa indicates that it also grinds or churns the contents mechanically to compensate for the whales’ lack of teeth. According to the American Society for Microbiology, "the fore stomach consists of … non glandular tissue and appears to be analogous to the tissue of the rumen." The forestomach is lined with a keratinized stratified squamous epithelium which is continuous of the oesophagus epithelium. Due to the reflux of stomach juices from the main stomach, minor digestive activity takes place in the forestomach.

The main stomach, also referred to as the fundic stomach, secretes mucus, protein digesting enzymes and hydrochloric acid to aid the breakdown of food and lined. The stomach lining is reddish–purple, highly convoluted and sometimes trabeculate epithelium.

The pyloric stomach secretes enzymes that digest fat as well as alkaline chemicals to neutralize the stomach acid. It is the combined actions of these different compartments that allow whales to digest the chitin in the exoskeletons of krill and prey swallowed whole.

Duodenal ampulla/ duodenum

The duodenal ampulla can be described as the atrium of the small intestine and is a dilated sac forming the beginning of the duodenum. The duodenum prepares many nutrients from the digested matter (chyme), for absorption, which takes place in the intestines, and is the principal site for iron absorption.

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