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|Regal 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 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 groups 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 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 5 feet (1.5 m). 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 compounds 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. Interestingly, ocular autohemorrhaging has been documented in other lizards  which suggests blood-squirting could have evolved from that 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.
A University of Texas publication notes that,
Horned lizard populations continue to decline and disappear throughout the southwest despite protective legislation. The species most often noted for declining numbers is the Texas horned lizard which has disappeared from almost half of its geographic range.
Population declines are attributed to loss of habitat; human eradication of the ant populations upon which the lizards prey; displacement of native ant populations by invading fire ants (aided by synergistic effects of native ant eradication); and predation by domestic dogs and cats.
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
- Pygmy short-horned lizard, Phrynosoma douglasii
- 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
Texas designated the Texas horned lizard (Phrynosoma cornutum), the official state reptile in 1993 and the "horned frog" is the mascot of Texas Christian University in Fort Worth, Texas. TCU is the only known athletic team with the "Horned Frog" as a mascot.
- 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.
- Sherbrooke, W. C. (2000). Sceloporus jarrovii (Yarrow’s spiny lizard). Ocular Sinus Bleeding. "Herpetological Review" 31: 243.
- Leache, A. D. and J. A. McGuire. 2006. Phylogenetic relationships of horned lizards (Phrynosoma) based on nuclear and mitochondrial data: evidence for a misleading mitochondrial gene tree. "Molecular Phylogenetics and Evolution" 39: 628-644.
- Pianka, Eric; Hodges, Wendy. "Horned Lizards, Part 2". http://uts.cc.utexas.edu. University of Texas. Retrieved 1 March 2015.
- "State symbols". Wyoming Secretary of State's Office. 2011. Retrieved January 22, 2011.
|Wikispecies has information related to: Phrynosoma|
|Wikimedia Commons has media related to Phrynosoma.|
- HornedLizards Yahoo Group: Horned Lizard Preservation and Discussion
- Horned lizard skulls and info at Digimorph.org
- Horned Lizard Conservation Society
- Phrynosoma.Org: HL Species, Husbandry, and Conservation Info
- Phrynosoma.Org: Forum
- Horned Lizards at UTexas.edu
- Argentine ants linked to declines in coastal horned lizards
- Texas Christian University mascot: What's a Horned Frog?
- Horned Toads - Field study of Short-horned Lizards by students of Waterville Elementary School