The corrupted, bloated form of these creatures is a testament to the harsh conditions of the Land of Shadow.These bats leave their caves at night to suck the fluilds condensed inside the corpses that litter Mordor. Rotten flesh sticks in their teeth, decaying over time, and creating a lethal toxin which is prized by the Uruks. The bravest of them journey into the caves to harvest the poisonous substance and coat their weapons.
Those few brave Uruks that meet skin-to-fang become horribly diseased and bloated, yet also take to wearing their deformities as marks of honor and prowess in hunting.
The Ghûls are the only known predators of these creatures. Those ones that survive their mealtime, naturally mix the poison with their caustic digestive juices, creating a deadly cocktail for projectile-vomiting.