One the first alien from Xen that you see on Earth after the Resonance Cascade is the parasitic Headcrab. This animal is a little larger than the human head has four legs and weighs 6 to 8kg. It has only an animal intelligence and has not shown the ability to even use a simple door handle. When attacking, it attempts to leap onto a person and dig in the claws to hold onto the person. This really does not sound all that dangerous compared to other creatures from Xen which can just about rip a human being limb from limb. That is until you consider what it does to a person after a successful attack.
When a Headcrab attacks it jumps at a person and tries to engulf the targets head with its body. Then the Headcrab impales the person (in the back of the neck?) with a number of sharp beaks. The Headcrab takes over the person’s nervous system giving the Headcrab total control over the person’s movement. The exact method for this is unknown. It could be that when the Headcrab simply hijacks the human nervous system and sends its own messages through it. A more exotic method would be that since it covers the head, the Headcrab effects the victim’s brain in some fashion. This method could explain why the victim’s speech is garbled and backwards. At the same time it is taking over the victim’s nervous system, it apparently injects some kind of fast acting mutagenic compound into human host’s body. The person very quickly (a matter of minutes in some cases) begins to change in a truly gruesome fashion. The arms elongate, hands grow into huge claws, the chest breaks open exposing the internal organs, there is blood all over the place, the skin color changes. It is very nasty. Also the host body has its strength and resistance to damage is increased. The worst part however is that the victim is at least partially aware of what has happened. The mutation certainly causes a great deal of pain (more like unending agony) to the victim. Being “Headcrabbed” could be a fate worse than death as the old saying goes. And there seems to be no way of removing the Headcrab without killing the victim. And even if the Headcrab could be removed there would be no way of undoing the horrific mutation the person has suffered. The only release that can be given to a Headcrab victim is a quick death.
After a Headcrab has successfully taken over a victim (the time period seems to vary somewhat) they are commonly called a “Headcrab zombie” or more often just plain “zombie”. This name does not refer to the Headcrab/victim combination being dead (or undead) but more to how they act. A zombie walks with a slow shambling walk and often limps. The Headcrab zombie unlike the undead ones makes sounds like grunts, strange inhuman cries and low growls. Many of the sounds/noises produced by a Headcrab zombie may be from the victim, not the Headcrab itself. Zombies are also inhumanly strong and can be hard to kill. Some zombies even survive for a time after being chopped in half at the waist. And even when you kill the zombie victim, the Headcrab may survive and attack another person in attempt to get a new host body.
When a zombie sees a target (and that seems to be any self propelled living thing other than a Headcrab or another zombie) it will head in a direct line towards it. If there are large movable objects (such as an empty barrel) in the way, it will strike them with great force so they fly in the direction of their target and injure it. If there is a barrier in the way the zombie will attempt to break it down and if it fails, it will wander around randomly in an attempt to find a way around the barrier. Zombies seem to attack animals and people in an effort to secure food, though Headcrabs can live on a vegetarian diet (Llammar eats melons).
If the zombie gets close enough to attack it lashes out with its oversized clawed hands. These attacks are capable of causing gave injury and even death with a single successful strike. If there is no food to be found, normal Headcrab zombies can put themselves into a state of hibernation where they appear to be dead. Of course if you get close to it or disturb it in some way the zombie will quickly “wake up” and attack. This behavior has not been observed in the Poison Zombie. While in this state of hibernation, zombies can be submerged in toxic/radioactive waste for extended period of time without any apparent harm.
A significant weakness of Headcrabs and their zombie victims is that they easily combustible. Hit them with a flare, explode something flammable (such as a gas canister) nearby or even trick them into walking though a fire (which is not hard to do since Headcrabs do not seem to recognize fire as a hazard) and the Headcrab or zombie will light up like a torch and quickly burn to death. Headcrabs might burn so easily due to their chemical makeup. Something in their biology produced a chemical that burns easily and quickly. The zombies would seem to have the same chemical(s?) present due to the massive mutations caused by the Headcrab.
Now I have been talking about the standard or classic Headcrab and zombie. Unfortunately for the human race in the Half Life universe there is more than one kind of Headcrab (and zombies). In Half Life: Opposing Force there was a type of zombie called a “Gonome”. A Gonome is faster (it can run) than a standard zombie and a little tougher as well. And for a ranged attack it can pull some toxic material from the mutated victim’s chest cavity to throw at a target. It’s been speculate that the Gonome is the next step the process of a Headcrab Zombie becoming a Gonarch.
In Half Life 2 there are two new types of Headcrabs/zombies that plague the battered Earth, the Fast Headcrab, which creates a Fast Zombie and the Poison Headcrab which is associated with the Poison Zombie. The Fast Headcrab looks similar to a standard Headcrab, the coloration and size are about the same. The main noticeable difference between the two is the Fast Headcrab has much longer legs and moves much faster. A good way to describe a fast Headcrab is to say that it’s a normal Headcrab on a huge dose of speed, which may be more accurate than it may seem.
The Fast Headcrab of course creates a Fast Zombie from its victim. This zombie looks a lot different than a normal zombie and acts different as well. The Fast Zombie is well, FAST. It runs more like a four legged animal than a human being and is able to jump great distances. They are even agile (and strong) enough to climb up rain downspouts on the sides of buildings. Since these downspouts are not usually reinforced, it is clear these zombies have lost a large amount of mass. It’s not too hard to figure out where that mass went though. Fast zombies don’t have any cloths, skin or internal organs (That means no intestinal track, no heart, no lungs). So when you look at a fast zombie all you see are bone, tendons and (damaged) muscle. It is unknown if the damage to the zombies body is the result of the mutation or caused by the zombies body consuming itself due to the high rate of activity (I favor the 2nd explanation). Given the condition of the body the host’s brain has likely been destroyed, leaving the Headcrab in total control.
While the Fast Zombie is quick and agile, it is not nearly as strong as a normal zombie. This means its attacks inflict much less damage and it is unable to hit objects and turn them into projectiles. It makes up for lower strength with prodigious leaping abilities. Fast Zombies seem to favor locations that are high up and open. This means that they will frequently be found on the roofs of buildings or running around the streets rather than underground. Unlike other zombies which tend to growl, the Fast Zombie frequently howls and screams when attacking. Though how it does this is unknown since the Fast zombie has no lungs.
This next Headcrab is the most dangerous of the ones we know of on Earth and it’s the Poison Headcrab. Visually the Poison Headcrab looks a lot different than other ones as it is charcoal black in color with what looks like strands of stiff hair poking out of it in random places (think a Headcrab that had been put though an Ugly Machine). What makes the Poison Headcrab so dangerous is that as the name states, it’s poisonous. Instead of trying to take over a person it jumps on, the Poison Headcrab inject the victim with a neurotoxin. If this does not kill the target, it will certainly weaken it and make it easy prey for other Headcrabs. Presumably a weakened target will be taken over to become a zombie. Another difference from normal Headcrabs is that Poison Headcrabs will travel in a “colony”. A single (Poison) zombie will carry three or four Poison Headcrabs around on its back. This is a level of cooperation not seen in any other type of Headcrab. When a target is spotted the zombie will throw one of the Headcrabs it is hosting in an attempt to poison the target. These thrown Headcrabs can fly ten or even twenty meters due to the enhanced strength of the zombie. The Poison Headcrab can also jump off the back of a zombie to make an attack on its own.
If the thrown Poison Headcrab misses the target or the target survives the first attack, the Poison Headcrab will continue attacking. Poison Zombies are significantly harder to kill than normal zombies and not just because the zombie itself is tougher, the Headcrabs carried by the zombie will act as armor for it, protecting it from harm. The Poison Headcrab is tougher than a standard Headcrab and when injured it will sometimes scuttle off to hide for a short time.
Normally the Poison Headcrab moves slowly (slower than a normal Headcrab), but is able to produce short burst of speed when it’s required, though it’s not as fast as the Fast Headcrab. The Poison Zombie has a somewhat different appearance than the standard Headcrab zombie, though not as an extreme as changes seen with the Fast Zombie. The Poison Zombie is bent at the waist to allow for more space to carry Headcrabs. The hands, while claw like, are not quite as large as those of a standard zombie. The Poison Zombie is very bloated and there is exposed bone on the back where the flesh has been eaten away by something. It seems that the Poison Zombie victim is at least partially aware of what is happening around them.
A variant on the standard Headcrab zombie is the so called “Zombine” (so named by Alyx Vance). As the name suggest it’s a Combine Soldier that has been Headcrabbed. Unlike the other Headcrab zombies in Half Life 2 the differences are purely the result of the victim being different, not because the Headcrab was different. Since the Zombine started out as a Combine Soldier, it has a number of advantages over a normal zombie. The cybernetic implants that are standard issue for Combine soldiers make the Zombine stronger, tougher and faster. Then the body armor the Zombine still wears increases it’s resistance to damage even more. The cybernetic nature of the Zombine may have a (potentially lethal) drawback for the Headcrab though. While the Headcrab can control a human body through its link with the nervous system, I don’t think that it would be able to control the Combine implants the same way. The short version is that the Headcrab may not have total control of the Zombine. This is a possible explanation for the most lethal of the Zombines attacks, pulling a hand grenade out and running towards the enemy. Of course since the Zombine is still holding the grenade when it detonates, it is killed in the explosion. I have seen a number of different explanations for this behavior and they include the human victim attempting to kill himself and take out an enemy of the Combine with him and the Headcrab realizing that the grenade was a weapon, but knowing enough to throw it at the enemy. We really don’t have enough information to decide which theory is correct, if any of them are.
Now let’s look a bit at the biology and ecology of the Headcrab and address a rather interesting puzzle about them. While the Headcrab is a creature from an alien dimension, its biology is clearly compatible with that of animals (and people) here on Earth. In fact humans can eat Headcrabs (the Vortigaunts seem to eat them as well). The main body of the Headcrab is composed of a soft, flexible but strong tissue. I doubt that the Headcrab has any bones in its main body, but to keep its shape there would need to have some sort of structure. A simple cartilage framework (similar to the shark or the human ear) would be enough to do the job. The structure that makes up the legs is certainly a hard substance, but I cannot tell if it’s bony or something more like chitin (similar to the exoskeleton on an insect, or the beak on a giant squid). The ends of the legs clearly end in claws. These claws would be used to help hold onto a victim after it attacks. On the underside of the Headcrab is a large cavity with a number of sharp beaks at the edge of it. When a Headcrab attempt to seize control of a target this cavity engulf the head and the beaks will impale the target. With little doubt the material(s) that cause a victim’s massive mutations are injected through one or more of these beaks. I strongly suspect that the Headcrab draws nutrition from the host body through one or more of these beaks as well, probably by consuming blood. The cavity is fairly large and limits how much space there is in a Headcrabs body for organs. This also explains why Headcrabs are not very smart, there is only a tiny space for a brain. One interesting note about the Headcrab and vortigaunts is we have never seen a Headcrabbed vort. This suggests that while vortigaunts can eat Headcrabs for nutritional value, they are different enough in biology that they cannot be Headcrabbed as humans can. When we were in Xen in Half Life 1, in areas where the Vortigaunts where present, there were few if any Headcrabs to be found.
It’s a little hard to determine what the ecology of the wild Headcrab would be. I know, I know “How can you say that? The Half Life series has Headcrabs all over it!”. The problem is that the Headcrabs from Half Life 1 have been teleported from Xen and you see them in them in (or near) the locations they arrived on Earth. This random arrangement gives no clue on what conditions they prefer to live in. In Half Life 2 (and the Episodes) things are only slightly better. And that is because most, maybe all of Headcrabs we see are created/cloned/grown for use by the Combine as living weapons. One thing we can say is that Headcrabs tend to like places with low light conditions. Of course these conditions are best for hiding and making surprise attacks. Standard Headcrabs have shown the ability to quickly burrow into soft earth and sandy soil, this may explain why we don’t see anything that resembles eyes on a Headcrab, they “see” by sensing vibrations. This explanation fits in with the standard Headcrabs not using its front legs sometimes when they walk. They cannot feel vibrations well enough with their legs to determine the exact location of a target. As useful as vibration sensing may be, I don’t think it the only way a Headcrab is able to see. Why? Because we don’t see Headcrabs trying to attack people through fences and other barriers. This indicates they can see in some fashion and know that attacking would be useless. I considered they might be able to sense heat, but if they did, they would not walk into fires all the time. Maybe they see in the ultraviolet wavelength?
The life cycle of the Headcrab is only partially known at this time, but it starts off with the live birth of the Headcrabs by a huge creature called the Gonarch. These baby Headcrabs are able to walk, run, jump and even make an attack. Of course a baby Headcrab would be incapable of taking over a human, in theory it might be able to headcrab a small animal. I would guess that the baby Headcrab matures to an adult Headcrab fairly quickly, but a poor food supply could slow this process down. While growing up a, Headcrab is likely omnivorous, eating both plants and meat. I would not be overly surprised if the Headcrab was more a scavenger than a hunter at this stage. Given that Headcrabs don’t attack each other (that we know of) they are probably not cannibalistic. Headcrabs seem to be able to stay in this adult state for years, probably even decades. Somewhere along the line a Headcrab goes from an adult to a matriarch, called a Gonarch. The Gonarch form is huge compared to that of an adult Headcrab, something like 20 feet tall, a weight of several tons and heavily armored. The cavity on the underside of the Headcrab vanishes and a large sac that produces baby Headcrabs (in huge numbers) hangs in between its four legs. The Gonarch also has a ranged attack as it can produce and expel toxic material upward in a ballistic arc so it will land on an enemy. It is unknown what causes an adult Headcrab to metamorphose into a Gonarch, there really are too many variables to even make a guess.
Despite the huge numbers of Headcrabs we see in Half Life 2 and the Episodes, I tend to think there are few Gonarch’s on Earth, if any. The Headcrabs we see where created by the Combine for use as weapons and are not natural Headcrabs. Also from the Combine’s viewpoint it would not be good if there where (more) huge multi-ton monsters running around. So I find it likely that the Combine created Headcrabs are “sterile” and cannot grow into a Gonarch. If the Gonome is an intimidate step for a Headcrab to turn into a Gonarch, it explains why we don’t see any them either. The Combines “sterilization” prevents the Headcrab from advancing to the Gonome stage. The Combines "tinkering” with the Headcrab may also be why it looks a little different than in Half Life 1. But these changes could also be the result of a different/better diet compared to Xen.
There are a couple of problems with Headcrab biology or rather things that don’t make sense if you think about them. The first problem is the emergence of two variant or subspecies of Headcrabs in just the space of twenty years. I know Headcrabs are not the product of evolution here on Earth, but that speed of evolution is way too fast for nature, no matter where the Headcrabs came from. So this leaves the Fast and Poison Headcrabs a product of genetic manipulation by the Combine. It’s an easy answer that works well. But according to VALVe, it’s wrong. The exact quote
“Some of the critters came early (immediately after the Black Mesa Incident) and adapted to Earth. I think the poison Headcrabs must have eaten something poisonous at one point, and liked it so much they added it to their repertoire.”. 1
So Combine manipulation is out. But there is still a possible answer to the origin of the new types of Headcrabs. It’s not that the Fast and Poisonous Headcrabs are genetically different from the standard Headcrab, rather they are Headcrabs that had natural dormant characteristics activated. This sort of activation of dormant characteristics can be found here on Earth. One place is the humble mouse. The coloration of mice can quickly change dependent on the surrounding conditions, mice that live in cold conditions will grow more hair (and in more places) than a mouse that lives in warmer climate. When a mouse moves from a warm climate to the inside of a large freezer (yes, mice can and DO live inside freezers. Think of all the fresh food it would have access to), it can grow the new hair surprisingly fast.
Sometimes characteristics that don’t ever show up in nature can come about when conditions change enough. I recall seeing something about animals being kept in controlled conditions (It might have been minx) and after a few generations these animals started showing patterns and colors in their fur that have never been seen before (blue for instance). Genetically they were the same as the wild animals, but apparently a few new genes had been activated by the different conditions they lived in. Change the environment, you change how the creature grows and what traits it has. A Headcrab that is exposed certain chemicals (steroids? methamphetamines?) may turn into a Fast Headcrab. Another Headcrab that gets exposed to certain toxins changes into a Poison Headcrab.
Another problem with Headcrab biology is the fact that it clearly is compatible with Earth biology (despite the yellow blood). In fact the ability of the Headcrab to interface with the human nervous system and mutate the human body is a huge statistical anomaly. It would be the same as if man travel to another star system, landed on a planet and discovered Headcrabs all over the place (and nothing that looks like a human being around). This is going to be it’s going to be VERY rare in the real world, (despite what TV shows like Star Trek portray) like a 100 million to one shot. So that leaves only a few viable explanations for the Headcrab’s abilities.
The first alternative is that the Headcrab is an artificial life form or has been heavily modified so that it can take over and mutate species over a wide range of biological possibilities. This would mean the Combine’s use of Headcrabs as biological weapons is actually what they were intended for.
The second alternative has some interesting implications. If the Headcrab is not a bio engineered life form, and that means it evolved its abilities naturally. In order to evolve the specific biological abilities it would need another species to prey for a long period of time. And this other species would have to have a very similar shape and biology to human beings. The chance of a species that is so close to humans arising through parallel evolution is very remote. A possibility a little more likely is that at some time in the distant past when man was still evolving, a portal to another dimension opened and humans or something fairly closely related ended up in another dimension. (maybe aliens from another dimension kidnapped people). A variation of this last possibility is that at least some of the other dimension are not actually other dimensions at all, but are alternate realities. Or maybe you can access alternate realities from Xen in some way. This would give Headcrabs access to human beings for the time period needed to evolve the specific traits/abilities needed to take over and mutate humans.
With the (apparent) defeat of the Combine on Earth the future of the Headcrab is not a bright one. Without the Combine “producing them” the numbers of Headcrabs will start to decline as they are killed by humans or eaten by Vortigaunts. A few Headcrabs might survive by being domesticated like Llamar has. But that would be the rare exception rather than a rule. If another Gonarch manages to arise it may not survive very long. As soon as people find out about it efforts would be made to destroy it and destroy it quickly. In the end though, the Headcrab will eventually be driven to extinction on planet Earth. The horrific threat the species posses makes it too big a threat to allow to exist. Though it will take decades and maybe even centuries before all the Headcrabs are destroyed.