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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", but 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 are morphologically similar to the Australian thorny devil (Moloch horridus), but are only distantly related. They also have other similarities, such as being sit-and-wait predators and preying upon ants, so the two species are considered a great example of convergent evolution.
Protection against predation
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 more difficult to swallow. At least four species are also able to squirt an aimed stream of blood (see Autohaemorrhaging) from the corners of the eyes for a distance of up to five feet. 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. 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 lizard.
Species and subspecies
- Giant horned lizard, Phrynosoma asio Cope, 1864
- Short-tailed horned lizard, Phrynosoma braconnieri Duméril, 1870
- Cedros Island horned lizard, Phrynosoma cerroense Stejneger, 1893
- Texas horned lizard, Phrynosoma cornutum (Harlan, 1825)
- Coast horned lizard, Phrynosoma coronatum
- Ditmars' horned lizard or rock horned lizard, Phrynosoma ditmarsi Stejneger, 1906
- Short-horned lizard, Phrynosoma douglassii
- Greater short-horned lizard, Phrynosoma hernandesi Girard, 1858
- Flat-tail horned lizard, Phrynosoma mcallii (Hallowell, 1852)
- Roundtail horned lizard, Phrynosoma modestum Girard, 1852
- Regal horned lizard, Phrynosoma solare Gray, 1845
- Mexican horned lizard, Phrynosoma taurus Dugès, 1873
- Gulf Coast horned lizard, Phrynosoma wigginsi Montanucci, 2004
- 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.
- "State symbols". Wyoming Secretary of State's Office. 2011. http://soswy.state.wy.us/SecretaryDesk/StateInfo_Symbols.aspx. Retrieved January 22, 2011.