I find that it is impossible for grunts to live in their methane-filled world.
These grunts, to avoid their entire planet exploding in an full-atomospheric fireball, would have to have sex, sit around, and die.
Rubbing two rocks would be dangerous, much less land a covenant carrier that has giant combustion points in the back to pick up grunts and train them; especially training with plasma weapons on their home world.
Isn't it?
[Edited on 11.05.2012 8:45 PM PST]
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Oxygen is flammable, yet we require it to live. Maybe the grunts have a similar situation to ours AKA the methane in their atmosphere only makes up a small part of it.