File:Yeerks.jpg A Yeerk is a parasite that takes over the body of another being by way of infestation through the ear canal. It resembles a small, grey-green slug about six inches long. Yeerks in their natural state have decent senses of smell and touch, but they are blind until they enter a host, as they do not have eyes. They possess antennae-like protrusions called palps, as well as osmosis nodes, small organs that process nutrients in the Yeerks' native pools. Although the presence of a circulatory system of some kind is debatable, apparently Yeerks have no hearts, as evidenced by the description of a human morphing into a Yeerk, in which the human heart stops and dissolves away completely.
Yeerks communicate in their natural state using a language of ultrasonic squeaks, and use sonar to get a basic picture of the surroundings. The process of infestation by a Yeerk is not totally painless, but it does not hurt as much as it seems like it should, as a numbing chemical is secreted to anesthetize and dilate the ear canal itself so that the slug-like Yeerk will not cause any physical harm to the host. It is able to manipulate and puncture a variety of alien ear canal structures including bone and membrane to reach the brain, although the mechanics of this are never fully described.
After the process of infestation, the Yeerk flattens itself into what has been described as "hammered-out gum" and sinks into all of the cracks and crevices of the brain. From there, all control is submitted involuntarily to the Yeerk (however, the host may fight and very rarely win). At this point, the host has no control over itself, no secrets, and no privacy. The Yeerk can even play with the memories of its former host. Although control can be returned to the host by the Yeerk, this does not occur often as most Yeerks prefer their parasitic relationship to a symbiotic one.
Yeerks cannot stay within their hosts for more than three Earth days. After this period of time, they need to bathe in a Yeerk pool and absorb Kandrona rays, the rays of their home sun. Without these rays, a Yeerk eventually starves.
However, there are two ways for Yeerks to survive without Kandrona rays, one terrible und cruel way being to eat other Yeerks. This cannibalism is not normal and a taboo and is only displayed by one Yeerk, Esplin 9466 the lesser (Visser Three's twin sibling; after being cut off from Yeerk society by his twin. He kill Human-Controllers, opening his head with brute force and eat the Slug with his maul.
Yeerks may also survive a lack of Kandrona rays by consuming instant maple and ginger oatmeal, although its effects are narcotic and cause the Yeerk to become insane and dependent and trapped within its host indeterminately, without the need to feed in a Yeerk pool. The host may regain some control of its body from time to time, while the Yeerk has lapsed into narcotic-induced lack of control. Usually the Yeerk is not intelligible after full addiction has occurred, since the chemicals in the oatmeal damage part of its brain stem. The Animorphs discover this upon investigating a man named George Edelman who attempts to commit suicide. Edelman is put in an insane asylum, where the Animorphs follow him and discover the Yeerks' weakness to the oatmeal. The Yeerk in his head has succumbed to this weakness, and Edelman is able to speak and move freely, except when his Yeerk seizes control and rants unintelligibly in alien languages. Yeerks may intake oat flakes by way of their hosts. How the dietary fiber exactly reaches the brain is unknown.
Yeerks reproduce through fusion. Three parent Yeerks merge into a single organism, which then breaks apart into hundreds of separate grubs, destroying the parents in the process. Sometimes twins are born when two Yeerks are born from one grub. So Yeerks have no living parents and don't know love and family.
Yeerks do not have different genders; presumably any three random Yeerks can reproduce. Nevertheless, the Yeerks are usually gender pronouns (he/she) in the novels based on their host's gender (usually by a human or Andalite, or its feminine personality Karen/Aftran).
So far, Yeerks have enslaved about 18 different kinds of alien species, completely or partially, including Hork-Bajir, Taxxons, Gedds, the Nahara (never seen in the series), Leerans, Garatrons, and of course, humans. According to a Yeerk named Temrash 114, the Yeerks may also have taken over creatures known as the Sstram and the Mak, neither of which is ever seen in the series.
The Yeerks also have the ability to infest Andalites; however, they are never successful in truly enslaving them. The only Andalite body that the yeerks have ever manage to take over was War Prince Alloran-Semitur-Corrass, turning him into Visser Three.
The Yeerks have a pseudo-diplomatic relationship with the metamorphic Skrit Na, but do not infest them due to the Skrit Na's lifestyle: The cockroach-like Skrit will sometimes unexpectedly cocoon itself and emerge as a Na over a year later, trapping any Yeerk inside, meaning death by starvation.
The Yeerks have also tried to enslave Hawjabrans and Ongachics, but they have failed on both accounts. One because the Ongachic race is completely nomadic and spread out through the entire galaxy. Two, because Hawjabran brains are spread out in little nodes throughout their bodies, so the Yeerks can never completely control them.
There are some species that the Yeerks do not know about, and therefore have not tried to infest, such as the Iskoort. The Iskoort species is actually related to the Yeerks, even though they do not look at all similar, but not a lot of them know that, or have even heard of them.
Since the Yeerk has access to all parts of the brain (except with Hawjabrans), it is able to pass flawlessly as its host (now called a Controller) so that nobody else suspects that there is, in fact, a secret invasion of their planet. Another result of this is that hosts and Yeerks can sense each others' thoughts and feelings; while this usually is the extent to the mental connection, it seems hinted that the mental bond between the two can result in identity confusion, as in the case of Taylor. Another case of notice is that of Visser One, who, after marrying and having children through her host, felt an actual romantic and maternal bond to her "family."
Evidently, morph-capable Yeerks morph with their hosts and fuse with the new body. This is shown in The Capture, when Jake, as a Controller, morphs into an ant. An ant is far smaller than a Yeerk, so otherwise Jake would have been killed by destruction of his head.
The Yeerk homeworld has various satellites, one of the natural ones is a moon called Madra. Yeerks inhabit a homeworld with a dark green and yellow, lightning-torn sky. The ground is largely dark rich soil, and roughly circular Yeerk pools dot the surface, presumably connected to each other somehow, but this is not explained.
Individual pools have names, such as Sulp Niar. Yeerk pool water is a sludgy grey, filled with nutrients Yeerks need besides Kandrona rays. There are no more than a hundred animal species on the Yeerk homeworld, among them the Gedds, which became the Yeerks' natural host, nearly a symbiotic relationship.
A Vanarx, also known as a "Yeerk-bane", is a big long purple tube-like creature that has a round mouth full of teeth, its native to the Yeerk homeworld, and appear to be the Yeerks' only natural enemy. It latchs itself onto the head of a Controller, sucking out the Yeerk inside and eating it. Visser Three morphs a Varnarx in Book #2: The Visitor as a means to eliminate useless subordinates without killing or hurting the host-body.
Sometime on or before the Earth year 1966 (Yeerk date: Generation 685, mid-cycle; each Generation is roughly two Earth years and divided into three cycles), the first Andalites arrived on the Yeerk homeworld, led by the Prince Seerow. Seerow pitied the Yeerks' physical drawbacks despite their capacity to learn, and set up the Andalite-Yeerk Peace and Cooperation Center on the Yeerk homeworld. He and other Andalites taught the Yeerks how to write down the Yeerk language phonetically, and taught them what lay beyond their green-clouded sky.
Once armed with the knowledge of space-faring technology and similar sciences, certain Yeerks rebelled against the Andalites, taking them off-guard and killing many people during the escape with several small Andalite ships and a few transports. The Yeerks stopped on the far side of their planet to siphon up large quantities of Yeerks from Yeerk pools before fleeing the planet, eventually to become the Yeerk Empire. Andalite forces began to patrol the Yeerk homeworld in orbit to make sure no more Yeerks entered, attempted contact, or attempted to leave.
The Yeerk Empire was gutted devastatingly with the death of seventeen thousand, three hundred and seventy-two Yeerks flushed from the Yeerk Pool Ship's onboard pool during the final blatant attack on Earth. The Empire's loss of Earth was caused largely by the Animorphs, and surrendered Yeerks were given amnesty by the Andalite military if they became nothlits, or trapped in one morph forever. A single Blade Ship (holding the morphing cube) escaped with the intent of creating a new Yeerk Empire, and allied with a dangerous, cult-like, otherworldly entity known only as The One. The One captured Ax when he boarded a dead-in-space ship which concealed the Blade Ship. Not much is known about The One except that it is possible he, or it, is allied with the Kelbrid.
Yeerk names end in numbers, which designate which number grub they are (i.e. Aftran 942 is the nine hundred forty second grub to emerge from the Aftran tri-parent). In the case of twins, both are given the same number, which ends in a double number. For example, Visser Three is Esplin 9466 primary, and his brother is Esplin 9466 secondary. The primary twin is considered to be superior to the other.
In the Yeerk military, each of them is ranked one after the other, and are referred to by title and number. The lowest rank apparently does not earn the person a title (for example, the Yeerk Essam 293 is only ever referred to by name). The next rank up is Sub-Visser, which is equivalent to a lieutenant or some such officer rank; after being promoted above sub-visser level, Yeerks may become one of 47 Vissers, which are equivalent to generals or to Andalite Princes.
As a note, earlier in the series, the name number and rank number were the same thing (for example, Temrash 252 was promoted to being Temrash 114) - Vissers were of the numbers from 1 to 47 and Sub-Vissers from 47 to 100; however, this was later retconned by K.A. Applegate with Yeerks remaining with the same number as their "name," no matter what their rank. For example, Visser Three's "name" is still Esplin 9466 primary, while Visser Three is his rank.
The head of the Yeerk Empire is the Council of Thirteen, the leader of which is the enigmatic Yeerk Emperor; however, to prevent assassination, his identity is never known by anyone but the Councilors. Of the thirteen council members, two remain always concealed. They wear very dark red, almost black, robes, and those Council members who have Hork-Bajir hosts wear a thin mesh under the robes to prevent them from tearing. Thirteen seems to be a symbolic number for the Yeerks, as trials also have thirteen guards on duty.
There is a faction of Yeerks who have formed an entity known as the Yeerk Peace Faction, which opposes the wars led by the Empire in secret and whose members, who may be Yeerks and/or their hosts, oppose involuntary infestation.
There exists an offshoot of the Yeerks that was taken from the homeworld millennia ago: the Yoort. The Yoort have long since changed from being parasites, and have become symbionts; they merged with a species created by them, the Isk, with whom they have a symbiotic relationship. The joint species is called Iskoort.
Before becoming symbionts, the Yoort (like the Yeerks) believed that conquest was the way forward. But over time, they realized that this wouldn't be beneficial, because the Yoort (Iskoort) are mostly concerned with profit and society. They made a symbiotic relationship so that the Isk cannot survive without the Yoort and the Yoort cannot survive without the Isk.
The Ellimist sent the Animorphs to their homeworld in hopes to save the species so that someday the Yeerks would meet Yoort (Iskoort), and the Yeerks would realize there is a better way to live than parasitism.
Yeerk technology is largely stolen from other races, although the Yeerks themselves appear to have a great capacity to learn and create new technology with pieces of alien technology. Yeerk technology and ships include:
The Sharing is the front organization for the Yeerks. The purpose of The Sharing is to persuade humans to submit to this infestation voluntarily. Some humans do submit voluntarily, while the Yeerks simply force infestation on those who do not. Members are not allowed to remain free indefinitely, and all members of The Sharing, bar those who have newly joined, are under the control of a Yeerk slug.
The Sharing uses psychological manipulations similar to those used by the cult-like organizations in the real world, although, unlike most cults, The Sharing is successful in making itself appear mainstream to the larger human society. The head of The Sharing is a high-ranking Controller named Hedrick Chapman.