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Horned lizard
Horned lizards are a genus (''Phrynosoma'') of lizards which are the type genus of the family Phrynosomatidae. The horned lizard is popularly called a "horned toad", "horny toad" or "horned frog", yet it is neither a toad nor a frog. The popular names come from the lizard's rounded body and blunt snout, which make it resemble a toad or frog (''Phrynosoma'' literally means "toad-bodied"). The spines on its back and sides are made from modified scales, whereas the horns on the heads are true horns (i.e. they have a bony core). Of 15 species of horned lizards in North America, eight are native to the United States. The largest-bodied and most widely distributed of the US species is the Texas horned lizard (''P. cornutum'').
Horned lizards use a wide variety of means to avoid predation. Their coloration generally serves as camouflage. When threatened, their first defense is to remain still to avoid detection. If approached too closely, they generally run in short bursts and stop abruptly to confuse the predator's visual acuity. If this fails, they puff up their bodies to cause them to appear more horned and larger, so that they are more difficult to swallow. At least eight species (''P. asio'', ''P. cornutum'', ''P. coronatum'', ''P. ditmarsi'', ''P. hernandesi'', ''P. orbiculare'', ''P. solare'', and ''P. taurus'') are also able to squirt an aimed stream of blood from the corners of the eyes for a distance of up to .〔Middendorf III, G.A.; Sherbrooke, W.C. & Braun, E.J. (2001): Comparison of Blood Squirted from the Circumorbital Sinus and Systemic Blood in a Horned Lizard, Phrynosoma cornutum. ''The Southwestern Naturalist.'', 46(3): 384-387.〕〔Sherbrooke, W.C. & Middendorf III, G.A. (2001): Blood-Squirting Variability in Horned Lizards (Phrynosoma). ''Copeia.'', 2001(4): 1114-1122.〕〔Sherbrooke, W.C. & Middendorf III, G.A. (2004): Responses of Kit Foxes (Vulpes macrotis) to Antipredator Blood-Squirting and Blood of Texas Horned Lizards (Phrynosoma cornutum). ''Copeia.'', 2004(3): 652-658.〕〔Hodges, W.L. (2004): Defensive blood squirting in Phrynosoma ditmarsi and a high rate of human-induced blood squirting in Phrynosoma asio. ''The Southwestern Naturalist'', 49(2): 267-270.〕 They do this by restricting the blood flow leaving the head, thereby increasing blood pressure and rupturing tiny vessels around the eyelids. This not only confuses predators, but also the blood tastes foul to canine and feline predators. It appears to have no effect against predatory birds. Only three closely related species (''P. mcallii'', ''P. modestum'', and ''P. platyrhinos'') are certainly known to be unable to squirt blood.〔 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.
== The blood-squirting mechanism ==
While previous thought held that compound were added to the blood from glands in the ocular sinus cavity, current research has shown that the chemical compounds that make up the defense are already in the circulating blood.〔〔 It is possible that their diet of large quantities of venomous Harvester Ants could be a factor; however, the origin and structure of the chemicals responsible are still unknown. 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 autohemorrhaging has also been documented in other lizards,〔Sherbrooke, W. C.(2000) . Sceloporus jarrovii (Yarrow's spiny lizard) Ocular Sinus Bleeding. "Herpetological Review" 31: 243.〕 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.〔Leache, A. D. and J. A. McGuire. 2006. Phylogenetic relationships of hores lizards
(Phrynosoma) based on nuclear and mitochondrial data: evidence for a misleading mitochondrial gene tree. "Molecular Phylogenetics and Evolution" 39: 628-644.〕
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