Strains are variations of the xenomorph you play as, changing up their play style to something either slightly or completely different to the normal xenomorph. This allows you to change your play style to experiment with.
You can pick your strain from the get go if your caste has any strains available to them. When you evolve you will lose the strain you previously picked. Each Xeno can purchase one Strain available to their caste and they are permanent.
Under the Alien tab you can buy strains for yourself with the Purchase Mutators verb.
Strains
Each Strain is permanent, will alter how you'll play the caste and can not be reverted.
Trade the ability to tail sweep for the ability to use headbutt while your crest is lowered. Additionally, your headbutt does more damage, gains 1 tile extra range and you're able to move slowly while fortified.
Trade the ability to fling and lunge for a lower cool down, increased damage and range on your punch ability. Punch loses the ability to immediately break bones. Gain a new ability, Jab: A stun ability that applies no damage but refreshes punch ability.
Trade the ability to spit in exchange for increased pheromones, the ability to tail sweep (as Defender), a shorter wind-up on your acid spray, and the boiler's acid spray. Your screech now buffs the damage of your fellow Xenomorphs.
Trade the ability to spit/spray acid and a hefty chunk of your armor for greatly increased speed and two new abilities: Dance and Tail Attack. Dance is an active ability that greatly buffs your speed and evasion. Tail attack is also an active ability, and has two modes: Impale, and Abduct. Impale deals direct damage and abduct stuns, then drags the targeted marine back a few tiles after a long windup. Using either of these abilities while dance is active will remove the dance buffs but buff the effect of the ability. Your screech now buffs the move speed of other xenos as well as debuffing the accuracy of nearby marines.
Trade most of your speed, the ability to spit, and the ability to spray acid for a ton of armor and the ability to toss a neurotoxin grenade that goes over barricades, as well as an empowered version of the punch the Warrior uses. Your screech now buffs the armor of other, nearby Xenomorphs.
Trade the ability to bombard with gas clouds in exchange for a single acidic projectile that deals more damage against structures with perfect accuracy. The firing process of the railgun strain will be much quicker. Railgun projectiles can pass through barricades as well as cover, but it is possible to hit them if a boiler targets the cover manually.
