Digestive Anatomy

Humpback whale baleen is made of palatal mucous membrane, and is used to trap krill and other small prey in their mouth as they filter out the water (Stevens and Hume, 1995). Humpback whales have a varying number of throat grooves (14-22) that allow the throat to expand to increase the volume and take giant mouthfuls (Clapham and Mead, 1999). Sperm whales have long narrow jaws with teeth along the bottom jaw. Their jaws are made to quickly grab and swallow cephalopods (Stevens and Hume, 1995), though they do not chew their food, so digestion occurs fully within the stomachs and small intestine (Slijper, 1962). Sperm whales swallow very large prey such as giant squid, and therefore have wide esophaguses compared to humpback whales (Slijper, 1962). Humpback and sperm whales have similar gastrointestinal tracts. The esophagus leads from the mouth into a three-compartment stomach. There is a forestomach, a main stomach, and a pyloric stomach. The forestomach and main stomach are very large and can hold a lot of food (Slijper, 1962). The forestomach can be compared to the gizzard of a bird, in that whales often swallow small stones, or use the shells and hard parts of their prey that rest in the muscular forestomach to help break down or ‘chew’ food (Slijper, 1962). The main stomach secretes hydrochloric acid and pepsin, which aids in digestion and breakdown of food (Slijper, 1962). Lipase can also be found in the main stomach, which is typical of carnivores such as whales, to help digest their fatty prey (Slijper, 1962). In sperm whales, the forestomach is the largest stomach compartment, but in humpback whales, the main stomach is the biggest (Slijper, 1962). This may be due to the size of their prey. Sperm whales swallow very large prey whole, and must physically break it down in the forestomach before it can be digested and absorbed further. Humpback whales eat small prey that do not need as much physical breakdown, so the main stomach is larger for chemical breakdown of their food. The stomach compartments are joined by narrow openings in sperm whales, and wider ones in humpback whales (Slijper, 1962). The pyloric stomach secretes pyloric acid, and leads to the small intestine for further digestion and absorption of nutrients (Slijper, 1962). Sperm whales have a longer small intestine to body size ratio than humpback whales, as baleen whales tend to have lower ratios. Humpback whales have a small intestine to body size ratio of 5.5:1. Sperm whales would have a ratio closer to 10:1. (Stevens and Hume, 1995). This translates to a small intestine length of around 150 metres. These ratios have not been found to have any indication of diet or size. (Slijper, 1962). Humpback whales have a short cecum (Slijper, 1962), and sperm whales lack a cecum altogether (Stevens and Hume, 1995). This means that the transition from small intestine to large intestine is not clear, which is typical of carnivores. Animal material is easily broken down and absorbed, compared to plant material, so a large cecum is not necessary.

A diagram of the digestive tract of a humpback whale or any similar baleen whale was not found, but the humpback whale and sperm whale have very similar gastrointestinal tracts, other than the differences discussed above.

Diagram of the digestive system of Physeter macrocephalus (Sperm whale).
Diagram of the digestive system of Physeter macrocephalus (Sperm whale).
Diagram of a dolphin stomach taken from Stevens and Hume, (1995) to show a similar multicompartmental stomach as the sperm and humpback whales.
Diagram of a dolphin stomach taken from Stevens and Hume, (1995) to show a similar multicompartmental stomach as the sperm and humpback whales.

Note that in relationship to the diagram of the dolphin stomach above, a sperm whale would have a larger forestomach, and a humpback whale would have a larger main stomach.

In conclusion, even though humpback whales and sperm whales are related, and live in the same parts of the ocean, there are reasons why they have different diets. Humpback whales have baleen, and are unable to physically break down large prey in their stomach. They rely on lots of small prey that they can filter feed, for their nutritional needs. Sperm whales have teeth that are able to grab large, fast prey, and can ‘chew’ them up in their large forestomach. Each of these types of whales is using a different part of the ocean ecosystem to survive. Humpback whales are adapted to eat lots of nutritious, small zooplankton, like krill, while sperm whales are adapted to eat larger cephalopods, like squid. Nevertheless, both whale species share many similarities in their caloric and nutritional needs, and they have found different ways to satisfy their hunger.

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