North American desert horned lizards have a wide range of predators within their habitat. One unusual defense mechanism involves the flooding of their ocular sinuses, tissues found below their eye, with blood. When a horned lizard feels threatened by a predator, its final defense response is to shoot blood from these flooded sinuses and out its eye sockets. As a result, the predator is often frightened and flees. The lizard also uses this mechanism to remove foreign particles from the surface of its eyes.
The horned lizard has two constricting muscles that line the major veins around its eye. When these muscles contract, they cut off blood flow back to the heart, while it continues to flow into the head. This floods the ocular sinuses with blood, building pressure, and causing them to bulge. By further contracting these muscles in a rapid manner, the pressure increases even more, eventually rupturing the thin sinus membranes. The result is a jet stream of blood that can shoot up to four feet from the eye socket, a process known as auto-hemorrhaging. Amazingly, this process can be repeated several times within a short period if necessary, though the mechanism for this rapid recovery is not completely understood.
The lizard uses the same mechanism to remove particles from its eye, without rupturing the sinus membranes completely. When dirt, soil, or other particles enter the eye, the lizard controls the pressure precisely, allowing the sinuses to swell, but not hemorrhage. It then sweeps a thin, transparent third eyelid across the surface of the eye. This membranous eyelid folds back to the front corner of the eye, leaving the debris at the rear corner. The horned lizard then uses the bulging ocular sinuses to draw the debris away from the rear of the eye and onto the eyelid. Once the lizard floods its sinuses with blood, as described above, the skin surrounding the eyelids expands, dislocating the debris where it falls off or is otherwise easily removed.
This summary was contributed by Sarah Dodge.
“Among the most famous, and spectacular, performers of autohemorrhaging are three species of North American desert-dwelling lizard, Phrynosoma cornutum, P. coronatum, and P. solare, which are commonly known (albeit inaccurately) as horned toads…If the predator is still not intimidated, however, and persists in its attack, the lizard has one final, and quite grotesque, defense mechanism. It uses a series of thin-walled, blood-filled spaces called sinuses found within its eye sockets. When the lizard rapidly increases the blood pressure within these sinuses, it causes the sinus walls to break suddenly. The blood is then forced out in jet-like squirts of crimson droplets. Sometimes, the force with which the lizard squirts this eye-ejected blood is so powerful that it can send sprays shooting up to distances of 4 feet (1.2 m). This bizarre squirting can be repeated several times if necessary, which is usually sufficient to frighten off any predator. Also, the squirted blood may contain a distasteful chemical, which would act as an additional deterrant [sic] to potential predators.” (Shuker 2001:128)
“The horned lizard can wipe intruding particles from the surface of the eyeball with an elastic, transparent sheet of tissue, the nictitating membrane, a structure also present in other lizards. This membrane is usually folded into the front corner of the eye, but it can sweep back over the surface of the eye to pick up foreign debris. The material thus picked up accumulates as a mucus-encapsulated pellet in the rear corner of the eye…At the base of the nictitating membrane is a blood sinus, a tissue that swells when its numerous capillary vessels are flooded with blood. When muscles around the major veins leaving the head are contracted, blood pressure in the eye area of the head increases. This increase fills the blood sinuses around the eye, forcing the enlarged pellet of dirt from its resting site onto the eye or eyelid (pl. 50). From there is either falls off or is deftly removed by a delicate flick of the lizards hind foot.” (Sherbrooke 2003:85)
“Contrary to earlier suggestions no antipredator substances, from glands around the eye, are added to the blood as it leaves the eye socket.” (Sherbrooke 2003:129)