Not a working one. The platypus has no functional stomach. Where a stomach would sit there is only a small, pouch-like widening of the gut that makes no acid or digestive enzymes, so food passes almost straight from the esophagus into the intestine. The platypus grinds its food in its mouth instead, and digestion happens in the intestine.
What the platypus has instead of a stomach
In most mammals, food goes from the esophagus into a stomach, which stores it and bathes it in acid and the enzyme pepsin before it moves on to the small intestine. The platypus (Ornithorhynchus anatinus) skips that step.
A 2008 genome study in Genome Biology describes the platypus stomach as completely aglandular and reduced to a simple widening of the lower esophagus. A 2024 study in Open Biology calls it small, amorphic and glandless, without a pyloric sphincter (the valve that normally closes off the stomach from the intestine). That makes it hard to tell the esophagus, stomach region and intestine apart.
The Australian Platypus Conservancy puts it this way: a small expanded pouch-like section is there, but it does not secrete digestive acids or enzymes. That is why the best answer to "do platypus have stomachs" is "not a functional one."
How does a platypus digest food without a stomach?
The work moves up front, into the mouth.
- Grinding pads. Young platypuses have teeth, but they lose them when they start eating solid food. Adults use hard keratin grinding pads at the back of the jaw. The Conservancy says these can reduce prey to a fine paste without help from other abrasive material.
- Cheek pouches. Platypuses forage underwater with their eyes, ears and nostrils closed. They store prey in cheek pouches, then surface and chew it while floating.
- The intestine. Because the food arrives already ground fine, it needs little further processing, and digestion and absorption happen in the intestine.
Contrary to a popular claim, platypuses do not grind food with gravel. The Conservancy says mud or sand found in cheek pouches is probably just an inconvenience picked up while feeding.
What do platypuses eat?
Platypuses feed mostly at night on aquatic invertebrates, especially insect larvae, along with freshwater shrimp, worms, mussels, snails and tadpoles. They forage for 10 to 12 hours a day on average, according to the Australian Museum. These are soft, bottom-dwelling prey that grinding pads can mash into paste.
How did the platypus lose its stomach?
The genes that build and run a stomach are gone or switched off. The 2008 study found the genes for pepsinogen A, pepsinogens B and C, gastrin (a hormone that stimulates acid) and part of the acid pump (ATP4A) missing from the platypus genome, while ATP4B and cathepsin E are damaged. The authors say a stomach had worked in most vertebrate lineages for more than 400 million years.
The 2024 study points to another gene, Nkx3.2, which was inactivated in the common ancestor of platypuses and echidnas before the two lineages split about 55 million years ago.
Why it happened is still debated. The authors of the 2008 study say the loss could have been an advantage, or the genes may simply have stopped mattering, and they do not settle the question. Other researchers note that diets rich in chalky shells or bottom muck can neutralize stomach acid, which would make acid digestion less useful. The 2024 authors propose that accumulating detritus in the platypus's bottom-feeding habitat may have driven the loss. Researchers say it is highly unlikely that a platypus lineage could ever regain a stomach.
Do echidnas have a stomach?
Sort of. Echidnas, the platypus's closest relatives, have a bulbous stomach, but like the platypus's it is glandless and does not make acid. Both monotremes (the egg-laying mammals) lost acidic digestion.
Other animals have lost their stomachs independently. Many bony fish, including the carp family, lack one, and estimates put it at up to 27 percent of bony fish species. Lungfish and chimaeras (ratfish) lack a stomach too. In the 14 species researchers examined, stomach loss went with complete loss of the genes for pepsin and acid digestion.
More platypus oddities
- Venom. Males have a spur about 12 mm long on each ankle linked to a venom gland. The venom is not considered life-threatening to people, but it causes severe pain that can last days or months. Wild platypuses should not be handled, and anyone spurred should get medical care.
- Electroreception. The bill has pressure-sensitive receptors and electroreceptors. Exactly how the platypus uses them to find prey is still unknown.
- Eggs and milk. Females lay one to three eggs, usually two. The young suckle milk from two patches on the mother's abdomen.
Conservation status
The IUCN lists the platypus as Near Threatened. The Australian Museum notes that its dependence on freshwater systems may lead to decline.
Sources
- Loss of genes implicated in gastric function during platypus evolution (Genome Biology, 2008)
- Pseudogenization of Nkx3.2 in monotremes and unique gastric anatomy (Open Biology, 2024)
- Platypus myths (Australian Platypus Conservancy)
- Platypus (Australian Museum)
- Why the platypus will probably never get its stomach back (NBC News)