How Do Horned Lizards Shoot Blood from Their Eyes?
Horned toads, also called horned lizards can contract the muscles around their eyes, cutting off blood flow back to the heart. Blood continues to flow into the eye area where it fills the ocular sinuses with blood. They then continue to contract those same muscles rapidly, increasing pressure on the thin sinus membranes until they rupture, releasing a stream of blood that can shoot up to four feet from the eye! Scientists call this process auto-hemorrhaging.
If necessary, horned lizards can repeat this process several times within a short period. It also has a practical use beyond defense. In the areas where they live, it’s common for them to get dirt and dust in their eyes. They can carefully control the auto-hemorrhaging process to remove irritating particles from their eyes without fully rupturing their sinuses.
There are 17 species of horned lizards that live in dry, desert areas from Guatemala and Mexico up through the western United States and even into southern Canada. They have a wide variety of predators, including bobcats, wolves, coyotes, foxes, snakes, and hawks. As a result, they have developed some unique defense mechanisms.
Their first line of defense is camouflage. Although the various species differ in coloring and the exact number and placement of horns, they usually blend in very well with their desert surroundings. When predators approach, they usually try to stay still to avoid detection.
If they are detected, they may or may not try to run away. They have wide, flat bodies with short legs, so they’re not very fast. Some predators, such as rattlesnakes, don’t usually chase their prey, so they may run away from these predators.
Many predators can easily outrun and catch a horned lizard. When caught, horned lizards will often stretch out and puff themselves up to appear as large as possible. They do this to deter predators who eat their prey whole. If the predators can’t get the whole lizard in their mouths, they may move on.
As a last resort, horned lizards may use one final defense mechanism that’s particularly effective against predators like bobcats, wolves, and coyotes. They shoot blood from their eye sockets! This usually frightens predators enough to make them flee. Fortunately for humans, horned lizards rarely shoot blood at people. They are often kept as pets but seldom live long in captivity; they slowly starve as a result of their specialized diet.
The blood-squirting mechanism increases survival after contact with canine predators; therefore, it is probable that, while unorthodox, the trait could have provided an evolutionary advantage. Ocular auto-hemorrhaging has also been documented in other lizards, which suggests blood-squirting could have evolved from a less extreme defense in the ancestral branch of the genus. Recent phylogenic research supports this claim, so it appears as though the species incapable of squirting blood have lost the adaptation for reasons yet unstudied.
To avoid being picked up by the head or neck, a horned lizard ducks or elevates its head and orients its cranial horns straight up, or back. If a predator tries to take it by the body, the lizard drives that side of its body down into the ground so the predator cannot easily get its lower jaw underneath.