January 24, 2012this was my idea :) i posted earlier a longer version of this idea,
planting, harvesting, a K3 civ version of farming, where every planet is a crop..
.. and using time dilation, they travel at near light speed from planet to planet making round trips, planting /returning to harvest, so from their POV the growing process is accelerated.
Beyond that, my idea is the alien life cycle is an offshoot of a mechanism they use not only to feed on us but to use our biomass to rapidly multiply their own (whilst absorbing fresh local DNA)