After reading ‘Your Inner Fish’ by Neil Shubin, I wanted to find out more about hiccups. A lot is still unknown about them, however, Shubin suggested a phylogenetic hypothesis.
Hiccups are caused by a disruption to the ventilatory central pattern generator resulting in a spasm in the diaphragm causing a sharp inspiration of air pulling the epiglottis shut, creating the ‘hic’ sound which characterises hiccups.
Although hiccups are only found in mammals, a link has been found to amphibians. Tadpoles are bimodal breathers because they have both lungs and gills. Therefore, they exhibit a motor pattern during gill ventilation, similar to that causing hiccups. Tadpoles gulp water allowing it to wash over their gills, however to do so without the water filling their lungs as well, their glottis has to snap shut immediately after inspiration. Further evidence shows that both hiccups and amphibian gulping can be stopped when the concentration of inhaled air is increased.
Although this seems like good evidence, it is very hard to prove phylogenetic connections. Other reasons for hiccups which have been suggested include to clear air from the stomach of a suckling infant to allow room for more milk. The air bubble could stimulate the contraction of the stomach and oesophagus, triggering the hiccup which creates suction in the chest, pulling the air from the stomach up and out. This is supported by the observation that hiccups are more common in infants.
Sometimes, hiccups can also be caused by infections such as pneumonia or nerve damage.