Vomiting, or emesis, is a forceful expulsion of stomach contents through the mouth, a protective reflex involving coordinated muscles and neural pathways. While common across many mammalian species, the ability to vomit is not universal in the animal kingdom.
Animals That Cannot Vomit
Several animal species cannot vomit, influencing their physiology and survival. Horses, for instance, cannot vomit, making them vulnerable to digestive issues like colic, a major cause of death.
Rats and mice, along with other rodents, also lack this reflex. This constraint has led to their use as models for studying nausea without the confounding factor of vomiting. Rabbits similarly cannot vomit, posing challenges for their digestive health. Guinea pigs, belonging to the rodent order, share this inability.
The Biological Reasons
The inability to vomit stems from anatomical and neurological factors. Horses possess a strong lower esophageal sphincter, a muscular valve preventing food return. The esophagus connects to the stomach at a sharp angle, tightening the sphincter when full, making upward expulsion nearly impossible. Their abdominal muscles and diaphragm are also not structured for the forceful contractions required for vomiting.
For rodents like rats, mice, and guinea pigs, reasons are multifaceted. They have a similarly strong gastroesophageal barrier, with a tight muscle band at the junction of the esophagus and stomach. Their esophageal muscles lack strength for reverse peristalsis, the wave-like contractions that move food upward. A factor is the absence of specific neural circuits in their brainstems that coordinate the emetic reflex. While they may experience nausea, their brains cannot trigger the necessary muscle actions for vomiting.
How They Cope Instead
Since these animals cannot expel harmful substances through vomiting, they have evolved alternative mechanisms. Horses rely on their liver, kidneys, and large colon to filter and remove toxins. Their digestive system is designed for continuous, one-way food movement, processing roughage efficiently. Ingesting problematic substances must be processed internally, which can lead to severe issues like colic.
Rats have developed sensitive food-avoidance learning, tasting small amounts of new foods and avoiding those that cause sickness. They may also engage in pica, consuming non-food materials like clay, which can bind toxins in the stomach and dilute their effects.
Rabbits, as hindgut fermenters, have specialized digestive systems that rapidly move indigestible fiber through their gut, preventing accumulation. They also practice coprophagy, re-ingesting nutrient-rich fecal pellets (cecotropes) to absorb vitamins and amino acids produced by gut microbes.