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They differ from their western counterparts in that they possess a yellow-green skin color, as opposed to the green or blue colors of their western cousins, as well as thick, bulging veins. Like all super mutants, the specimens of the Vault 87 variant appear to be powerfully-built, hairless humanoids standing approximately eight feet tall (2.44 m) and clad in heavy armor made from car parts and scrap metal. If a super mutant of this type eludes death for prolonged periods of time, they may become a considerable threat, reaching the size of a small building. Another unforeseen side effect is that the unfinished virus strain codes the body for continued growth. While some semblance of cognition is retained, exhibited by the ability to speak, operate machines and use basic tactics in combat, super mutants of this type are typically characterized by significant mental retardation. Within fourteen days of initial exposure, subjects regress to an almost feral state, exhibiting extreme aggression at every non-mutant. However, a key problem with this batch is that the mutation does not stop. At ten days, skin pigmentation becomes a mixture of yellow and green, while its thickness and resistance to radiation and other environmental hazards rapidly increases. When exposed, phenotypical changes occur within three days and include increased muscle mass, changes in skeletal structure and atrophy of all visible sexual characteristics, creating a uniform body shape devoid of genitalia or another sexual dimorphism. Test subjects were exposed a single time to FEV in isolated chambers, where the virus was deployed as airborne particulate. The inferior FEV strain used at Vault 87 retains the recursive growth patterns characteristic to FEV but has a different effect, which may be also linked to how the infection was performed in the labs. However, by 2277, the mutants are running out of FEV and are looking in desperation for a new source of the virus. They attempt to capture live humans as often as possible, to infect them with FEV, or "green stuff," at Vault 87.
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Their inability to reproduce (possibly bolstered by the FEV batch and/or human procreation instincts) led to an obsession with preserving their species. Their routine behavior included raiding to kidnap people for conversion with the EEP mutagen.
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The first super mutants of this variety were created in 2078, becoming a permanent feature of the Capital Wasteland - not intelligent enough to overrun it, but too powerful to die off. This modified strain of FEV was used to create these mutants and resulted in a new strain of super mutant which, in addition to becoming sterile, grow larger and stronger as they age. These super mutants were created in the " Evolutionary Experimentation Program," an experiment assigned to Vault 87.
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