Imma Perfetto
Cosmos science journalist
Monotremes are weird animals. They’re famously the only mammals to lay eggs instead of live young.
But did you know that they also have bizarre stomachs more akin to some fish than other mammals?
Platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus) gastric systems are abnormally small, and lack the glands for secreting enzymes and acidic juices.
Platypus even go without a pyloric sphincter – the ring of smooth muscle that acts like a valve to regulate the flow of partially digested food from the stomach to the small intestine. This makes it difficult to tell the different between the oesophagus and intestines!
Now, Australian researchers have pinpointed a single gene, the NK3 homeobox 2 (Nkx3.2), as the likely cause for all this weirdness. The gene probably became inactivated tens of millions of years ago in the most recent ancestor of modern monotremes.
“Work from our lab previously had shown that the platypus and echidna had lost the genetic instructions for proteins that break down food and secrete stomach acids, but to me this didn’t explain the drastic shift in their stomach anatomy relative to other animals,” says Jackson Dann, a PhD student at the University of Adelaide and lead author of a new study detailing the discovery in the journal Open Biology.
“Thanks to novel repositories of genetic data, and physical specimens we had at the lab, we were able to discover that Nkx3.2 wasn’t functional in monotremes and that this inactivating event contributed to the evolution of their unique body plans.”
Apart from monotremes, only some aquatic and semi-aquatic fishes have lost their stomachs over the course of their evolutionary past.
The Australian ghostshark (Callorhinchus milii), Japanese puffer (Takifugu rubripes), zebrafish (Danio rerio), and Japanese rice fish (Oryzias latipes) all lack the same hydrochloric acid and gastric enzyme genes as seen in monotremes.
“It’s likely there is some overarching ecological factor we’re missing as to why these species have lost their stomach,” says Dann.
“It’s otherwise surprising that we would see these drastic shifts in stomach anatomy in monotremes and then the next closely related species are loose groups of fish.”
Dann and co-authors suggest that if this was a trait associated with aquatic and semi-aquatic organisms, their evidence would support a semi-aquatic ancestor of the short-beaked echidna and platypus.
Subsequent adaptations to the echidna lineage would have then allowed them to become specialised for terrestrial environments.
According to Dann, the more we know about monotremes, the more we can appreciate their role in their ecosystems and celebrate their eccentricities.
He highlights ecology endeavours like EchidnaCSI, which is an Australia-wide citizen science project helping conserve wild echidnas.
“A better understanding of these unique and iconic species provides a significant contribution to Australian ecology and culture, they help us understand more about mammals, including ourselves,” says Dann.
