The Ghost of Gills

It begins mid-sentence, or mid-meal, or mid-thought. A small involuntary lurch in the chest, followed by a noise that no one has ever found dignified. Hic. Then another. Then, if you are particularly unlucky, a third and a fourth, spaced with the metronomic cruelty of a leaky faucet. You hold your breath. Someone offers you a glass of water and suggests you drink it upside down. Someone else tries to startle you. None of it really works, and yet everyone present feels compelled to participate, as if the hiccup were a small community emergency.

The strange thing about the hiccup is not its persistence but its pointlessness. Almost every reflex in the human body has a job. The cough clears the airway. The sneeze evicts irritants from the nose. The blink protects the cornea. The gag reflex stops you from choking to death on something you should not have swallowed. These are useful inheritances, sharpened by millions of years of natural selection into reliable defensive routines.

The hiccup defends nothing. It clears nothing. It aids no act of digestion or respiration. It arrives without warning, accomplishes no visible task, and departs on its own schedule. In medical literature it carries the Latin name singultus, which sounds vaguely Roman and important but only means, more or less, a sob. For most of the twentieth century, physicians regarded it as a nuisance with no story attached. It was a tic of the diaphragm, the sort of thing a textbook mentioned in a single paragraph and then moved on from.

And then, in the early 2000s, a small group of French neurologists working at the Pitié-Salpêtrière in Paris asked a question that turned out to have an unexpectedly long answer. If the hiccup is useless, they wondered, why does every human being on earth still have one? Reflexes that survive 370 million years of evolutionary editing usually had a reason to survive. The body does not, as a rule, preserve elaborate neural circuits for the fun of it.

The Anatomy of a Small Spasm

To understand why the hiccup is so peculiar, it helps to slow it down. From the inside it feels instantaneous, a single jolt. In fact it is a small two-act drama with very strict choreography.

The first act is muscular. The diaphragm, the wide sheet of muscle that separates the chest from the abdomen, contracts sharply and without permission. The intercostal muscles between the ribs join in. The chest expands. A gulp of air rushes down the throat. This is the part you do not hear. ¹

The second act, about 35 milliseconds later, is the part everyone else does. The glottis, the small valve at the top of the larynx that opens and closes around the vocal cords, snaps abruptly shut. The incoming column of air hits the closed door. The collision is what we call the hic. ²

Thirty-five milliseconds is an interesting number. It is too fast for conscious control: by the time you notice the spasm, the door is already closing. But it is also weirdly specific. The interval does not vary much between people, or between hiccups in the same person, or even between adults and newborns. The reflex runs on a fixed timer, set somewhere below the level at which thought happens.

That somewhere turns out to be the brainstem, the oldest part of the brain, the part shared, in its essentials, with reptiles and amphibians and fish. The signal travels along the phrenic nerve, which runs from the neck down to the diaphragm, and the vagus nerve, which wanders through the chest and gut. None of this circuitry is new. None of it is particularly human. The hiccup is generated by hardware that was already in place long before humans existed.

A Question Nobody Was Asking

Christian Straus, a respiratory physiologist at the Pitié-Salpêtrière, was studying breathing disorders when he became interested in the hiccup almost by accident. He and his colleagues, including Marie-Noëlle Fiamma and Thomas Similowski, had been mapping the neural rhythms that drive normal respiration. The hiccup kept turning up in their data as an anomaly, a rhythm that did not match anything else the diaphragm did.

It also did not seem to match anything any other mammal did. Dogs do not hiccup as a regular feature of life. Adult cats hiccup rarely. The reflex is most reliably observed in human infants, in mammalian fetuses, and, with a few exceptions, in young animals that are still developing their breathing apparatus. In adult humans it appears mostly when something unusual is happening to the digestive tract: a fast meal, a carbonated drink, a sudden swallow of air, a stretched stomach.

Straus’s group published a paper in 2003 in the journal Bioessays titled, with the dry grandeur of a French academic title, “The whys of the hiccup.” The paper made a striking proposal. The hiccup, they argued, was not a malfunction of the mammalian respiratory system. It was a surviving fragment of a different respiratory system entirely. ³

To make their case, they did something most studies of human reflexes do not do. They went looking for the closest mechanical match in the rest of the animal kingdom. The match they found was not in any mammal. It was in a tadpole.

The Tadpole in the Diaphragm

Tadpoles, like the larvae of many amphibians and the embryonic forms of certain fish, breathe through gills. The process is not as passive as it sounds. Water does not simply drift across gill tissue and deposit its oxygen there. The animal has to actively move water through the chamber where its gills sit, and it does this by a maneuver that biologists call buccal pumping or gill ventilation.

The sequence runs as follows. The tadpole opens its mouth and draws in a quantity of water. Then it closes a flap at the back of the throat, the glottis or its functional equivalent, to prevent the water from running on into the lungs. Then the muscles of the throat and the trunk contract to force the water sideways across the gills, where dissolved oxygen is extracted before the water is expelled. ⁴

In outline, that is precisely the hiccup. A sharp inhalation, a sudden glottal closure, a fixed brief delay between the two. The same muscles, more or less; the same nerve pathways, more or less; the same timing. The mammalian version no longer moves water across gills, because the gills are gone and the animal in question is breathing air. But the underlying motor pattern, the rhythm written into the brainstem, has not noticed. It still runs the old routine.

Straus and his colleagues argued that this was not coincidence. The hiccup, they proposed, was a vestigial echo of gill ventilation, preserved in the central pattern generators of the brainstem because the circuitry was never costly enough for evolution to bother dismantling. Lungs were grafted on top, but the old wiring stayed. Every time you hiccup, you are running, however briefly, a program written for an animal that lived in water.

The oldest creatures known to have had this kind of dual respiratory system, capable of both gill ventilation and primitive air breathing, lived in the late Devonian period, roughly 370 million years ago. They were lobe-finned fish, the lineage from which the first four-limbed land animals would eventually emerge. They are, in a meaningful sense, your great-grandparents, several hundred million times removed. The reflex you cannot stop after dinner is one of the things they passed down.

The Womb Rehearsal

A vestigial-organ story would be tidy on its own, but Straus had a second hypothesis, and other researchers have since extended it. The hiccup may not be quite as useless as it looks, at least not for everyone.

Fetuses hiccup. They hiccup constantly. Ultrasound studies have observed hiccupping in human fetuses as early as nine weeks of gestation, well before the lungs are functional and well before there is any air to breathe. By the third trimester, hiccupping accounts for a measurable fraction of fetal activity. Some estimates suggest that a late-stage fetus may spend roughly two and a half percent of its day producing hiccup-like contractions of the diaphragm. ⁵

This is a great deal of effort for an animal that has, by all reasonable measures, nothing to gain from it. The fetus is not breathing. There is no food in its stomach to displace. The gill-ventilation circuit, if that is what the hiccup truly is, has no water to move. And yet the reflex fires, hour after hour, week after week, all the way through development.

In 2019, a team led by the neuroscientist Lorenzo Fabrizi at University College London proposed a possible reason. Fabrizi’s group studied 13 newborn infants, some born prematurely, using electroencephalography to record the electrical activity of their brains while they hiccupped. Each hiccup, they found, produced a substantial wave of activity in the cortex. The signal was not random noise. It was a discrete, repeatable response, the kind that would arise if the brain were treating each hiccup as a piece of information. ⁶

Fabrizi’s interpretation was that the developing brain may be using the hiccup as a kind of internal training signal. Each contraction of the diaphragm sends a sensory pulse upward into the cortex, which is in the slow process of learning where the body’s parts are and how they move. The hiccup, on this view, is a clean, regular, easy-to-isolate input. It tells the brain something like: here is the muscle that controls breathing, here is what it feels like when it fires, here is where it sits in the body’s map. The fetus is rehearsing.

This does not necessarily contradict the gill-ventilation theory. It may be that the reflex originated, hundreds of millions of years ago, as a way to move water across gills, and that it was then quietly conscripted by mammalian development into a second, later role, calibrating the diaphragm during the months before birth. Evolution is famously parsimonious about throwing things away. Useful old machinery often acquires new jobs without losing its original shape.

What all of this means is that you spent a non-trivial portion of your prenatal life performing, over and over, a motion first invented by something with gills. By the time you were born, the rehearsal was no longer strictly necessary. The wiring, however, did not go anywhere.

Why It Returns

After infancy, the hiccup loses whatever developmental usefulness it may have had. The diaphragm is calibrated. The cortex has its map. The reflex, however, remains on standby, easy to provoke and difficult to suppress, lurking in the brainstem like an old subroutine no one bothered to delete.

The triggers are familiar to anyone who has ever eaten too quickly. A stretched stomach. A swig of carbonated water. A large gulp of air swallowed alongside food. Sudden changes in temperature, like a glass of ice water on a warm afternoon. Strong emotions, surprisingly, can do it too. Alcohol is a particularly reliable culprit, both because it relaxes the diaphragm and because people drinking it tend to swallow air. In each case, sensory information from the gut and the chest reaches the brainstem and, for reasons that are not entirely clear, jostles the dormant pattern generator into firing.

Most hiccup episodes are brief. The reflex runs for a few minutes, the trigger passes, the circuit quiets, and life resumes. Folk remedies for stopping hiccups exist in nearly every culture and almost certainly outnumber the genuine cures. Holding the breath raises carbon dioxide levels in the blood, which can sometimes suppress the diaphragmatic rhythm. Drinking water from the far side of the glass requires unusual breath control, which may do the same. Being startled probably works mostly by distraction. Pulling on the tongue, swallowing dry sugar, gargling vinegar, sneezing on command: these are mostly traditions, occasionally effective, mostly not.

For a small number of unfortunate people, however, the hiccup does not stop. It runs for hours, then days, then weeks. Doctors classify hiccups lasting longer than 48 hours as persistent, and those lasting longer than a month as intractable. The causes vary. Some intractable hiccups are produced by tumors or lesions pressing on the vagus or phrenic nerves. Some follow strokes that damage the brainstem itself. Some appear without any identifiable cause at all. ⁷

The most famous case in the medical literature is that of Charles Osborne, an Iowa farmer who began hiccupping in 1922, reportedly while trying to weigh a hog before slaughter, and continued to hiccup almost without interruption for the next 68 years. At its peak, his hiccup rate was about 40 a minute. Over the decades it slowed to around 20. He married, fathered children, lived a more or less normal life, and was treated by dozens of doctors with everything from sedatives to experimental surgeries. Nothing worked. The hiccups stopped on their own in 1990, about a year before his death. No one ever fully explained why they started, or why they ended. ⁸

Cases like Osborne’s underscore the most uncomfortable truth about the hiccup, which is that it is genuinely difficult to switch off. You cannot easily reason with it. You cannot easily medicate it. It lives below the level of language and intention, in tissue much older than the parts of the brain that do thinking. When the circuit decides to fire, the rest of you mostly has to wait for it to finish.

An Inheritance We Do Not Get to Refuse

It is worth pausing on what this means about the body more generally. The hiccup is not the only ancient routine running quietly inside a modern animal. Goosebumps are a vestige of fur, an old reflex meant to puff up a thick coat against the cold or against a rival, now firing on hairless skin to no visible effect. The appendix appears to be a much-reduced relic of a longer gut adapted to digesting a coarser diet. The plantaris muscle in the leg, useful for grasping with the foot in tree-dwelling primates, is now so functionally minor that surgeons routinely harvest it for tendon repair without anyone noticing it is missing.

The body, in other words, is not a clean design. It is an accumulation. Each layer was added by natural selection on top of layers that already existed, and the older layers were rarely removed when their original purpose disappeared. They were too entangled with the wiring that worked, too cheap to maintain, or simply too far down to reach without breaking something else. The result is a creature like the human being: largely functional, partly redundant, occasionally absurd, and full of small reflexes whose explanations lie deep in the past.

The hiccup is one of the more endearing examples, because it is so completely visible and so completely involuntary and so completely unmoored from any current purpose. When it arrives at a dinner party, it is doing nothing useful. It is not protecting you. It is not warning you. It is not, despite the persistent suspicion of dinner companions, a comment on what you have just eaten. It is the brainstem, briefly and mistakenly, running an old program for an animal that no longer exists.

Which is, in a quiet way, a remarkable thing to know about yourself. Most of what the body does feels like the body’s own. Heartbeat, breath, hunger, sleep: these are processes that belong, in some intuitive sense, to the person having them. The hiccup belongs to someone else. It belongs to a lobe-finned fish in a Devonian estuary, learning the trick of pulling oxygen out of water by closing a flap at the back of its throat. That fish is long gone, and so is the water, and so are the gills. The motion, somehow, is still here. It is still in you. It surfaces, uninvited, at the worst possible moments, and there is nothing you can do about it except wait for the old circuit to settle back down into the dark.

The next time it happens, the polite thing is to apologize. The honest thing is to listen. Somewhere very deep in the architecture of being alive, an animal you have never met is still breathing.

Sources

1. Straus C, Vasilakos K, Wilson RJ, Oshima T, Zelter M, Derenne JP, Similowski T, Whitelaw WA. A phylogenetic hypothesis for the origin of hiccough. BioEssays, 2003. — https://onlinelibrary.wiley.com/doi/10.1002/bies.10257
2. Howes D. Hiccups: a new explanation for the mysterious reflex. BioEssays, 2012. — https://onlinelibrary.wiley.com/doi/10.1002/bies.201100194
3. Whitelaw WA, Derenne JP. Origin of the hiccup. Canadian Respiratory Journal, 2003. — https://www.hindawi.com/journals/crj/2003/650725/
4. Brainard J. Why do we hiccup? Scientists may finally have an answer. Science (News), 2019. — https://www.science.org/content/article/why-do-we-hiccup-scientists-may-finally-have-answer
5. Whitehead K, Jones L, Laudiano-Dray MP, Meek J, Fabrizi L. Event-related potentials following contraction of respiratory muscles in pre-term and full-term infants. Clinical Neurophysiology, 2019. — https://www.sciencedirect.com/science/article/pii/S1388245719312337
6. Chang FY, Lu CL. Hiccup: mystery, nature and treatment. Journal of Neurogastroenterology and Motility, 2012. — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3325297/
7. Associated Press. Charles Osborne, 96; Hiccupped for 68 Years. Los Angeles Times, 1991. — https://www.latimes.com/archives/la-xpm-1991-05-06-mn-1804-story.html
8. Provine RR. Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond. Harvard University Press, 2012. — https://www.hup.harvard.edu/books/9780674284111
9. Clack JA. Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press, 2012. — https://iupress.org/9780253356758/gaining-ground-second-edition/