The Chestburster is a Xenomorph infant (or Nymph because it resembles the adult and is not seen forming a cocoon to reach maturity). An egg is laid in the victim by a Facehugger (Chet, a Predalien (caste) , was known to insert embryos directly into a host without a Facehugger). It is speculated that it develops in a manner similar to that of the immature heartworm (Dirofilaria immitis) that migrates through the body absorbing nutrients before establishing itself. Far from the pharynx it's unknown if the embryo travels to the host's stomach or to the bronchial tubes. Another theory says that the Alien begins its life in the host body not as an embryo but as a tumor that later takes the shape of an immature Alien. In this theory, what the Facehugger injects in the host is not an infant Xenomorph but a small egg that inserts its proteins in the host DNA and commands its cells to "create" the chestburster which has both Xenomorph proteins and those of the host.
When fully developed, the creature releases enzymes that softens the bone and surrounding tissues and begins chewing through the sternum. This causes the victim great pain, shock, and blood loss. Then, as its name suggests, it emerges from the chest. If far away from the hive, it cannot depend on the adults and so it actively searches for a safe location and/or food source (examples include a nursery, chicken coop, and food locker). The newborn alien requires a location with a large amount of organic or inorganic matter (such as metals) which it softens with its acidic fluids and directly converts into body mass. It also will attack and kill humans and feed on them. They will consume any part of the human victim (such as facial extremities or genitals), but prefers internal organs such as the brain, liver, or heart. Then after a few hours to a few days, it reaches maturity as an adult Alien.
The host's characteristics determine the chestburster's features. The Predalien chestburster, for example, sports the signature Yautja mandibles (and grows the dreadlocks later), while quadrupedal hosts produce quadrupedal chestbursters (whereas chestbursters spawned from humans may not even have forelimbs). AVP Extinction suggests that genetic alteration might allow a Runner chestburster to split into identical twins after escape from a quadruped.
An embryo implanted in a pregnant woman will feed directly on the fetus and/or placental fluids, and the chestburster will erupt from the stomach of the woman. They often raise themselves to a foot high and are 36 inches long. Some chestbursters are born with arms as seen in Aliens, but most are not. They are very quick and in tune with their surroundings once emerged. If in a populated room they will immediately seek an escape (i.e. through open doors, air ducts or vents; possibly going as far as burning a hole in the wall or floor or ripping off or through grate coverings). Aliens: Steel Egg also shows that they do have regular body processes. One of the crew found a pile of scat with a human tooth inside of it while exploring the ventilation system. The Predalien have the mandibles of their Predator hosts after they emerge and eventually grow the characteristic dreadlocks as well.
Appearance and characteristics
In Alien, it looked like a worm or snake though it had stubs where presumably the arms could be, it had metallic looking teeth and was very fast despite having no arms or legs. In the sequel Aliens, it possessed arms which Cameron said would help it push it's way out of the body, in-universe it could be explained by proper development. Alien 3 had the chestburster fully formed, with arms and legs, it was near enough just a smaller version of what it later becomes. Alien: Resurrection has the chestburster similar to the original but with a wider mouth, this design would be carried over to the AVP films.
/ Alien (novel) (first appearance)
/ Aliens (novel)
- Aliens (Series 1)
- Aliens: Earth Hive
- Aliens (Series 2)
- Aliens: Nightmare Asylum
- Aliens: Earth War
- Aliens: The Female War
- Alien Resurrection
/ Alien Resurrection (comic) / Alien Resurrection (novel)
- Aliens: Steel Egg
- Aliens vs. Predator: Extinction