Which two species are known not to vomit?

The correct answer identifies that rats and horses are species known not to vomit. This characteristic is due to anatomical and physiological differences in their digestive systems and the overall biology of these animals.

Horses have a unique digestive system; their stomachs are relatively small and designed to hold food temporarily without strong muscle contractions that would lead to vomiting. The esophageal sphincter in horses is highly toned, which prevents the backward movement of food from the stomach to the esophagus, making vomiting almost impossible.

Rats also lack the ability to vomit due to their anatomical structure. Their stomachs are divided into a glandular and non-glandular portion, but the muscular contractions necessary for vomiting are absent. Instead, they must rely on other bodily functions to expel toxins or undesirable material, such as through urination or defecation.

While cows, pigs, sheep, goats, dogs, and cats all possess the capability to vomit, they do so due to different anatomical and physiological structures that allow for the reflex of vomiting. Thus, understanding the distinct digestive adaptations in rats and horses highlights why they are particularly noted for this inability.