They were bred and trained to become the prey in a ritual hunt for another race, known as the Hunters. Tosk were genetically engineered by the Hunters to be sentient. The meaning of life for a Tosk was to outsmart the Hunters for another day. Being captured and returned to their homeworld alive was a great dishonor for the Tosk - the only greater shame was to seek protection from another race. They took an oath of silence regarding the hunt preventing them from even talking about it with anyone else. The Tosk were treated with respect and admiration by the Hunters. Tosk were well equipped to outsmart their hunters, presumably to make the hunt as exciting and interesting for the Hunters as possible. Tosk could make themselves invisible to the naked eye. They also had great agility and only required 17 minutes of rest per rotation. Tosk kept liquid nutrients stored in plasmic fibers throughout their bodies.
In 2369, a Tosk became the first visitor to the Alpha Quadrant through the Bajoran wormhole. He was convinced to dock at Starbase Deep Space 9 for repairs. The Tosk's ship was a one-person vessel, with standard systems. Instead of letting him be captured alive only to be dishonored, Chief O'Brien helped the Tosk evade the Hunters so that he could retain his honor. (DS9 episode: "Captive Pursuit")
In 2376, a Tosk discovered an Eav'oq vanishing crystal which gave him a vision of Opaka Sulan. He followed the vision to the planet Ee in the Gamma Quadrant where he informed Opaka of the vision. His vision showed him bringing her to the place he found the crystal. (DS9 novel: Rising Son)
- Accordingh to DS9 production sources, the Hunters were scripted to appear in "The Search" as members of the Dominion, but their role was cut from the final episode. The writers also established, unofficially, that "whoever breeds the Tosk for the Hunters also breeds the Jem'Hadar, explaining some of the similarities between both reptilian, genetically engineered species, such as the "shroud" and other abilities.