"Mankind wants them for their genes? I can think of a more valuable attribute: They're apparently able to generate biomass out of thin air. The baby born at the beginning of the film weighs maybe five pounds. In a few weeks the ship's cargo includes generous tons of aliens. What do they feed on? How do they fuel their growth and reproduction? It's no good saying they eat the ship's stores, because they thrive even on the second ship--and in previous movies have grown like crazy on desolate prison planets and in abandoned space stations. They're like perpetual motion machines; they don't need input."