Correct answer: They can't vomit These creatures’ inability to throw up can be explained by evolution – it’s all about adaptation. Carnivores have to be able to get rid of some of the things they eat, like fur or bones, because they can't digest them. Herbivores like rabbits, however, can digest everything they eat and, in theory, they know the difference between what is poisonous and what isn't. Like other species, horses and rabbits have a muscle located at the entrance to the stomach known as the cardiac sphincter to make sure food goes into the stomach and stays there. In horses and rabbits, this muscle is hypertrophied, which means it does not contract enough. So these animals are, in fact, designed to ensure that their food goes into the stomach and through to the intestine without ever having a chance of going the other way. In horses, the soft palate also plays an important role. It serves as a sort of valve when swallowing and prevents food from going back up into the mouth cavity. If food in the esophagus is regurgitated, it has to come out through the nose. Rabbits also have a specially designed larynx. Their epiglottis is a different shape from that of a cat or dog. And if a rabbit's food were to come back up again, it would have to go through the trachea and out through the nose. Because they can't vomit or burp, pet rabbits often develop stomach problems, which can quickly lead to serious consequences. As for rats, the muscles between their esophagus and stomach are so strong that reflux is impossible. Rats don't have the strength to break through this muscular barrier, neither do they have the neural connections needed to separate the two muscles of the diaphragm to make them throw up, which is what humans do. Rats are highly intelligent, so they have developed other strategies to avoid getting poisoned, given that their anatomy prevents them from vomiting. They use their sense of smell to avoid food that would make them sick. They have also been known to ingest substances such as clay, which is highly absorbent, or to make an old rat try unknown food. http://www.agorat.org/ Other animals that can't vomit include domestic mice and guinea pigs. Thanks to veterinarian Joseph Palmieri for his input.