I have long wondered about the evolutionary or adaptive value of hiccups. As a child I used to hiccup a lot and they seemed strange and inexplicable. Some recent experiences helped me to see that hiccups may play a role in swallowing. More precisely they may serve to help mammals recover from choking by clearing the esophagus. On several occasions I have had a mouthful of food block my esophageal airway. When this happens with a particularly large mouthful, it can partially occlude the tracheal airway making it hard to breathe. Several times now I have been saved by a hiccup. The hiccup literally squeezed the food down my esophagus and into my stomach.
A hiccup is like a cough in reverse. During a cough the diaphragm expels air from the lungs as the glottis opens. During a hiccup the diaphragm pulls air into the lungs while the glottis is closed and I firmly believe that this can pull food down the esophagus. Most people experience several hiccups in a row usually not coinciding with a meal or with choking. Perhaps these normal hiccups are the body practicing this important reflex. I would be very interested to know how many other people have experienced a hiccup while choking and had their airway cleared because of it.It happened to me again today. I was eating way too much food, way too fast and I had an uncomfortable feeling in my throat. It is a strong sensation that usually induces a bit of panic. This often happens when I eat something dry without drinking anything. Have you ever eaten a hard-boiled egg first thing in the morning? When this happens I always hiccup, but only a couple of times. The hiccuping tends to push the food down my esophagus, relieving me of the choking feeling. Could most cases of the hiccups represent misfirings of this important, hair-trigger reflex meant to keep our airway clear?
Important reflexes come out sometimes even when they are not needed. At times we salivate despite the fact that we are not about to eat. We often stress out, releasing adrenaline and cortisol despite the fact that we are not about to engage in strenuous physical activity. For a long time now I have concluded that random bouts of hiccuping are triggered accidentally but represent a means to constrict the middle respiratory tract forcing large boluses of food down so that they do not obstruct the trachea. I can think of many ways to test this hypothesis experimentally, anyone want to help?
Here are some of my favorite books that explain how common human traits are actually adaptive biological responses: