Rodents, oddly, can’t throw up. While scientists and pest managers have known this for years, they’ve just discovered why. LiveScience explains the findings of a group of neuroscientists from the University of Pittsburgh Cancer Institute, which were first published in the journal PLoS One.

First, the researchers wanted to find out if this singular trait is found in all rodents. The reason that rat poison works so well is that the pests can’t throw the poison back up, but most people don’t go around poisoning shrews, chinchillas and beavers. The researchers selected species from the animal kingdom’s three major rodent groups, including mouse-related rodents, squirrel-related rodents and guinea pig-related rodents. They gave all of the animals vomit-inducing drugs, but to no effect.

Universal lack of barfing confirmed, they decided to get to the bottom of this rodent characteristic by investigating the animals’ physiology and neurology. LiveScience contributor Charles Choi explains:

They found rodents had bodily constraints that would limit how much they could vomit even if they could attempt it. This included reduced muscularity of the diaphragm, the thin sheet of muscle underneath the lungs, as well as a stomach that is not structured well for moving contents up the throat. The researchers also investigated the brainstems of lab mice and rats. When given compounds that normally trigger nausea in other animals, the researchers saw less nerve, mouth, throat and shoulder activity normally linked with vomiting. This suggests they lack the brain circuits for throwing up.

Most mammals do throw up, Choi points out, making rodents the exception to the rule. Scientists reason that the furry little guys most likely lost their ability to vomit at some point in evolutionary history in favor of other defensive strategies.

For instance, rodent responses to taste may make them better at avoiding toxins that can sicken or kill them. Rodents also eat clay when sick, which apparently can latch onto dangerous materials and keep their bodies from absorbing them, said.

Incidentally, horses don’t throw up either. USA Today explains why not:

Horses have a band of muscle around the esophagus as it enters the stomach. This band operates in horses much as in humans: as a one-way valve. Food freely passes down the esophagus into the stomach as the valve relaxes but the valve squeezes down the opening and cuts off the passage for food going back up. Horses, however, differ from us because their valve really works. Humans can vomit. Horses almost physically can’t because of the power of the cut-off valve muscle.

Normally, USA Today concludes, if a horse does vomit, it is because its stomach has completely ruptured, which in turn means that the poor horse will soon be dead.

More from Smithsonian.com:

Rodents of Unusual Size Do Exist

Watch How Fast the Insane Snout of a Star-Nosed Mole Can Move