Bleeding is an ability that the Acrocanthosaurus and Coahuilaceratops have.
Rather than do health damage over time, bleed will stop healing of injury damage, health, and comfort. This way, an Acrocanthosaurus can pursue its prey over time rather than go for a quick kill. It also deters larger predators like Tyrannosaurus rex from attacking an Acrocanthosaurus.
While bleeding, you cannot heal injury, health, venom, or increase comfort. Bleeding can only heal while being stationary or resting - moving around prevents it from going away. It's meant to be a debuff that disables a foe from recovering during a fight. Since the Apatosaurus is such a long hunt, it could be particularly effective in disallowing it to continue healing so a pack of carnivores may have a better time wearing it out. The Spiky Shield Ability of the Saichania also decreases bleed.
Bleeding will get worse while you move, and it disappears somewhat rapidly when you rest, staying stagnant while you're standing still.
The serrated teeth and sharp horns talent increases the amount of bleed damage applied.