The Cancri are known to live in a swamp-like environment, athey need to coat human food with meal worms otherwise they will not be able to digest it. There is also a significant population of this species on earth, and they went to war with earth at some point for unknown reasons. There is an old tradition amongst their people that after a war has ended two groups exchange surviving warriors to make up for each other's losses. The family who lost a son gets a replacement to ease their grief, and at least the other family knows their son is still alive.