Bleeding

The health UI changes appearance after being inflicted with bleed.

Bleeding is an ability that Acrocanthosaurus has.

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.

Bleeding will get worse while you move, and it disappears somewhat rapidly when you rest, staying stagnant while you're standing still.