The Sontarans are cloned species, although in the past they did use a primary and secondary reproductive system. The Sontarans are a warrior species who have been waging a war with the Rutans for centuries. In attempting to gain a strategic advantage in the war the Sontarans occasionally come into contact with other planets and races. The gravity on their home planet of Sontar was higher than that of Earth, giving them a great strength advantage.
The Sontarans' sole purpose is to win the war with the Rutans. They have even attempted to invade Gallifrey in an attempt to steal the Time Lords' secret of time travel. When this failed they attempted to ally themselves with the Androgums Chessene and Dastari and use the Kartz/Reimer time travel module.
The Sontarans appear as a stubby, armour wearing bipedal creature. When the helmet is removed the head is almost the same shape. The Sontarans have a strict code of honour, and will never turn down the opportunity to fight. Despite their strength they have one major weakness; a probic vent juts out from the back of the collar. If this is struck, or blocked, the Sontaran will fall unconscious. Despite their armour, coronic acid is fatal to a Sontaran.
Each clone of a Sontaran can produce up to a million hatchlings. Each Sontaran is identical to the one that was cloned, although there is occasionally some deviation in genetic structure.
When the tenth Doctor encountered the Sontarans they had upgraded the armour with Cordolaine signals. These signals caused the copper in UNIT troops' ammunition and weapons to expand, rendering the guns useless and unable to fire.