How the 'Alien' Xenomorph Works

For the crew of the Nostromo, it all began with a single egg. So, in order to chart the xenomorph's complex and vicious life cycle, we'll begin there as well.

At first glance, xenomorph eggs closely resemble the amniotic eggs of birds, reptiles and amphibians here on Earth. These vessels protect and nourish an organism during the earliest, frailest stages of development. The xenomorphic egg also seems to perform this function.

The tough shell offers a level of padded protection, and its leathery flesh (which resembles that of certain fish, amphibian and reptile eggs) may help it to survive exceedingly dry conditions. The interior of the xenomorph egg also appears to contain large quantities of fleshy, nutritious yolk, or deutoplasm. This material sustains the life-form until hatching occurs.

For all these similarities, xenomorph eggs are still thoroughly alien. Terrestrial organisms hatch by breaking, slicing or pushing through their egg's outer membrane. In the case of xenomorph eggs, external stimuli (the presence of a suitable host organism) cause four lips at the top of the egg to peel back like a flower, opening an aperture for the emerging organism.

In the absence of such stimuli, xenomorph eggs remain dormant for extended periods. The protrusion of ropy tendrils from the base raises additional questions: Do these structures merely anchor the egg to the spot, or do they gather nutrients for the growing embryo? Are they secreted resin, or are they alive?

And what emerges from those strange eggs? The alien grotesqueries only multiply.

Terrestrial organisms tend to hatch from their eggs as juveniles, which are immature forms that resemble the adult, or as larvae, such as tadpoles or caterpillars, which don't. Many crustaceans even boast prelarval and postlarval forms on the long, transformative road to maturity.

Larvae are morphologically distinct from adults and undergo metamorphosis to reach full adulthood. They often feature limbs and even entire organs absent in the adult stage.

As such, it's no great surprise that the prelarval xenomorph differs drastically from its adult self. While a butterfly exists only to mate and breed, its larval form operates solely to eat and transform. Likewise, the fledgling xenomorph that emerges from the egg pursues a single purpose: the forced impregnation of a suitable host organism.

As we'll explore in the pages ahead, the typical adult xenomorph boasts four limbs. The prelarval xenomorph, or facehugger, however, uses eight fingerlike appendages. The crablike creature lacks anything resembling a true head -- and likewise seems to boast only the simplest of neural systems. It does, however, possess a tail similar to that of an adult. In an interesting adaptation, the creature coils this appendage like a spring to propel its body through the air to a potential host.

As the name implies, the facehugger attaches itself to a host's face and implants what can best be described as an embryo. It does this by wrapping its deceptively strong, triple-jointed legs around the victim's skull, coiling its tail around the neck and inserting a proboscis down the throat.

Once secured, the facehugger cuts off the victim's breathing and feeds just enough air down the throat to keep the host alive but unconscious. It's not attached long enough to have to supply additional sustenance.

To prevent forced removal during this vulnerable time, the facehugger tightens its grip around the victim's neck if disturbed. As the creature has concentrated acid for blood, attempts to remove it surgically often prove fatal. In a curious adaption, observers have also noted the facehugger's ability to secrete its corrosive fluids in order to penetrate helmets and armor.

With the embryo successfully implanted, the facehugger itself detaches and dies.

While most encounters with facehuggers involve human hosts, there are reports of canine, bovine and even extraterrestrial hosts as well, indicating that facehuggers attach easily to a variety of cranial and pulmonary arrangements.

Interestingly enough, however, domestic cats are universally rejected as hosts, perhaps due to size.

The facehugger raises several tantalizing questions about the xenomorph life cycle. Is the facehugger truly a prelarval stage at all? The creature that emerges from the egg is little more than a highly aggressive delivery system for the embryo, which must then gestate inside a host body while the facehugger itself crumples up and dies.

Perhaps the facehugger is no more a true xenomorph than a sperm cell is a human being. Is it merely a mobile sex organ intent on penetrating a suitable mate/host?

This fantastic notion isn't entirely without precedent among terrestrial organisms. Here on Earth, the male argonaut (or paper nautilus) uses a specialized, detachable tentacle called a hectocotylus to deliver sperm to a female. Once detached, the spermatophore-laden tentacle can swim freely but is unable to seek out a female on its own [source: Orenstein]. Fittingly, marine biologists initially mistook the hectocotylus for a parasitic worm.

Of course, the "embryo" that a facehugger delivers is rather atypical. As we'll discuss, the ultimate form of the xenomorph depends greatly on the host organism in which it gestates. While many Earth species of parasitic wasps implant eggs or larva inside the bodies of host insects and spiders, the young wasps merely feed on the host organisms -- they don't benefit from them genetically.

The implanted xenomorph embryo, however, blurs the line between parasite and offspring.

At this point in the xenomorph life cycle, we find the embryo gestating inside the chest of its host, as some terrestrial insects have been known to do.

The parasitic wasp Cotesia glomerata uses its ovipositor (stinger) to pump eggs into the bodies of caterpillars. Up to 60 of these eggs hatch into hungry larvae, which eventually account for 30 percent of a host organism's body weight [source: Fabricius]. The larvae drink the caterpillar's fluids but avoid damaging vital organs. The Dinocampus coccinellae wasp goes even further, chemically modifying a host ladybug to "guard" the larva that bursts from its abdomen.

Why does the host's immune system allow this to happen? Parasitic wasp larvae use a virus-mimicking poison to shut down a host's immune system, thus keeping the caterpillar's natural defenses from fighting the intruders [source: Callaway]. The xenomorph embryo seems to work in a remarkably similar way, fooling the host's body into ignoring or accepting the foreign organism.

As the xenomorph embryo grows inside the host and feeds off its blood, it also virally assumes genetic characteristics of the host. For instance, the bipedal xenomorph encountered by the Nostromo crew displayed attributes of both its human host and the extraterrestrial pilot encountered aboard the derelict spaceship [source: Nathan]. Subsequent xenomorph encounters demonstrated that embryos gestated inside quadrupedal hosts such as dogs or cattle emerged as four-legged adults.

In any event, this gene-swapping always benefits xenomorph survival. While the emergent adult retains all its inherent strengths, it also takes on host qualities that may aid it in navigating a new environment and overcoming prey.

Once an embryo develops into its larval chestburster stage, it abandons the host organism and flees to safety. Since it's nestled inside the chest cavity, the creature lacks easy access to a bodily orifice and therefore emerges by violently punching through the host's rib cage.

The Nostromo chestburster featured a serpentine body, lacked legs and possessed only rudimentary arms -- a true larval stage. However, the chestburster encountered on the penal colony Fiorina 161 emerged from its quadruped host as a smaller, adolescent adult. In this scenario, we can only assume that host genetics played some role in dictating the creature's emergent form.

These differences lead to a question we'll explore again later: To what extent is the xenomorph formless? Is it a true species or something much stranger?

Once free of its host, the chestburster xenomorph undergoes a rapid growth spurt, shedding skin and accumulating mass at an astonishing rate.

The Nostromo xenomorph reached full adulthood and a height of 8 feet (2.4 meters) in a matter of hours. Even more remarkable, the creature managed this growth without feeding on its previous host or devouring a single crew member. This fact would seem to violate the law of conservation of mass, so we can only assume the chestburster gorged itself on food stores found elsewhere in the ship.

Fast growth spurts are common among terrestrial life-forms. The sooner an organism reaches full size, the sooner it can fend for itself in a hostile world. For instance, a newborn blue whale grows at a rate of 200 pounds (91 kilograms) per day consuming its mother's milk [source: National Wildlife Federation]. Given enough food, the alien's rapid maturity is well in keeping with terrestrial biology.

Once fully mature, the xenomorph can certainly fend for itself thanks to several adaptive advantages. For starters, it seems to benefit from a dual skeletal system. Terrestrial skeletons fall into three categories: endo- (internal), exo- (external) and hydrostatic (fluid-based). Each system comes with certain strengths and weaknesses. Exoskeletons offer enhanced protection and muscle leverage, while endoskeletons are stronger and allow for more flexibility.

There's also a size limit for exoskeleton systems. Despite Earth's vast populations of chitin-armored creatures, no specimen approaches the size of an adult xenomorph. After all, a true exoskeleton isn't just a suit of armor -- it's a scaffolding for the creature's internal systems. The larger the organism, the thicker the exoskeleton required, until further growth becomes morphologically impossible.

So how can an adult xenomorph even stand, let alone move at such incredible speeds? According to Chapman University biologist Frank Frisch, the creature likely possesses partial endo- and exoskeletal systems [source: 20th Century Fox]. While the upper torso benefits from an exoskeleton's defensive durability, the lower body depends on an endoskeleton's strength and flexibility.

That skeleton imbues the xenomorph with amazing durability, but its circulatory system offers an even greater defense: highly acidic blood. Once spilled, this fluid burns rapidly through inorganic material and liquefies human flesh on contact. The acidity neutralizes shortly after exposure, but not before dishing out extensive damage. Naturally, the xenomorph is impervious to harm from its own corrosive fluids in the same way that our stomach linings protect us against our own stomach acid.

The xenomorph's intelligence also grants it a key defensive advantage. In various encounters, they've been proven to possess not only a sharp animal cunning but also a certain capacity for vicarious learning and problem solving. When xenomorphs overran a terraforming colony on LV-426, colonial marines reported that the creatures manipulated doors and elevators and may have intentionally cut a power cable.

Although the creature's defenses are impressive, its offensive weapons truly define the species.

The adult xenomorph is a deadly creature. In addition to superior strength, speed, dexterity and cunning, the organism also demonstrates an array of offensive adaptations that include sharp claws, a spiked tail, razor-sharp teeth and a pharyngeal jaw.

This secondary jaw emerges from an organism's pharynx, or throat. Here on Earth, we see this adaptation in the aquatic moray eel. The eel catches prey in its outer jaw and then extends a secondary set of teeth to grasp the food and pull it down the throat. This pharyngeal jaw normally remains folded away when not in use.

The xenomorph's pharyngeal jaw works similarly. The alien's secondary set of teeth rest on the end of a bony appendage that extends from the mouth and retracts into the throat. The xenomorph can extend this "tongue" with terrific force, easily puncturing the flesh of an adversary. The creature can then pull back the deadly tongue (leaving a gaping wound in its prey), or it can bite down with the small pharyngeal jaws and reel its victim into the primary jaws.

Xenomorphs also occasionally spit their corrosive blood at adversaries, another tactic in keeping with terrestrial biology. When threatened, vultures spit stomach acid and camels spit partially digested cud. Horned lizards spray blood from their eye sockets, and the armored ground cricket hemorrhages it through cracks in its exoskeleton.

Other species on Earth take projectile weapons to even crazier extremes. Spitting cobras lob venom directly at an aggressor's eyes, and bombardier beetles can squirt an explosive, 100 degrees C (212 degrees F) stream of heated venom with formidable accuracy.

Of course, as the xenomorph inevitably acquires genetic attributes from its hosts, various other offensive and defensive adaptations are possible. What would happen if a facehugger implanted an embryo inside an elephant? Let's hope we never have to find out.

The genetic mandate of any organism is propagation. It must spread its genes, and eons of evolution have populated Earth with myriad strange and complex life cycles.

Asexual creatures produce offspring that are essentially genetic clones of themselves, while sexual reproduction involves the combination of genes from two organisms. Some creatures can even reproduce both ways. Complex life cycles and social structures build up around these reproductive methods, leading to everything from intricate honeybee hives to the complex, 13-stage, two-pronged life cycle of the Micromalthus debilis wood beetle [source: Taylor].

The xenomorph life cycle clearly begins with an egg, but just where does that egg come from? The lone alien encountered aboard the Nostromo apprehended two living crew members and cocooned them in a nest of secreted resin. According to some accounts of the Nostromo incident, Warrant Officer Ellen Ripley found both crew members in a state of metamorphosis. By all appearances, they were transforming into xenomorph eggs.

The Nostromo incident seems to suggest a very strange form of reproduction, in which a xenomorph plants an initial parasitic proto-embryo inside a host, which then virally and totally transforms the host's body into a facehugger-gestating egg.

This life cycle would look like this:

proto-embryo → cocoon egg → facehugger → embryo → chestburster → adult

The notion isn't as far-fetched as it sounds, as various terrestrial parasites alter host behavior or physiology. Worms in the Leucochloridium genus, for instance, grow a colorful, pulsating broodsac up through a host snail's eyestalk and force the zombified creature to lure in predatory birds -- all so the parasite can continue its multihost life cycle.

Of course, not every account of the incident mentions the cocoon, and the subsequent events at the LV-426 terraforming colony revealed a far more traditional means of egg production.

By the time colonial marines arrived on LV-426, xenomorphs had killed or cocooned most of the colonists and multiplied into a full hive society.

These xenomorphs were anatomically distinct from their Nostromo counterpart, despite originating from the same cache of eggs and having gestated inside human hosts. These differences might be due to genetic variation, rapid adaptation to LV-426 surface conditions (rather than the environment aboard the Nostromo), differing age or differing caste.

The caste theory is especially interesting, given that the LV-426 xenomorphs displayed eusocial organization. They boasted a hive society with a central queen -- a lone, reproducing female analogous to those of ants, bees, termites and mole rats here on Earth.

The queen herself differed dramatically from the rest of the hive. More than twice the size of a typical xenomorph, she also boasted a longer tail, an armored cranial crest and an extra set of arms.

Most notably of all, the queen developed an enormous distended abdomen (not unlike that of a termite queen) and ovipositor with which to deposit fully formed eggs. Anchored in place by secreted resin, the queen's abdomen rendered her immobile -- though she later proved capable of shedding this portion of her body when threatened.

How do xenomorph populations produce a queen? Dubious accounts speak of a royal facehugger, which carries the queen embryo -- though this explanation raises as many questions as it answers. For a more likely answer, let's look at how terrestrial creatures produce a queen.

In the termite world, winged reproductive male and female termites called alates fly away from a thriving termite mound. The female lands, sheds her wings and mates with a king. Then the royal couple digs its way into the ground to start a new mound, where she'll lay one egg every 3 seconds for the next 15 years [source: Nelson and Silva]. Honeybees create a new queen by feeding larvae a diet of royal jelly, which worker bees secrete. Female naked mole rats simply fight it out to see who is worthy to breed.

With the xenomorph, we're forced to dismiss gender-based theories. All xenomorphs are essentially female -- though it might be more accurate to view them as asexual or hermaphroditic. At any rate, there are no documented xenomorph kings to parallel termites or mating drones to parallel honeybees.

Amazonian ants on Earth actually provide a useful comparison, having gradually abandoned sexual reproduction and become an all-female species that breeds through asexual cloning [source: Gill]. As a downside, the Amazonian ants suffer from reduced genetic diversity, leaving whole populations vulnerable to parasites or disease. The xenomorph, however, seemingly obtains genetic diversity through the parasitic impregnation of host species -- making the existence of males unnecessary.

In the end, the origin of xenomorph queens remains a mystery, but this much seems reliable: While a single xenomorph is capable of propagating its species through the cocooning of a host, larger populations produce a queen in order to ensure mass production of eggs. Their means of queen production may lie in violent completion, specialized diet, hormonal triggers or something utterly incomparable to terrestrial modes of life.

If xenomorphs evolved on another world, then how nightmarishly violent would their native environment have to be in order to sustain them? And what if they ever made it to Earth? Whole ecosystems would surely perish as this ultimate invasive species chased humanity into extinction.

Much like the vampires of legend, the xenomorph doesn't seem a very sustainable creature. On any given world, it's only a matter of time until they extinguish all viable food and host organisms -- and that just might be the point.

While reports are shrouded in mystery, a 2089 exploratory mission to the distant moon LV-223 provides some possible answers. There, the crew of scientists, mercenaries and dreamers encountered an ancient and highly advanced extraterrestrial civilization -- the same civilization, in fact, responsible for the derelict ship the Nostromo crew discovered on LV-426. These cosmic Engineers wielded staggering biomechanical and genetic technology, including something that can only be described as weaponized evolution.

The crew of the Prometheus found a vast storehouse of ampules, each containing an exceedingly contagious viral mutagen. Within hours, this inky black slime infected human crew members, as well as a native species of worm. The slime quickly spawned several incredibly hostile life-forms, all of which displayed a pattern of parasitic impregnation like that of the xenomorph species.

While details of the doomed Prometheus mission are still emerging, the biological weapon eventually impregnated and absorbed genetic characteristics of both humans and Engineers to produce a slender, phallus-headed biped with a pharyngeal jaw.

Was this being, code-named the deacon, a precursor to the xenomorph? Or does the Prometheus disaster simply refute any species-oriented understanding of the xenomorph at all? Perhaps it is less a true creature than it is death made manifest -- a cancerous, genetic force that exists only to extinguish all life, save its own, and then perish amid the bones of decimated worlds.

For now, these mysteries remain unanswered.

I've been a fan of the "Alien" films for as long as I've been old enough to watch them -- probably a little longer to be honest. So it was a huge thrill to explore the xenomorphs in true HowStuffWorks fashion.

Elsewhere on the Net, you'll find any number of fan-generated articles on speculative alien biology, as well as some excellent resources about the making of the films. I tried to steer somewhere between the two and turned to real-world biology whenever possible, rather than the conflicting world of comics, novelizations, video games and "Predator" crossovers.

There's also quite a bit of fascinating film study on the "Alien" franchise out there, discussing everything from the sexual aspects of the creature itself to academic debate over Ripley's underwear. If you'd like to catch up on some of that, do check out the Stuff to Blow Your Mind episode "The Science of Prometheus."

I'd like to personally thank H.R. Giger, who was very generous in sharing information and artwork for this article. Be sure to visit the H.R. Giger Museum, in Gruyeres, Switzerland, where the artist's original "Alien" paintings can be viewed in person year-round. And if you want to dive into every possible detail regarding the making of "Alien," then check out the artist's own book, "Giger's Alien," Valaquen's blog Strange Shapes and Ian Nathan's book "Alien Vault: The Definitive Story of the Making of the Film."

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