They had developed, over the course of generations, powerful leg muscles to aid in their survival in the cliff-filled regions of their home planet. Abathur discovered their unique essence and wished to incorporate their genetic material into the base zergling strain. Plot branch : Kerrigan chooses to evolve the zerglingAfter Kerrigan chose to evolve the zergling strain with the karak's essence, Abathur incorporated the genetic material from the corpses of the Calus karaks into the Swarm creating the raptor zergling strain.