The Ulk-Ra were a species native to a wooded planet with copper-sulphate oceans.
Though the Ulk-Ra evolved from an arboreal species, the only sign of their ancestry was a spine on their backs. They were hairless except for a thin strip of wind- and atmospheric-pressure-sensitive fur on the spine. They had monochrome vision and human-level hearing, but their noses were sensitive to all types of chemicals.
By examining Ossu-hir's body, Zoe Heriot determined they were proto-mammalian. She guessed Ulk-Ra were either monotreme or diatreme and probably laid eggs, similar to the platypus. (PROSE: War Crimes)
The males of the Ulk-Ra were mindless warriors who could ignore their sense of pain. Females were highly rational thinkers, decision-makers, and battle strategists. They were physically smaller than the males.
Upon their coming to age, each male Ulk-Ra child would be sent into the forest to kill a beast and return wearing its pelt. Children that succeeded at this task were allowed to remain male and grow into warriors, whereas children that failed had their male organs removed and were forced to eat Issu-mul berries until they developed female characteristics. After initiation, the individual's name would be appended with the name of their new sex.
At some point in their history, some Ulk-Ra were removed from their planet and tested by the War Lords for possible use in their War Games. The Ulk-Ra were rejected for the games and sent back to their planet after being modified by the War Lords. One such returned Ulk-Ra, Ossu-male, was forced by his implant to kill a young male who was conducting his initiation. (PROSE: War Crimes)