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.