Their parents died from fighting when the cubs were newborns, I know that sounds really horrible but they were'nt taught to be reckless, they had to live by themselves in return though ( end of prologue and brb )
I'll make a prologue part 2 if you want, but I'll just say the rest here, the tribes fought constantly killing and killing and killing, they thought their cubs to be vicious, but it wasn't their fault, it was the scientist's fault, even the tribe members