Because of this peculiarity, they look like creatures out of some disturbed mind, but they are very real, both they and their bloody “weepings”. They do not “weep” blood because they have seen something horrifying, but because it is an extremely curious defense mechanism.
This behavior has been recorded mainly in some species of horned lizards of the genus Phrynosoma sp. distributed throughout North and Central America, although it is not the only genus of lizards that expel blood from their eyes. The trigger is a predator that irritates the lizard, causing the increasing of the blood pressure in the veins of the orbital sinuses and the expulsion of blood through the conjunctival sac and outward. However, not every potential predator stimulates this reaction. It is especially canids (foxes and dogs), suggesting a very context-dependent mechanism (obviously, the ejaculation of blood is a very important loss that could compromise the survival of the animal).
Some studies point to touch as the key sense that guides these animals in triggering their anti-predator response, although sight and smell would also be important.
It already seems clear that this behavior would serve to increase the probability of survival of the lizards, since negative and rejection reactions have been frequently recorded in canids that have received these bloody shots (also, a couple of people who received the blood in their eyes suffered ocular irritation). It is possible that the fluid contains some irritant substance, which, according to the investigations, would be present in the systemic blood, i.e., it would not be incorporated later when the blood passes through the orbital sinus. It is not known what it could be, but one possibility is that it is the venom of the ants that constitute its diet. Instead of metabolizing it, they incorporate it as a defensive system.
Middendorf III, G.A., Sherbrooke (1992). Canid elicitation of blood-squirting in a horned lizard (Phrynosoma cornutum). Copeia 2, pp. 519-522.
Middendorf III, G.A., Sherbrooke, W.C., Braun, E.J. (2001). Comparison of blood squirted from the circumorbital sinus and systemic blood in horned lizards, Phrynosoma cornutum. Southwest. Nat. 46, pp. 384-387.
Sherbrooke, W.C., Middendorf III, G.A. (2001). Blood-squirting variability in horned lizards (Phrynosoma). Copeia 4, pp. 1114-1122.
Sherbrooke, W.C., Middendorf III, G.A. (2004). Response of kit foxes (Vulpes macrotis) to antipredator blood-squirting and blood of Texas horned lizards (Phrynosoma cornutum). Copeia 3, pp. 652-658.