Rats can’t vomit. They can’t burp either, and they don’t experience heartburn. Rats can’t vomit for several related reasons:
(1). Rats have a powerful barrier between the stomach and the esophagus. They don’t have the esophageal muscle strength to overcome and open this barrier by force, which is necessary for vomiting.
(2). Vomiting requires that the two muscles of the diaphragm contract independently, but rats give no evidence of being able to dissociate the activity of these two muscles.
(3). Rats don’t have the complex neural connections within the brain stem and between brain stem and viscera that coordinate the many muscles involved in vomiting.
One of the main functions of vomiting is to purge the body of toxic substances. Rats can’t vomit, but they do have other strategies to defend themselves against toxins. One strategy is super-sensitive food-avoidance learning. When rats discover a new food, they taste a little of it, and if it makes them sick they scrupulously avoid that food in the future, using their acute senses of smell and taste. Another strategy is pica, the consumption of non-food materials (particularly clay), in response to nausea. Clay binds some toxins in the stomach, which helps dilute the toxin’s effect on the rat’s body.
The muscle that acts like a one-way valve where the esophagus connects to the stomach actually does its job, unlike in humans where it can let food travel the opposite direction, back to the mouth. Another reason that horses can not vomit is because the esophagus is connected to the stomach at an angle where it is forced shut when the horses stomach bloats. This can cause colic when the horses stomach bloats, and untreated, the stomach will rupture and the horse will die.