Why do we have hiccups?
The first thing to bear in mind is that only mammals hiccup. The amphibians do not hiccup, the lizards do not hiccup, the birds do not hiccup (though parrots can perfectly imitate human hiccups). A bird cannot hiccup as it does not have the diphragm; it has air sacs. Their lungs do not expand and contract as they are ventilated, they do not have alveoli, rather they have air capillaries; they also have two respiratory cycles instead of one. It is a very advanced design, much more efficient than mammalian breathing in transfering oxygen, and at slower respiration rate. http://icb.oxfordjournals.org/cgi/content/abstract/28/2/739
The reptiles do have the diaphragm, but they do not have the diaphragm muscle, so there is nothing to contract causing a hic. The contraction of the lungs is by moving the rib cage or the viscera (in turtles). The frogs have no diaphragm in their coelom; they do not have the ribs either. All internal organs are in a single compartment. So only mammals have a diaphragm and the muscle that can be used to have a hic - and so we have hiccups, even in utero. Why? Lately, there has been this greately advertised explanation which I find utterly ridiculous
...The hiccup reflex is a stereotyped twitch that involves a number of muscles in the body wall, diaphragm, neck, and throat. A reflexive firing of one or two of the major nerves that control breathing causes those various muscles to contract. This results in a very sharp inspiration of air. Then, about 35 ms later, a flap of tissue in the back of the throat (the glottis) closes the top of the airway. The fast inhalation followed by a brief closure of the air tube produces the "hic."
...The problem is that the brain stem, originally controlling breathing in fish, has been jury-rigged to work in mammals. Sharks and bony fish respire using muscles in the throat and around the gills. The nerves that control those areas all originate in a well-defined portion of the brain stem. That works well in fish, but it is a lousy arrangement for mammals. In fish, the nerves that control breathing do not have very far to travel from the brain stem. The gills and throat generally surround that area of the brain. We mammals have a different layout; our breathing is carried out by muscles much farther away. For example, the major nerve that controls contraction of the diaphragm--the phrenic nerve--exits the brain stem from the base of the skull, just as it does in fish. Then, however, it extends all the way down through the neck and the chest cavity to reach the diaphragm. That long path through soft tissue is exposed and vulnerable; a rational design would have the nerve travel through the protective spinal column and emerge nearer the diaphragm.
..Hiccups seem to be controlled by their own pattern generator in the brain stem: stimulate it, and you stimulate hiccups. It turns out that this pattern generator is virtually identical to one found in tadpoles. Tadpoles use both lungs and gills to breathe, and this pattern generator is active when they breathe with gills. In that circumstance, a tadpole needs to pump water into its mouth and throat and then out across the gills, but it must also prevent the water from entering its lungs. To keep out the water, it closes the glottis, the flap that can seal off the breathing tube. The central pattern generator in the tadpole brain stem ensures that an inspiration is followed immediately by a closing glottis. They can breathe with their gills thanks to an extended form of hiccup. (Shubin)
http://www.scientificamerican.com/article.cfm?id=what-causes-hiccups&page=2
So, why only mammals hiccup? One idea is that we need it during the development
...hiccups and amphibian gulping are inhibited by elevated CO2 and can be completely stopped by the drug Baclofen (a GABAB receptor agonist), [possibly] illustrating a shared physiology. These proposals would explain why premature infants spend 2.5% of their time hiccuping, indeed they are gulping just like amphibians, as their lungs are not yet fully formed. http://gut.bmj.com/cgi/content/extract/41/5/712
Alternatively, it is
...the habit adapted to a new use - helping mammals learn to suckle. The sequence of movements during suckling is similar to hiccuping, with the glottis closing to prevent milk entering the lungs. http://www.ncbi.nlm.nih.gov/pubmed/12539245
I believe these phylogenetic rationales to be incorrect. Baclofen is a general antispastic. Elevated CO2 stops a lot of other things: it is a life treathening situation, and the priorities change. Adult frogs do not hiccup and contraction of the diaphragm muscles has no analogy in animals lacking it. It is astonishing that such an obviously incorrect idea is repeated time and again; this shows the level of despair. Naturally, nobody knows how powered diaphragm has evolved. One idea is that it allowed higher metabolical rates, see
http://compphys.bio.uci.edu/bennett/pubs/76.pdf
That does not explain its origin, though; it just identifies the need it served in warm-blooded mammal-like reptiles. Another idea is that it allowed disconnection betweenthe rib cage movements and lungs during walking. Yet another idea is that it had a nontrivial original role, and this brings us to the title subject. Nobody seemed to make the connection so far. In mammals, the diaphragm serves two functions: moving air and stopping gastric contents from refluxing into the oesophagus.
...Did a respiratory muscle become recruited by the digestive system, or did a gastrointestinal muscle become adapted for ventilating the lungs? One might intuitively assume that the respiratory function came first; the contraction of the crural diaphragm during inspiration exists precisely because of the negative pressure caused by the contraction of the costal diaphragm. Although it is tempting to assume this apparently obvious evolutionary history of the diaphragm, the alternative case, that the diaphragm initially had a purely gastrointestinal function, must be considered.
...Some hints that the latter case may be true come from a somewhat unlikely source; the pipid frogs. Keith (1905) felt that this was where the precursor of the mammalian diaphragm is to be found. He suggested that two muscles found in Xenopus laevis corresponded to what we now call the crural and costal diaphragm in mammals. These muscles were innervated by nerves emerging from the cervical plexus, much as the phrenic nerve of mammals does. Keith also believed that these muscles were not involved in respiration. Neither did he seem to consider them to have a gastrointestinal function. Instead, he believed that they were involved in the circulatory system, acting in such a manner as to increase venous pressure and aid the return of blood from the abdomen to the heart. Snapper concluded that the function of this muscle is to move air out of the lungs during expiration. However, electrical stimulation of the muscle between breaths was found to produce a very slight increase in pulmonary pressure... During the emetic act Xenopus reduces abdominal volume by sliding the sacrum rostrocaudally along the ilium. It is of interest that one attachment of the ‘crural muscle’ in Xenopus is the ilium. Therefore, this muscle could quite conceivably provide the mechanics to re-internalize the prolapsed stomach. The crura may also have a role in keeping live prey in the stomach... If it is a homologous structure, it would allow us to gain an understanding of the evolutionary origins of the diaphragm, which will in turn lead to insights into how the diaphragm functions in modern mammals. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1570921
Perhaps hiccups have little to do with the functioning of the lungs and the nonexistent tadpole stage; it is not the primary function of the diaphragm and the reflex. Its primary function might be making sure that live prey does not escape from the stomach. It bodes well with my experience. Eating too much or too quickly creates the imitation of movements of live prey in the stomach. The prey has to be subdued before it gets out. Hiccups are a means to this worthy aim. Holding breath and counting to ten works because it suggests the body that the prey is dead. Drinking water works because it drawns the prey. Frightening also works, because subduing prey, while important, is not everything to life.
The reptiles and the amphibians have teeth, but almost never use it for chewing; the preferred method is swallowing one's food whole; the teeth are mainly to prevent the escape of the prey. Chewing is largely a mammalian innovation. Well, perhaps that's not how it started. The ancestral way of eating was also swallowing whole.
Why do we hiccup?
The reptiles do have the diaphragm, but they do not have the diaphragm muscle, so there is nothing to contract causing a hic. The contraction of the lungs is by moving the rib cage or the viscera (in turtles). The frogs have no diaphragm in their coelom; they do not have the ribs either. All internal organs are in a single compartment. So only mammals have a diaphragm and the muscle that can be used to have a hic - and so we have hiccups, even in utero. Why? Lately, there has been this greately advertised explanation which I find utterly ridiculous
...The hiccup reflex is a stereotyped twitch that involves a number of muscles in the body wall, diaphragm, neck, and throat. A reflexive firing of one or two of the major nerves that control breathing causes those various muscles to contract. This results in a very sharp inspiration of air. Then, about 35 ms later, a flap of tissue in the back of the throat (the glottis) closes the top of the airway. The fast inhalation followed by a brief closure of the air tube produces the "hic."
...The problem is that the brain stem, originally controlling breathing in fish, has been jury-rigged to work in mammals. Sharks and bony fish respire using muscles in the throat and around the gills. The nerves that control those areas all originate in a well-defined portion of the brain stem. That works well in fish, but it is a lousy arrangement for mammals. In fish, the nerves that control breathing do not have very far to travel from the brain stem. The gills and throat generally surround that area of the brain. We mammals have a different layout; our breathing is carried out by muscles much farther away. For example, the major nerve that controls contraction of the diaphragm--the phrenic nerve--exits the brain stem from the base of the skull, just as it does in fish. Then, however, it extends all the way down through the neck and the chest cavity to reach the diaphragm. That long path through soft tissue is exposed and vulnerable; a rational design would have the nerve travel through the protective spinal column and emerge nearer the diaphragm.
..Hiccups seem to be controlled by their own pattern generator in the brain stem: stimulate it, and you stimulate hiccups. It turns out that this pattern generator is virtually identical to one found in tadpoles. Tadpoles use both lungs and gills to breathe, and this pattern generator is active when they breathe with gills. In that circumstance, a tadpole needs to pump water into its mouth and throat and then out across the gills, but it must also prevent the water from entering its lungs. To keep out the water, it closes the glottis, the flap that can seal off the breathing tube. The central pattern generator in the tadpole brain stem ensures that an inspiration is followed immediately by a closing glottis. They can breathe with their gills thanks to an extended form of hiccup. (Shubin)
http://www.scientificamerican.com/article.cfm?id=what-causes-hiccups&page=2
So, why only mammals hiccup? One idea is that we need it during the development
...hiccups and amphibian gulping are inhibited by elevated CO2 and can be completely stopped by the drug Baclofen (a GABAB receptor agonist), [possibly] illustrating a shared physiology. These proposals would explain why premature infants spend 2.5% of their time hiccuping, indeed they are gulping just like amphibians, as their lungs are not yet fully formed. http://gut.bmj.com/cgi/content/extract/41/5/712
Alternatively, it is
...the habit adapted to a new use - helping mammals learn to suckle. The sequence of movements during suckling is similar to hiccuping, with the glottis closing to prevent milk entering the lungs. http://www.ncbi.nlm.nih.gov/pubmed/12539245
I believe these phylogenetic rationales to be incorrect. Baclofen is a general antispastic. Elevated CO2 stops a lot of other things: it is a life treathening situation, and the priorities change. Adult frogs do not hiccup and contraction of the diaphragm muscles has no analogy in animals lacking it. It is astonishing that such an obviously incorrect idea is repeated time and again; this shows the level of despair. Naturally, nobody knows how powered diaphragm has evolved. One idea is that it allowed higher metabolical rates, see
http://compphys.bio.uci.edu/bennett/pubs/76.pdf
That does not explain its origin, though; it just identifies the need it served in warm-blooded mammal-like reptiles. Another idea is that it allowed disconnection betweenthe rib cage movements and lungs during walking. Yet another idea is that it had a nontrivial original role, and this brings us to the title subject. Nobody seemed to make the connection so far. In mammals, the diaphragm serves two functions: moving air and stopping gastric contents from refluxing into the oesophagus.
...Did a respiratory muscle become recruited by the digestive system, or did a gastrointestinal muscle become adapted for ventilating the lungs? One might intuitively assume that the respiratory function came first; the contraction of the crural diaphragm during inspiration exists precisely because of the negative pressure caused by the contraction of the costal diaphragm. Although it is tempting to assume this apparently obvious evolutionary history of the diaphragm, the alternative case, that the diaphragm initially had a purely gastrointestinal function, must be considered.
...Some hints that the latter case may be true come from a somewhat unlikely source; the pipid frogs. Keith (1905) felt that this was where the precursor of the mammalian diaphragm is to be found. He suggested that two muscles found in Xenopus laevis corresponded to what we now call the crural and costal diaphragm in mammals. These muscles were innervated by nerves emerging from the cervical plexus, much as the phrenic nerve of mammals does. Keith also believed that these muscles were not involved in respiration. Neither did he seem to consider them to have a gastrointestinal function. Instead, he believed that they were involved in the circulatory system, acting in such a manner as to increase venous pressure and aid the return of blood from the abdomen to the heart. Snapper concluded that the function of this muscle is to move air out of the lungs during expiration. However, electrical stimulation of the muscle between breaths was found to produce a very slight increase in pulmonary pressure... During the emetic act Xenopus reduces abdominal volume by sliding the sacrum rostrocaudally along the ilium. It is of interest that one attachment of the ‘crural muscle’ in Xenopus is the ilium. Therefore, this muscle could quite conceivably provide the mechanics to re-internalize the prolapsed stomach. The crura may also have a role in keeping live prey in the stomach... If it is a homologous structure, it would allow us to gain an understanding of the evolutionary origins of the diaphragm, which will in turn lead to insights into how the diaphragm functions in modern mammals. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1570921
Perhaps hiccups have little to do with the functioning of the lungs and the nonexistent tadpole stage; it is not the primary function of the diaphragm and the reflex. Its primary function might be making sure that live prey does not escape from the stomach. It bodes well with my experience. Eating too much or too quickly creates the imitation of movements of live prey in the stomach. The prey has to be subdued before it gets out. Hiccups are a means to this worthy aim. Holding breath and counting to ten works because it suggests the body that the prey is dead. Drinking water works because it drawns the prey. Frightening also works, because subduing prey, while important, is not everything to life.
The reptiles and the amphibians have teeth, but almost never use it for chewing; the preferred method is swallowing one's food whole; the teeth are mainly to prevent the escape of the prey. Chewing is largely a mammalian innovation. Well, perhaps that's not how it started. The ancestral way of eating was also swallowing whole.
Why do we hiccup?