Science fiction: The biology of the alien in Alien

Abstract

Parasites serve as a source of threatening outcomes for humans in many science fiction plots. Perhaps the most notable is the Xenomorph of the first Alien film (1979). Here, we use the film as the sole source of direct information to hypothesize its life cycle. We recognize a distinctive infective stage, the face-hugger. To further its development as an internal parasite in its human host, we conceive features of its physiology. It has an astonishing ability to manipulate the behaviour of its doomed host, before emerging as the famous chest-burster. It is clearly a parasitoid, requiring the death of its host. A further metamorphosis completes its development to the adult predator that roams the doomed spaceship Nostromo. The Xenomorph adult stage bears an uncanny resemblance to a parasitoid of salps, pelagic invertebrates. Conceptualizing its mythic biology offers insight into the physiology and biochemistry of real parasites.

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Science ction: The biology of
the alien inAlien
Parasites serve as a source of threatening outcomes for humans in many science ction plots. Perhaps
the most notable is the Xenomorph of the rst Alien lm (1979). Here, we use the lm as the sole
source of direct information to hypothesize its life cycle. We recognize a distinctive infective stage, the
face- hugger. To further its development as an internal parasite in its human host, we conceive features
of its physiology. It has an astonishing ability to manipulate the behaviour of its doomed host, before
emerging as the famous chest- burster. It is clearly a parasitoid, requiring the death of its host. A further
metamorphosis completes its development to the adult predator that roams the doomed spaceship
Nostromo. The Xenomorph adult stage bears an uncanny resemblance to a parasitoid of salps, pelagic
invertebrates. Conceptualizing its mythic biology oers insight into the physiology and biochemistry
of real parasites.
Armand M. Kuris
(University of California,
Santa Barbara, USA)
Mona Y. Luo (Divadelni
14, Pilsen 301 00, Czech
Republic)
Science ction becomes science fact
Fandom disclaimers: Before we say anything, one of us,
AMK, mused about the calamitous loss of the Martians
in the Martian Chronicles. But, catching Alien, soon aer
it came out in 1979, in Santa Barbara’s magnicent movie
palace, the Arlington eatre, he was hooked on the
biology of extraterrestrial life forms from that moment.
MYL, having attended AMK’s lectures as an impres-
sionable undergraduate, vividly recalls his impassioned
discussion of the Alien movie from a biological stand-
point. She has been hooked on parasites ever since.
Much science ction engages humans with alien
forces. inking about these from an ecological
perspective oers a glimpse about interactions that could
reveal biochemical insights. Common themes include
competition, predation, parasites and pathogens. In an
ultimate sense, these interactions ask us to consider a
most basic question. What is life, and how will we know
alien life when we see it?
oughts of alien worlds oen revolve around
competition. One species, the dominant species, seeks the
good things in the world of the subordinate species. Avat a r
is a classic example, humans questing for unobtainium.
Competition is a pretty harsh ecological process. It could
involve a specic resource, like unobtainium, or more
generally space, up to and including an entire world.
Absent countervailing biology, one species will run the
resource down to the point that the other species cannot
survive. Stepping back, this is a numbers game, where
the dominant species out- reproduces the subordinate
species. Not much biochemistry here.
Predation is the most visible consumer–resource
interaction on earth. Speaking precisely, a predator, as
distinct from other types of consumers, feeds on multiple
individuals during the course of its consumer life history
phase, such as the adult. Jaws does a marvellous job
holding us enraptured. Predation does not seem so
frequent in science ction lms. Aer one or two attacks,
it may lose its lmic fascination. e ‘predators’ in the
Predator series are not ecological consumers, requiring
humans for food. ey are just trophy hunters. Not much
biology there. For life forms that are truly predators,
universally fearsome, consider the mature creature in
Alien or the Martians in e War of the Worlds.
e War of the Worlds lm presents another
common science ction theme: a lethal native pathogen
kills the exotic predator, implying immunocompetence
on the part of the earthlings but incompetence on the
part of the Martians. A more interesting example is the
pathogen that seemingly came from nowhere to kill
the humans on the extractive asteroids in the series
e Expanse. e absence of a natural host for that
pathogen seems unrealistic, but actually our Earth
holds many such pathogens. While naturally free- living
absorbing nutrient in water or dust, these microbes
can, on entering a human, cause serious, sometimes
fatal, diseases. Examples include valley fever in arid
California and Arizona and histoplasmosis in caves. e
most frightening is the ‘brain- eating’ amoeba, Naegleria,
almost always fatal to a swimmer with the misfortune of
having them enter the nose or eyes while swimming in
a pond.
Much has been written about Alien, the original 1979
lm. Here we will look at it from the standpoint of the life
cycle of the alien creature, the Xenomorph (Figure1). e
humans will just be hosts. is oers a lot of interesting
exobiological surmise and speculation. Arriving at a
derelict spaceship, the crew of the Nostromo encounters
a chamber with egg- like structures. Approaching them,
one of the eggs ‘explodes’, releasing its infective stage
that adheres to the face of Kane, one of the crew. We
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Science ction becomes science fact
can speculate about how the ‘egg’ was able to detect a
potential host. If it is only about humans or earth life
forms, perhaps it detected the high concentration of
CO2 and other gases exhaled by animal respiration. How
might this have been detected? ere are quite a few
parasites that use a sort of detection system to increase
opportunities to infect a host. Dog ticks are compulsively
attracted to CO and CO2. Questing ticks accumulate
alongside roads, seeking the sources of those gases. A
roadside may be a good place to nd a dog. It is also a
good place for a tick to get squished.
e infective larva of an insect- parasitic roundworm
becomes quite active in the vicinity of caterpillars,
increasing its opportunity to enter the spiracles,
openings to the respiratory system of the insect host.
As will happen to the unfortunate crew member,
Kane, with the face- hugger, the insect host must die to
continue the life cycle of the roundworm. Interesting,
relatively unexplored, questions include evaluating the
biochemistry of these recognition systems. How do
these detection systems operate? Does the host have any
capacity to reduce its chances of infection?
Returning to the face- hugger, the ‘organism’ in the
lm is a glopped up giant spider crab, Loxorhynchus
grandis, a species with a limited shery along the
Southern California Bight. e market for these crabs is
quite local, in the harbours and o the docks of shers
from Santa Barbara to San Diego. e Hollywood prop
designers likely discovered it, as a prop element, on a
SoCal pier.
We can surmise that the face- hugger, essentially a host
attack larval stage, pierces the head of Kane and injects
the next larva stage. Whatever enters Kane’s head, it must
have been very small, because, later, when Kane arises, his
face is apparently unmarked. An attacking larva injecting
the next larval stage is realistic for earthling parasites. e
attacking kentrogon larva of the famous barnacle parasitic
castrator of crabs, Sacculina, is a living hypodermic
needle. It injects a microscopic worm- like stage that
develops as a root- like absorptive internal parasite,
branching throughout the organs of the crab. It will fully
control the physiology and behaviour of the crab host. It
is a body- snatcher. When a host is parasitically castrated,
it is eectively dead from an evolutionary standpoint. e
sterilized host will only produce Sacculina ospring, not
crab ospring.
e internal larval Xenomorph developed very
rapidly. at indicates ecient metabolism, converting
host tissue to parasite over a short period of time.
We suggest that the chest- burster stage was not the
morphological form that had been consuming Kane’s
tissues. e chest- burter appears to have a cuticle and
a mouth full of teeth – highly unlikely to be an ecient
internal parasitic consumer, absorbing host tissues.
e chest- burster, erupting from Kane’s chest to
scream and slither out of the Nostromo dining room,
is clearly adapted for rapid movement as a free- living
stage. We soon realize that it is now a predator. e
chest- burster scene in Alien is widely recognized as one
of the greatest movie scenes of all time. Now free- living,
Figure 1. Xenomorph life cycle. A speculative life cycle of the Xenomorph based solely on Alien (1979). Eggs are dormant
and viable for a long time. On detecting proximity of a suitable host, the attack face- hugger is activated. The Xenomorph has
a single- host life cycle with obligate parasitoid and free- living life stages. The internal parasitic form of the organism has not
been observed (nor is depicted in the lm). Also unknown is how the chest- burster metamorphoses into the bipedal beast
that terrorizes the ship.
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3
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Science ction becomes science fact
the alien escapes into the dank interior of the spaceship.
More to come.
It is noteworthy that just before he dies, Kane arises
and seems to behave normally. is is a characteristic
of many parasitoid insect wasps. For some species of
parasitoid wasps, the host is eectively paralysed upon
being infected. ey can then feed on it at will, consuming
its organs without much regard for its continued life.
For other types of parasitoids, as for the Xenomorph, its
fatally infected host is active, behaving relatively normally.
For those parasitoid wasps (Figure2), also for horsehair
worms and the now famous fungus, Cordyceps, their use
of the host involves a highly sophisticated feeding strategy.
Its organs are consumed in sequence. In some parasitoid–
host interactions, the host is even able to continue feeding.
e dying host retains neural and motor abilities. It is
capable of complex behaviours. ese usually enable the
host to oer its parasitoid the best chance for successful
emergence to its next, always free- living, life history stage.
Famous examples include infected ants climbing to a high
point to release Cordyceps spores. Crickets infected with
fully developed horsehair worms, seeking water, dive in
and drown. is enables the mature horsehair worm to
emerge as an adult, swimming away to seek a mate and
reproduce.
e physiology of these precise feeders, called
koinobiont parasitoids, is remarkable, deserving
considerable study. Selective removal of host tissues to
produce a parasite that can be almost as large as the dying
host must require precise selective uploading and down-
regulating genes to produce this sophisticated physiology.
Energetic studies of parasitoids indicate that they can
convert ~70% of the host tissues to parasitoid biomass and
up to 90% of its nitrogen mostly to build parasitoid protein.
ese conversion eciencies are far above the conversion
eciencies of free- living consumers. eir eciencies
average about 10%.
Once loose in the Nostromo, there is one further
metamorphosis. e adult alien appears arthropod- like,
with jointed appendages, and it is cephalized – it has a
well- dened head. Now it is clearly a predator, consuming
all the rest of the crew save Ripley, actor Sigourney
Weaver, in her iconic role. Film bus have noted that its
morphology bears a striking resemblance to Phronima, a
marine crustacean parasitoid of salps (planktonic oceanic
tunicates). Intention or coincidence?
An evident feature of both the face- hugger and the
adult Xenomorph is that they are slimy. is attribute
must be a physiological challenge for those stages. In
the presumably arid climate of space, and in aerial
environments in general, mucus dries up rapidly. is
suggests these Xenomorph stages are continuously
secreting slime. Alternatively, they are producing a
form of slime with a powerful ability to retain moisture.
Development of such a substance would have remarkable
applications here on Earth.
To sum up, a careful consideration of the life cycle
of the Xenomorph in Alien reveals some interesting
parallels with Earth’s living creatures. What we can
glean of its biology suggests avenues for physiological
and biochemical exploration of some of Earth’s most
fascinating parasitic organisms and their complex life
cycles.
Before we leave Alien, we note that its most important
cultural contribution is its powerful feminist message.
It is a cinematic pioneer, revealing the central role of
women in a crisis. As the alien devours the crew of the
Nostromo, it slowly emerges that it is a woman, Ripley
(accompanied by her cat, Jones), who has the courage and
the smarts to dispatch the devastating Xenomorph. It is a
great movie.■
Figure 2. A real- world parasitoid. An earthling parasitoid
adult jewel wasp (Ampulex compressa), erupting, fully
formed from its cockroach host. These wasps incapacitate
their hosts with well- placed venom injections before laying
their eggs. The larvae feed on the internal organs of the still-
living cockroach over several days. The adult wasp emerges
to seek a new host, inject an egg and renew its parasitoid
cycle.
Armand Kuris trained as a marine biologist. In a twist of fate, he became a disease ecologist. He holds the
Charles A. Storke Chair in Ecology, Evolution and Marine Biology at the University of California Santa Barbara.
The mission statement of his research group is 'to reveal the role of infectious diseases in ecosystems'. His
research has targeted control of human schistosomiasis in Kenya and Senegal. He has a particular fondness
for parasitic crustaceans, blood ukes and the concept of parasitic castration. Email: alien@ucsb.edu
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Science ction becomes science fact
Mona Luo has a background in both art and biology, receiving degrees in scientic illustration and insect
biotechnology. She currently works at the intersection of art and science for a biotechnology company in
the Czech Republic. The thread which connects this California native to her new Czech home is her love
of parasites and the friendly Czech parasitologists that helped her along the way. Although not currently
working with parasites, every year she makes parasite- themed Valentine’s cards. Email: mona.luo144@
gmail.com
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