Hiccups, or singultus, are a common, sudden, and involuntary reflex involving the respiratory system. They are characterized by a rapid, spasmodic contraction of the diaphragm muscle, the thin, dome-shaped muscle separating the chest cavity from the abdomen. The resulting quick intake of air is abruptly stopped by the sudden closure of a structure in the throat, creating the distinctive “hic” sound. Though often considered a harmless nuisance in humans, the exact purpose of this reflex, which occurs across the animal kingdom, remains a long-standing biological puzzle.
The Physiological Mechanism of a Hiccup
The mechanism of a hiccup is governed by a precise, involuntary reflex arc involving specific nerves and muscles. The process begins with an irritation along this arc, which includes the phrenic nerve (controlling the diaphragm) and the vagus nerve (carrying sensory information from the digestive tract). This irritation sends a signal to the brainstem.
This irritation triggers an efferent signal that causes the diaphragm to contract sharply and suddenly. This spasm forces a rapid, unscheduled inhalation of air into the lungs. Almost instantaneously, about 35 milliseconds later, the glottis—the opening between the vocal cords in the larynx—slams shut.
The closure of the glottis prevents the rush of inhaled air from entering the lungs, and the air hitting the closed vocal cords produces the characteristic “hic” noise. This sequence is an automated, rhythmic motor action that the body cannot consciously override, making it a true reflex.
Which Animals Experience Hiccups?
Hiccups are a reflex primarily observed in mammals, a biological limitation rooted in respiratory anatomy. The diaphragm is the defining muscular structure required for this specific reflex, and it is a feature unique to the mammalian class. Therefore, nearly every species of mammal, including pets and livestock, has the capacity to hiccup.
Documented instances range from common animals like dogs, cats, horses, and rats, to more exotic species such as squirrels and otters. Kittens and puppies often experience bouts of hiccups, sometimes because they eat or drink too quickly. Animals like birds, reptiles, and fish lack a true diaphragm and have fundamentally different respiratory systems, meaning they do not exhibit the same physiological hiccup reflex.
The Evolutionary Theories of Why Hiccups Exist
The persistence of the hiccup reflex in adult mammals, despite its apparent lack of function, has led to two dominant evolutionary hypotheses. One theory, known as the “Ancient Reflex” or “Amphibian Hypothesis,” suggests that the hiccup is a vestigial remnant of our aquatic ancestry. The neural circuitry involved in hiccupping resembles the motor pattern used by early tetrapods, such as tadpoles, who possess both lungs and gills.
This reflex in amphibians coordinates the simultaneous act of breathing air and swallowing water. It uses gill slits for water flow while sealing the glottis to prevent water from entering the developing lungs. Proponents suggest that the mammal hiccup is simply a conserved, functionally useless neural program left over from this transitional stage of evolution.
A second, more mammal-specific hypothesis is the “Stomach Air Clearance” theory, which posits that the reflex evolved to benefit suckling young. In infant mammals, the complex coordination of breathing, swallowing, and suckling often results in air being gulped into the stomach along with milk.
When triggered by an air bubble irritating the stomach lining, the hiccup reflex functions to forcefully contract the diaphragm. This creates a vacuum that helps expel the trapped air, essentially acting as a burp. Clearing the air maximizes the space available for calorie-rich milk, offering a nutritional advantage to young mammals.
Fetal and Infant Hiccups: A Distinct Developmental Function
Though hiccups may seem pointless in adults, they play a distinct role during early development, long before birth. Hiccups are one of the earliest motor patterns established, observed in fetuses as young as nine weeks old within the womb.
In newborns, particularly premature infants, hiccups are frequent, sometimes occurring for up to 15 minutes a day. Research shows that each hiccup contraction triggers a large wave of electrical activity in the brain’s cortex. This response helps the developing brain create and strengthen the neural pathways necessary to monitor and control the respiratory muscles.
The hiccup reflex is believed to serve as practice for the diaphragm, ensuring the muscle is functional and ready for sustained breathing after birth. By linking the physical sensation of the muscle contraction with the sound of the “hic,” the brain learns to process these multisensory inputs, aiding the development of voluntary breathing control and neurological maturation.