Horned lizard

Mexican Plateau horned lizard (Phrynosoma orbiculare) near Xalapa de Enríquez, Veracruz, Mexico, showing blood squirted from eye as defensive behavior (20 April 2011)

Horned lizards use a 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.[1]

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 up to 5 ft (1.5 m).[2][3][4][5] They do this by restricting the blood flow leaving the head, thereby increasing blood pressure and rupturing tiny vessels around the eyelids. The blood not only confuses predators, but also 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.[3]

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.[3][4] 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;[4] the trait may provide an evolutionary advantage. Ocular autohemorrhaging has also been documented in other lizards,[6] 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 the species incapable of squirting blood apparently have lost the adaptation for reasons yet unstudied.[7]

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.

A University of Texas publication notes that horned lizard populations continue to disappear throughout their distribution despite protective legislation. Population declines are attributed to a number of factors, including the fragmentation and loss of habitat from real estate development and road construction, the planting of non-native grasses (both suburban and rural), conversion of native land to pastureland and agricultural uses, and pesticides. Additionally predation by domestic dogs and cats place continued pressure upon horned lizards.[8]

Fire ants (Solenopsis invicta) introduced from South America via the nursery industry's potted plants, pose a significant threat to all wildlife including horned lizards. Phrynosoma do not eat fire ants. Fire ants kill many species of wildlife and are fierce competitors against native ants which horned lizards require for food (with their specialized nutritional content). Fire ants have given all ants a bad reputation and human attempts to eradicate ants, including invasive species and the native species on which the lizards prey, contribute to the continued displacement of native ant species and decline of horned lizards.[8]

The Texas horned lizard (Phrynosoma cornutum) has disappeared from almost half of its geographic range. Their popularity in the early to mid 20th century pet trade, where collectors took thousands from the wild populations to sell to pet distributors, without provision for their highly specialized nutritional needs (primarily formic acid from harvester ants), resulted in certain death for almost all the collected lizards. In 1967 the state of Texas passed protective legislation preventing the collection, exportation, and sale of Phrynosoma, and by the early 1970s most states enacted similar laws to protect and conserve horned lizards in the USA. However, as recently as the early 2000s the state of Nevada still allowed commercial exploitation of Phrynosoma. Despite limited federal protection in Mexico, horned lizards are still offered in Mexican "pet" markets throughout the country.[8]

In 2014, the Center for Biological Diversity in Tucson petitioned the Oklahoma Department of Wildlife Conservation to have the Texas horned lizard put on the endangered species list due to the massive declines of its population in Oklahoma, where it was once plentiful. The center said it may later seek protection for the animal on a federal level; it also said that reptiles in general are dying off at up to 10,000 times their historic extinction rate, greatly due to human influences.[9]

Phrynosoma douglasii

The following 21 species (listed alphabetically by scientific name) are recognized as being valid by the Reptile Database, three species of which have recognized subspecies:[10]

• Giant horned lizard, Phrynosoma asio Cope, 1864
• Baur's short-horned lizard, Phrynosoma bauri Montanucci, 2015 (disputed; may be conspecific with P. hernandesi)
• San Diego horned lizard or Blainville's horned lizard Phrynosoma blainvillii Gray, 1839
• Short-tailed horned lizard, Phrynosoma braconnieri A.H.A. Duméril, 1870
• Great Plains short-horned lizard, Phrynosoma brevirostris (Girard, 1858) (disputed; may be conspecific with P. hernandesi)
• Cedros Island horned lizard, Phrynosoma cerroense Stejneger, 1893
• Texas horned lizard, Phrynosoma cornutum (Harlan, 1825)
• Coast horned lizard, Phrynosoma coronatum (Blainville, 1835)

• Cape horned lizard, P. c. coronatum (Blainville, 1835)
• California horned lizard, P. c. frontale Van Denburgh, 1894
• Central peninsular horned lizard, P. c. jamesi Schmidt, 1922

• San Luis Valley short-horned lizard, Phrynosoma diminutum Montanucci, 2015 (disputed; may be conspecific with P. hernandesi)
• Ditmars' horned lizard or rock horned lizard, Phrynosoma ditmarsi Stejneger, 1906
• Pygmy short-horned lizard, Phrynosoma douglasii (Bell, 1828)
• Sonoran horned lizard, Phrynosoma goodei Stejneger, 1893
• Greater short-horned lizard, Phrynosoma hernandesi Girard, 1858
• Flat-tail horned lizard, Phrynosoma mcallii (Hallowell, 1852)
• Roundtail horned lizard, Phrynosoma modestum Girard, 1852

Comparison of P. modestum and P. platyrhinos

• Mexican Plateau horned lizard or Chihuahua Desert horned lizard, Phrynosoma orbiculare (Linnaeus, 1758)

• P. o. bradti Horowitz, 1955
• P. o. cortezii (A.H.A. Duméril & Bocourt, 1870)
• P. o. dugesii (A.H.A. Duméril & Bocourt, 1870)
• P. o. orbiculare (Linnaeus, 1758)
• P. o. orientale Horowitz, 1955

• Desert short-horned lizard, Phrynosoma ornatissimum (Girard, 1858) (disputed; may be conspecific with P. hernandesi)
• Desert horned lizard, Phrynosoma platyrhinos Girard, 1852

• Southern desert horned lizard, P. p. calidiarum (Cope, 1896)
• Northern desert horned lizard, P. p. platyrhinos Girard, 1852

• Guerreran horned lizard, Phrynosoma sherbrookei Nieto-Montes de Oca, Arenas-Moreno, Beltrán-Sánchez, & Leaché, 2014
• Regal horned lizard, Phrynosoma solare Gray, 1845
• Mexican horned lizard, Phrynosoma taurus Dugès, 1873

Note: In the above list, a binomial authority or trinomial authority in parentheses indicates that the species or subspecies was originally described in a genus other than Phrynosoma.

• 
  Blainville's horned lizard (P. blainvillii)
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  Short-tailed horned lizard (P. braconnieri), Oaxaca, Mexico (20 May 2013).
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  Texas horned lizard (P. cornutum), Armstrong County, Texas, USA (28 April 2013)
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  Coast horned lizard (P. coronatum), San Luis Obispo County, California, USA (June 14, 2008)
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  Coast horned lizard (P. coronatum) (25 April 2009)
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  Greater short-horned lizard (P. douglassi brevirostre), Sweetwater County, Wyoming, USA (15 June 2016).
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  Pygmy short-horned lizard (P. douglasii), Washington, USA (5 June 2014)
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  Mountain short-horned lizard (P. hernandesi), Culberson County, Texas, USA (19 May 2018)
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  Flat-tailed horned lizard (P. mcallii) (9 November 2010)
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  Round-tail horned lizard (P. modestum), Municipality of Tula, Tamaulipas, Mexico (15 August 2004)
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  Mountain horned lizard (P. orbiculare orientale), Municipality of Miquihuana, Tamaulipas, Mexico (24 September 2009)
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  Desert horned lizard (P. platyrhinos), San Bernardino County, California, USA (9 July 2019)
• 
  Regal horned lizard (P. solare), Apache Junction, Arizona, USA (25 March 2007)

The genus of horned lizards is the official state reptile of Wyoming.[11] Texas designated the Texas horned lizard (Phrynosoma cornutum) as the official state reptile in 1993[12] and the "TCU Horned Frog" is the mascot of Texas Christian University in Fort Worth, Texas. TCU is the only known athletic team with the horned lizard as a mascot.

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  The TCU mascot is commonly known as Super frog to TCU fans and students. He is usually present at TCU sporting events.
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  Horned lizard on a necktie at El Paso, Texas, in 1940

Wikispecies has information related to Phrynosoma.

Wikimedia Commons has media related to Phrynosoma.

• Horned lizard skulls and info at Digimorph.org
• Horned Lizard Conservation Society
• Horned Lizards at UTexas.edu
• Argentine ants linked to declines in coastal horned lizards
• Horned Toads - Field study of Short-horned Lizards by students of Waterville Elementary School