The natural world is full of specialized defense mechanisms, but few are as startling as the ability to voluntarily shoot blood from the eyes. This bizarre action, known as autohemorrhaging, serves as a powerful deterrent against potential threats. Only a specific group of reptiles employs this dramatic tactic to avoid becoming a meal.
Meet the Horned Lizard
The reptile responsible for this feat belongs to the genus Phrynosoma, commonly known as the horned lizard. They are native to North America, with a range extending from southern Canada down into Mexico. These lizards thrive in environments like deserts, scrublands, and shortgrass prairies, where their coloration provides excellent camouflage against the sandy and rocky terrain.
The horned lizard is characterized by a wide, flattened body and a crown of bony, spiny projections, or “horns,” on its head. This spiky armor and cryptic coloration are the animal’s first lines of defense. There are approximately 15 species in the genus, and at least eight have been documented using this blood-squirting defense. The Texas horned lizard (Phrynosoma cornutum) and the Greater Short-horned lizard (Phrynosoma hernandesi) are two of the most well-known species that exhibit this behavior.
The Physiological Mechanism of Blood Projection
The process of ocular blood projection is a voluntary and controlled physiological action that begins when the lizard is severely threatened. The lizard initiates the process by constricting the major veins that carry blood away from its head and neck. This restriction causes a rapid increase in blood pressure within the head.
The blood is redirected and pooled into specialized vascular cavities located around the eyes, known as the ocular sinuses. These sinuses are essentially large blood-filled pouches with thin walls. As the pressure mounts, the lizard precisely controls the rupture of tiny capillaries situated near the eyelids.
Muscular contractions surrounding the eyes then act to forcibly eject the accumulated blood through these ruptured vessels and tear ducts. This results in a directed stream of blood that can travel a significant distance. The process is fast and does not cause permanent damage to the lizard’s eyes; the vessels quickly seal and repair themselves afterward.
The Defensive Purpose of Ocular Spray
The act of squirting blood is not the horned lizard’s primary defense; it is a last-resort measure used after camouflage and stillness fail. This defensive spray is highly effective because it is aimed directly at the face and mouth of the attacker, creating a startling effect.
The blood’s efficacy lies not just in the surprise, but in its chemical composition, which is particularly noxious to certain predators. The foul-tasting compounds in the blood are especially repulsive to canine predators, such as coyotes, foxes, and domestic dogs. Research suggests that these chemical deterrents are derived from the lizard’s specialized diet of harvester ants (Pogonomyrmex species).
The lizard incorporates defensive compounds from the ants into its bloodstream. These plasma-borne compounds cause the intense aversion in mammalian predators. When a coyote or fox tastes the blood, they are often repelled and immediately drop the lizard, trying to clear the repugnant flavor from their mouth. While highly effective against canids, the blood spray is less effective against avian predators like raptors.