Defensive vomiting refers to the use of emesis to defend against ingested pathogens or, in animals, against predators.
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In humans
Vomiting serves an evolutionary purpose for humans by preventing the ingestion of something harmful, and by expelling noxious substances once ingested.
Vomiting excessive amounts of alcohol is an attempt by the body to prevent alcohol poisoning and death. Vomiting may also be caused by other drugs, such as opiates, or from toxins found in some foods and plants. Food allergies and sensitivities, such as lactose intolerance, can cause vomiting.
Even morning sickness, nausea and vomiting common to most pregnant women but no other mammals, has a defensive purpose. Morning sickness discourages pregnant women from eating meat and strong-tasting vegetables, which may contain toxins and microorganisms. If ingested, the fetus might be harmed. After around the 18th week, the fetus becomes less vulnerable.
In animals
Turkey vultures will vomit to dispel any disturbing animal. They can propel their vomit up to 10 feet.
The European roller, a much smaller bird found in parts of Africa, Europe, Asia, and the Middle East, uses vomit in a different way. A baby European roller will vomit a foul-smelling orange liquid onto itself to turn away a predator. The smell also warns the parents to return to the nest.