Churn it over: This beaked whale has 13 stomachs

13!

On the floor of the visitor center in Glacier Bay Lodge lies the skeleton of a Baird’s beaked whale (Berardius bairdii), said to be the largest of all the beaked whales.

Beaked whales, about five genera of toothed whales in the taxonomic family Ziphiidae, are deep-diving predators on fish and squid. They can stay underwater for an hour and dive deeper than a thousand meters, but of course they have to come to the surface to breathe. Very little is known about their ecology and behavior, although studies of Baird’s diet in the western north Pacific have shown some variation with location, or season. The prey was mostly squid in one place and mostly fish in two other places.

There is an interpretive sign next to the skeleton that says Baird’s beaked whale has thirteen (!) stomachs. What would they do with thirteen of them? That piqued my curiosity, and so I tried to get more information. Easier said than done, but thanks to John Moran, whale biologist at NOAA, I did get a useful paper on whale stomach anatomy.

Beaked whale stomach compartments can be considered in four categories, in order from esophagus to intestine, i.e., the order in which food passes: forestomach, main stomach, several small connecting chambers and pyloric stomach. Most active digestion is thought to occur in the main stomach. The confusing thing (or one of them, anyway) is that each of those four “stomachs” is typically subdivided into smaller compartments, sometimes also called “stomachs.” The total number of compartments differs among the species of beaked whales, although some of the differences could be due to differences in the state of preservation of beach-stranded specimens or to differences among investigators.

[Juneau scientist helps discover new whale species]

However, for Baird’s beaked whale, some fresh specimens were inspected by marine mammal scientists at a Japanese whaling station, where commercial whalers brought their catch. Their counts are presumably more reliable than those from beach carcasses in various stages of decomposition. All 30 fresh specimens had no forestomach, two main stomach compartments, and two pyloric stomach compartments. But the number of connecting chambers between main and pyloric stomachs varied from seven to 10. So the total number of stomach compartments varied from 11 to 14.

Thus, the number of stomachs (or compartments) varies not only among species but also within a species (Baird’s, and others too). And I found no information at all about why there should be so many stomach compartments. It’s not simply related to the fishy diet, because dolphins, which are also fish-eating toothed whales, have only two or three stomachs.

In the Aleutians and Bering Sea, there is a population of beaked whales that’s been assumed to be B. bairdii, but recent work on mitochondrial DNA has revealed significant differences between that population and the one in the western North Pacific. So there may be a new species in the genus Berardius, but that one is even less well-studied than the other.

Humpback whales are commonly seen in our local waters. They, along with other baleen whales, are considered to have three stomachs — or four if one counts a swelling at the start of the small intestine. The fore stomach churns things up but does little breakdown. The main stomach is glandular, secreting mucous, enzymes and acids to breakdown proteins and carbohydrates, and muscular, to mix it all well. The pyloric stomach neutralizes the incoming acids and secretes more enzymes that digest fats. Then the mix goes to the intestine.

Humpback whales in Juneau. (Michael Penn | Juneau Empire File)

Humpback whales in Juneau. (Michael Penn | Juneau Empire File)

Whales are considered to be descended from terrestrial two-hoofed herbivores (other descendants include cows, deer, sheep, goats and giraffes). All those animals have a four-chambered stomach that digests plant material. The first chamber is called the “rumen,” where chunks of vegetation are worked into a “cud” that is regurgitated for further chewing, before being re-swallowed and passed on to the next chamber. They are collectively known as ruminants. That ruminant ancestry has occasionally been used to explain the complexities of whale stomachs; the idea was that, since the ancestors had a complex stomach, the whales inherited that arrangement. However, that is not a satisfactory explanation, because tens of millions of years have passed since the origin of whales from those ancient ancestors. That’s plenty of time for whale stomachs to evolve in new directions, as indeed they have, several times, without such constraints from the past.

Once again, there is much still to be learned, but understanding whale stomachs is not an easy thing. It might be a dull world indeed if we knew it all!


• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Wednesday.


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