Slave Planet by Laurence M. Janifer

Cadnan had learned much in a very short time. Everyone was hurried now, as the time of mating approached more and more quickly and as the days sped by: knowledge was thrown at Cadnan and at Dara in vast, indigestible lumps, and they were left to make what they could of it, while the others went about their normal assigned work.

He learned about the invasion, for instance -- or as much about it as Marvor, the elders and a few other late arrivals could piece together. Their explanations made surprisingly good sense, in the main, though none of them, not even Marvor, could quite comprehend the notion of masters having masters above them: it appeared contrary to reason.