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The Yeerks are a species that features in Animorphs.




In appearance, the Yeerk were described as a parasite that takes over the body of another being by infesting a host's brain through their ear canal. It resembles a small, grey-green slug about five or 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. As the Yeerk reaches the eardrum and other ear canal structures, it pushes them aside.

After the process of infestation, the Yeerk flattens itself into what has been described as "3 parts, two wrinkled, one smooth", which is 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 and cruel way being to eat other Yeerks. This cannibalism is not normal, 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 kills Human-Controllers, opening their heads with brute force and eats the Yeerk inside with his maw.

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, 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.



  • Aftran 942 :
  • Akdor 1154 :
  • Carger 7901 :
  • Derane 344 :
  • Edriss 562 :
  • Efflit 1318 :
  • Eslin 359 :


  • The Yeerk were created by Katherine Applegate where they featured in the setting of Animorphs.


  • Animorphs:

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