Their intestinal flora has characteristics of that of both carnivorous mammals and ruminants. Bacteria similar to those that can degrade the cellulose walls of plants are found in the intestines of baleen whales, which are carnivores. The latter share a common ancestor with terrestrial herbivores, which roamed the Earth 60 million years ago. This age-old trait may have persisted to help break down the chitinous shells of crustaceans that these whales gorge themselves on.
Herbivorous terrestrial ancestor
The first cetaceans appeared 50 million years ago as land mammals sharing a common ancestor with modern-day ruminants (cows, hippos, pigs, etc.). During their transition from a terrestrial to an aquatic environment, their diet changed from a plant-based diet to a carnivorous one. Nevertheless, ancestral traits still testify to their terrestrial herbivorous past: their digestive tract is compartmentalized into different chambers, just like that of their distant cousins, the ruminants. In the latter, one of these chambers features a bacterial community capable of digesting plant cellulose and facilitating the extraction of sugars. Cellulose is a complex carbohydrate which makes up the walls of plant cells, the sugars of which cannot be extracted naturally by mammals. The presence of these bacteria is essential in the digestion of plants and the absorption of nutrients.
A team of researchers from Harvard University, the University of California – Los Angeles, and the University of Vermont, led by Peter Girguis, collected feces samples of baleen whales in New Brunswick’s Bay of Fundy and analyzed their DNA sequences. They found that the microbial flora of these whales is unique. Specifically, it contains bacteria similar to those of large carnivores with meat-rich diets, but also bacteria similar to those that ruminants depend on to digest plants. Their results were published in the journal Nature Communications.
Bacterial flora that has evolved
The intestinal bacterial flora in mammals plays an important role in bodily functions. Its composition depends on the diet of the animal, but the influence of evolutionary traits is also recognized, albeit poorly studied. The case of baleen whales supports this idea. But why has this ancestral trait persisted for millions of years? Although whales do not feed on plants, these bacteria similar to those present in the digestive tracts of ruminants might be used to digest another complex carbohydrate: chitin. Baleen whales consume nearly one tonne per day of small crustaceans such as krill, whose chitinous shells cannot be digested naturally by mammals.
What about toothed whales?
Toothed whales feed more on fish than crustaceans. Their intestinal microbial flora should thus be different from that of baleen whales. Further research will shed light on the influence of diet and ancestral descent on the diversity of the bacterial community in their digestive tracts.
The role of vestigial pelvic bones (in french)
The relationship between hippos and whales (in french)
Archaeoceti had an asymmetrical skull (in french)