Why Do We Get Hiccups?

You are halfway through a sentence, or a glass of fizzy water, and it happens: a sharp jolt in the chest, an undignified little squeak, and then, a few seconds later, another one. Hiccups arrive without asking, ignore every instruction to stop, and usually leave just as mysteriously as they came. For something almost everybody has had, they are surprisingly hard to explain.

The mechanics, at least, are well understood. According to the Cleveland Clinic, a hiccup is a sudden spasm of the diaphragm, the dome-shaped muscle under your lungs that does most of the work of breathing. It pulls down sharply between breaths, so you gulp in air, and a split second later the glottis, the gap between your vocal cords, snaps shut to stop the air coming in. That snap is the hic. A 2012 paper in the journal BioEssays puts the delay at roughly 35 milliseconds, which is why the gulp and the squeak feel like a single event.

The usual triggers are ordinary ones. Eating too much or too fast, fizzy drinks, very hot or very cold food, swallowing air and alcohol all turn up on the Cleveland Clinic's list. Most bouts are over within minutes. Hiccups that last longer than 48 hours count as persistent, and the clinic advises seeing a doctor if they go on for more than two days, because a long bout can be a sign of something else going on.

What about the cures? Holding your breath, a spoonful of sugar, a sudden fright, drinking from the far side of the glass: every family has its favourite. The Cleveland Clinic is frank about them. Experts have not found a cure, and the evidence for the home remedies is anecdotal. They may work for some people some of the time, which is exactly how a folk remedy survives.

The deeper puzzle is why the reflex exists at all. It starts astonishingly early. The BioEssays paper, by the physician Daniel Howes, notes that hiccups appear in the unborn baby, before swallowing or breathing movements do, and babies hiccup often. Hiccups have also been studied in cats, rats and rabbits, and are seen in horses and dogs. Yet there appear to be no documented observations of hiccups in reptiles, amphibians or birds.

That pattern points to two competing explanations. One looks a long way back. Writing in Scientific American, the biologist Neil Shubin describes tadpoles, which breathe with both gills and lungs. When a tadpole breathes with its gills, it shuts the glottis to close off the breathing tube while taking a sharp breath, which in Shubin's words is an extended form of hiccup. On that view, our hiccups are an echo of ancestors who lived in water. Howes proposes something closer to home. Since hiccups belong to mammals, and mammals are the animals that suckle, he suggests the reflex evolved to help babies get rid of air swallowed while feeding, a kind of built-in burp that makes room for more milk.

Nobody has settled which idea is right, and both may hold part of the answer. What is clear is that the reflex is old, that it arrives before we are born, and that it has outlasted whatever job it was first built for. So the next time a hiccup ambushes you mid-sentence, you are not malfunctioning. You are running a very old piece of equipment that simply switched itself on at the wrong moment.