November 15th, 2022
Written by: Sophie Liebergall
What’s your go-to remedy when you get a case of the hiccups? Do you hold your breath for 10 seconds? Or 30? Do you try to chug a glass of water? Or even try to hang upside down? Despite our best efforts, hiccups generally persist until they finally seem to peter out. Hiccupping, perhaps more so than any other human behavior, feels like something distinctly out of our voluntary control. And we usually don’t know exactly what sets it off.
So what exactly is hiccupping and what is the reason for this strange involuntary behavior? The answer may actually lie in the mysteries of the brain! Our brain controls breathing by sending signals to a large, flat muscle underneath our lungs called the diaphragm. When the diaphragm contracts, it pulls down our lungs so that they inflate with air. Usually this process occurs smoothly, often without having to consciously think about it. But occasionally the brain will tell the diaphragm to abruptly contract. This sucks air down towards the lungs so fast that the muscles in our throat get caught off guard. Our vocal cords then snap shut, causing what we call a hiccup.1
We know that hiccups can be caused by swallowing excessive air, acidity in the stomach, and even prolonged bouts of laughter. And sometimes very serious and persistent spells of hiccups can be caused by strokes and tumors in the brain.2 But, unlike more intuitively understood reflexes like blinking and sneezing, neuroscientists remain unsure about the reasons why we have this often-uncomfortable diaphragmatic reaction. Nevertheless, this hasn’t stopped scientists from coming up with evolutionary theories for the reason why we hiccup, and one of the leading theories might actually reach as far back as our gilled, water-inhabiting ancestors.
Hiccupping is a phenomenon that occurs in many mammals, including cats, rabbits, and horses. And though hiccupping occurs relatively infrequently for most mature humans, human fetuses (who spend their time in an essentially aquatic environment in the womb) spend up to 2.5% of their day actively hiccupping. The frequency of hiccupping then slowly decreases through infancy into childhood.
In the early 2000s, a group of researchers proposed that there is, in fact, no purpose for hiccupping in modern humans.3 Instead, hiccupping is a vestigial behavior that is a remnant of the movement patterns that occur when breathing through the gills of our fish- or frog-like ancestors. A vestigial behavior is something that originally served a purpose in one of our ancestors, but that has lost this purpose over the course of evolution.4 These researchers argue that hiccups have many properties that are similar to the breathing patterns of organisms with gills. Amphibians such as tadpoles take a breath through a short, sharp contraction of their breathing muscles that closely resembles hiccupping. And both hiccups and breathing through gills can be inhibited by high concentrations of carbon dioxide and by muscle relaxant drugs that bind GABA-B receptors.3
There are other competing theories of the origin of hiccupping, such as the “burping” hypothesis. The burping hypothesis argues that hiccups serve a function similar to a burp: they help expel air that accumulates in the stomach when a mammalian infant is suckling milk from their mother.2 It is still under debate whether hiccuping is a vestigial behavior. But if it is, it is the not only behavior humans borrowed from our evolutionary ancestors.
Another example of a vestigial behavior in humans is called the hypnic jerk. Hypnic jerks (also called hypnagogic jerks or myoclonic jerks) occur when someone experiences a sudden contraction of a part of their body – usually their arms or legs – just as they’re drifting off to sleep.5 After a hypnic jerk people often feel the eerie sensation that they are falling before abruptly waking up. Hypnic jerks are merely disruptive and scary for the modern human that is settling onto their cushy mattress for an expectedly stable night of sleep. But some scientists have postulated that the hypnic jerk is a protective reflex that aided the survival of pre-modern humans. When pre-modern humans made a habit of sleeping in trees, they may have developed a startle reflex so that they could quickly act to grab a branch if their body sensed that they were falling. It is possible that hypnic jerks may be a remnant of this branch-grabbing reflex. As the body relaxes as it drifts off to sleep, the brain sometimes mistakes this relaxation for falling and instructs the limbs to reach out to save itself from a fall.
A recent study from a group of audiology researchers revealed another vestigial reflex in humans.6 You may have noticed that your pet cat or dog or horse often perks their ears in the direction of a sound, especially in response to startling or interesting sounds. Throughout evolution, humans have lost this ability to point their more rigid ears. These researchers, however, showed that humans may indeed retain a remnant of this ability from their distant ancestors. In their study, the researchers measured the electrical activity of the small head muscles that attach to the ears. Surprisingly, when a participant was presented with an unexpected sound through a speaker, the researchers recorded more electrical activity in the ear closest to the speaker. The muscles of the left ear were more active when the speaker was placed on the left side of the participant, and correspondingly the muscles of the right ear were more active when the speaker was placed on the right. This study served as evidence that the brain reflex that underlies ear perking is preserved in humans, even if it serves no clear purpose because we have lost the ability to noticeably move our ears.
Though the brain is an incomprehensibly complex and astoundingly efficient machine, it is important to remember that it is the product of a long and winding evolutionary process. Our brains have evolved to carry out sophisticated tasks such as naturally producing a shared language, creating medical technology that has conquered nearly all of our biological predators, and probing the depths of the nanoworld and the cosmos. Nevertheless, we may still sometimes fall victim to a spell of the hiccups if we let a bout laughter get out of control!
References
1. Becker, D. E. Nausea, Vomiting, and Hiccups: A Review of Mechanisms and Treatment. Anesthesia Progress 57, 150–157 (2010).
2. Howes, D. Hiccups: A new explanation for the mysterious reflex. Bioessays 34, 451–453 (2012).
3. Straus, C. et al. A phylogenetic hypothesis for the origin of hiccough. Bioessays 25, 182–188 (2003).
4. Sarnat, H. B. Transitory and Vestigial Structures of the Developing Human Nervous System. Pediatr Neurol 123, 86–101 (2021).
5. Vetrugno, R. & Montagna, P. Sleep-to-wake transition movement disorders. Sleep Medicine 12, S11–S16 (2011).
6. Strauss, D. J. et al. Vestigial auriculomotor activity indicates the direction of auditory attention in humans. eLife 9, e54536 (2020).
Cover image made with BioRender.com.
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