Slave owners could, if they wanted good offspring from healthy and strong slaves in their household, create a kind of "living together situation" for their slaves, a so called "contubernium". Especially on the large "latifundia" (big farms with wide spread farm lands and lots of animal stock) many strong slaves were needed and lots of them were got from living- together- arrangements of slaves who were already working there. The work was hard, so the masters needed strong slaves out of strong slaves, and cheap ones. The contubernium wasn't necessarily a bond for a lifetime between slaves, but was initiated and finished by their masters, as soon as the necessity ran out. Obviously slaves living together in bonds were able to "get together" and form a threat for the masters.