Species bond

Species bond is when each Lupar has 1 designated human to save them (and Earth) against an event called the Flux (TV: The Halloween Apocalypse). Karvanista took Dan Lewis for a species bond because he got to Earth Earlier than anyone else. Also, When they were species bonding, the Sontarans got behind the Battalion Wall in the milisecond before it was made and launched temporal attacks across time (TV: War of the Sontarans).