Sure, vomiting can be gross, but it’s also an essential survival reflex that’s saved countless lives. Humans are very fortunate to able to vomit, unlike other species like rats. They don’t burp or experience heartburn, either. In fact, it seems like rats have little to any reflexes — not the kind that saves your gut from poisons, bad drugs, motion sickness, radiation or hearing Donald Trump talk on television.

rats

Credit: Wikimedia Commons

What's vomiting

Vomiting, or emesis, is the reflexive act of ejecting stomach contents forcefully through the mouth by coordinated muscular contractions.

Vomiting, or emesis as doctors call it medically, is the body’s reflex of ejecting stomach contents forcefully through the mouth. Vomiting is distinct from regurgitation. Unlike regurgitation which is a passive, effortless expulsion of the stomach’s content, vomiting is an active reflex which involves complex muscle coordination. Occasionally rats will regurgitate, but they can’t vomit.

In humans, the vomiting signal is triggered  by a group of nuclei in the brainstem. This instructs the muscles surrounding the stomach to contract, the diaphragm to spasm inward and downward, and the esophagus to open.

By lacking gut reflexes, rats may have trouble swallowing food. At times, a rat might seem like it’s choking, straining intently by pulling the chin down toward the throat and flatting the ears. The rats are still able to breathe though and true choking is very rare. Sometimes, a rat will expel some of the swallowed food, but that’s not vomiting.

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The body's protection system against toxins

 

  • First line of defense: Avoidance of certain foods due to smell or taste cues
  • Second line of defense: Detection of toxins in the gut followed by nausea (prevents further consumption) and vomiting (purges the body of already ingested toxin)
  • Third line of defense: Detection of toxins in the circulation by a sensor in the central nervous system, also followed by vomiting.

Biologically, a rat is unable to vomit because of a powerful and effective gastroesophageal barrier, research shows. This barrier consists of crural sling, the esophageal sphincter, and the intra-abdominal esophagus. Researchers found that the pressure at the two ends of this barrier is greater than the pressure found in the thorax during any phase of the breathing cycle. This pressure, thus, makes it impossible for rats to reflux.

More specifically, rats are unable to vomit because they can not open the crural sling at the right time. Rats also lack the neural wiring required to coordinate the muscles involved in vomiting mentioned earlier.

While they do lack the ability to vomit, an integral part of many species’ defence mechanisms against toxins, rats seem to have adapted by strengthening their first line of defence. Researchers note that rats have a very keen sense of smell and taste and will easily avoid foods which might cause a vomiting response in other species. Some speculate that vomiting has become redundant and lost over time because rats seem to avoid dangers at the hand of toxins so well. Alternatively, rats developed a hyper-sensitive food avoidance to compensate for the inability to vomit. It’s not clear at the moment which came first.

Rat toxin avoidance isn’t full proof, though (remember, rat poison!). At times, a rat will become intoxicated and experience nausea. Luckily, the rat developed an alternative to vomiting by consuming non-nutritive substances. This behaviour is called pica. When rats feel nauseous, they start eating things like clay, dirt, hardwood bedding, all sorts of materials they wouldn’t consider ingesting in normal conditions. These non-nutritive substances may help dilute the toxin’s effect, so pica can be thought of as part of the rat’s second line of defence.

As a parenthesis, some human communities also engage in pica practice. When researchers went to Madagascar to study  760 participants from the Makira Protected Area,  they found 63 percent of adult males engaged in pica and amylophagy.

Other common animals that can’t vomit include rabbits, horses, guinea pigs or the Japanese quail. So, next time you throw up consider yourself blessed.