On temporal structure of human hiccups: ethology and chronobiology.

During a study on the phylogeny and ontogeny of human hiccups, essentially all hiccups to occur over continuous periods of 3m - lly were recorded from 20 healthy people of either sex and various ages. Hiccups displayed considerable structure in time. If more than a few occurred together, they became "established" and a hiccup bout ensued, which did not stop until a certain "minimum" number of hiccups had occurred. For many subjects, two or more bouts frequently occurred on the same hiccup day, even though days on which bouts occurred were weeks apart. Number of hiccups was correlated with age, sex, reproductive status and stage of menstrual cycle, time of day, length of time since previous hiccup day, length of time since previous hiccup bout, and length of hiccup-hiccup intervals within a bout. Findings support the hypothesis that human hiccup is a "fixed action pattern" (FAP)--a central theoretical concept in ethology--and provide leads for further studies aimed at learning the biological function(s) of human hiccup and its possible homologues in other species.