Horses Cannot Vomit

While many mammals can expel a toxic (Review) meal, this life-saving reflex is physically impossible for a horse. The reason lies in its unique anatomy, primarily an incredibly powerful band of muscle called the cardiac sphincter at the entrance to the stomach. This muscle acts as a strict one-way valve, clamping shut with such force that stomach contents cannot be pushed back up the esophagus. The esophagus itself also joins the stomach at a much sharper, lower angle than in humans, further preventing any reverse flow. This design is a product of their evolution as grazing animals, whose digestive systems are adapted for a constant, slow, forward-moving intake of fibrous food.

This unforgiving anatomy is precisely why the digestive condition known as colic can be so devastating. If a horse (Review) ingests something poisonous, or if food ferments and produces excess gas, the resulting pressure has nowhere to go. The stomach can distend to a dangerous degree, causing extreme pain. In the most severe cases, the pressure can build until the stomach actually ruptures, a condition that is almost always fatal. This biological feature is a dramatic reminder of how a specialized evolutionary trait can also create a critical vulnerability.