A high-pressure defense
In the arid landscapes of the Sonoran Desert, horned lizards (Phrynosoma) have a defense mechanism that is startling and effective. Known scientifically as ocular autohemorrhaging, at least eight species of horned lizard can forcefully eject streams of blood from the corners of their eyes. This is not passive bleeding but a controlled, high-pressure spray. The lizard accomplishes this by deliberately increasing the blood pressure in its head. It restricts the veins that carry blood away from its head, while blood continues to be pumped in, causing the ocular sinuses around the eyes to swell. The pressure builds until tiny, specialized vessels in the conjunctival area rupture, releasing a targeted jet of blood that can travel up to five feet.
This defense is reserved for specific threats. The blood-squirting response is used almost exclusively against canid predators like coyotes and foxes. Studies have shown that canids trigger the response with near-certainty, while other predators like birds of prey do not. The lizard can even aim the stream, orienting its head toward the attacker before firing. After the event, the ruptured vessels heal quickly, and some lizards have been observed firing multiple times in a short period with no apparent injury. While it may seem extreme, losing a small amount of blood is a small price to pay for escaping a predator's jaws.
You are what you eat
The blood itself is effective because of chemical compounds. Its effectiveness comes from chemical compounds derived directly from the lizard's diet. Horned lizards are specialized myrmecophages—they primarily eat ants. Specifically, their diet consists largely of venomous harvester ants of the genus Pogonomyrmex. These ants produce defensive chemicals, including formic acid. The lizards are adapted to consume these toxic insects, coating them in mucus to avoid being stung during ingestion.
the lizard's body sequesters chemical compounds from it into its bloodstream. These foul-tasting compounds are what give the ocular spray its power. When a coyote or fox gets a mouthful or face-full of this chemically-enhanced blood, its reaction is one of immediate disgust. Predators are observed shaking their heads, gaping, and drooling, distracted long enough for the lizard to make its escape. This makes the horned lizard's defense a example of how an animal's diet can be directly weaponized for survival. The blood of lizards that do not eat harvester ants lacks the same deterrent effect.