TO CONTINUE, since carnivores need only one stomach in order to digest meat (we include not only the meat of mammals and birds but also of fish, crustaceans, insects, worms, and other animals), it follows that we would not expect them to possess multiple stomachs. This is especially true for hypercarnivores, whose diet is almost all easy-to-digest meat
And indeed, hypercarnivores such as cats and seals only have one stomach. But there is one exception. One of the most hyper-carnivorous clades of all mammals possess multichambered stomachs. Namely, cetaceans, which are the dolphins and whales. This has been known since biologists first dissected whale carcasses from the whaling industry. Cetaceans have four ‘stomachs’ more or less.
Huh? Cetaceans eat nothing but animals – crustaceans, bottom dwelling annelids and mollusks, fish, and even sharks, mammals and birds (for orcas and false killer whales). They are hypercarnivores.
Replay the paragraph above, noting crustaceans. Many cetaceans eat crustaceans, from small copepods to krill to shrimp to pelagic crabs. The largest animals on earth, the baleen whales are noteworthy for filtering and eating small crustaceans. Smaller dolphins, given the chance, will eat shrimp.
Here’s the point. Crustaceans are covered by chitin. Chitin chemically is similar to cellulose. Thus, chitin is hard to digest. But with multiple stomachs full of fermerting microbes, chitin can be digested. Since crustaceans may be up to 10% chitin by weight, cetaceans are advantaged by possessing multichambered stomachs.
Thus, in marine ecosystems, the possession of multiple stomachs gives cetaceans an advantage over the other big group of marine mammals – the pinnipeds, better known as the seals. Seals are also hypercarnivores, also eating fish and invertebrates such as crustaceans. But they can’t digest chitin.
Seals are members of the mammalian order carnivora. It’s the same order that our familiar carnivorous cats and dogs belong to. (Indeed, seals look like dogs.) Now why don’t seals have multichambered stomachs?
It’s probably because the ancestral seal, like all carnivora, had only one stomach. Modern seals retain this condition.
But cetaceans have multiple stomachs. And the main clade of placental mammals that possess multiple stomachs are the artiodactyls. No way cetaceans can be artiodactyls, right?
Wrong.
In the past several decades, it has become increasingly clear that cetaceans are deeply nested in the order artiodactyla. Whales and dolphins are artiodactyls. The order is often referred to nowadays as cetartiodactyla.
A number of studies on the molecular phylogeny of cetaceans has identified their closest relatives to be hippopotamuses. These are studies on genes and their sequences, ubiquitous and important proteins such as fibrinogen and cytochrome, RNA sequences. But that wasn’t enough to reclassify cetaceans as artiodactyls. What convinced most biologists are fossil findings. (To be continued)/PN
