Despite belonging to the same lineage that gave rise to giraffes and bison, modern whales are true carnivores, feasting on a range of animals from fish to tiny krill. So it would make sense that the communities of bacteria that live in their gut show a resemblance to those of other meat-eaters. But after sampling the microbiomes of right whales, researchers have found that these creatures have managed to cling onto a relic of their herbivorous past.
“From one point of view, whales look like carnivores,” explains Peter Girguis, who led the study published in Nature Communications. “They have the same kind of microbes that we find in lions and tigers that have very meat-rich diets. But they also have abundant communities of anaerobic bacteria, similar to those that ruminants use to break down cellulose.” Considering cellulose, the carbohydrate found mainly in plants, doesn’t normally feature in the whales’ diet, this seems a little odd.
Like blue whales and humpbacks, right whales are part of a group known as baleen whales – those that have large bristle-like plates that are used to filter out food from the water. The researchers think that it is this food – primarily small animals called copepods, such as krill – that hold the key to why the whales' guts still retain bacteria most usually associated with ruminants.
Krill have hard exoskeletons to protect their soft bodies, and this is made from a type of carbohydrate similar to cellulose, called chitin. “What our paper suggests is the whale foregut is much like a cow's gut, and we posit that chitin-degrading anaerobic microbial community thrives in there, breaking down that material and making it available to the whale,” says Girguis.
The researchers think that as the chitin exoskeletons make up as much as 10% of the whales' total food intake, rather than excreting it, their unusual mix of carnivore and herbivore microbiome allows them to access the extra nutrition. But, according to Girguis, the implications of this study are wider than just what is happening in the guts of whales.
When you study an animal's microbiome, you can, to some degree, work out what other species it is related to. This is because, in general, organisms that are related tend to have similar microbiomes, even though they may now live in different environments. “So the question is,” says Girguis, “how different does your environment need to be before it changes your microbiome?”
While the study might not provide a definitive answer to this question, it does suggest that if the community of bacteria is in some way still beneficial to the animal, it could be retained even over 55 million years of evolution in a completely different environment.