Horned lizards (Phrynosoma), also known as horny toads or horntoads, are a genus of North American lizards and the type genus of the subfamily Phrynosomatinae. The common name is a metaphor prompted by their flattened, rounded body and blunt snout.
The genus name Phrynosoma means "toad-bodied". In common with large true frogs and toads, horned lizards tend to move sluggishly, making them easy to catch; this may also avoid triggering attacks by predators. They are adapted to arid or semi-arid areas. The spines on the lizard's back and sides are made from modified reptile scales which prevent the water loss through the skin, whereas the horns on the head are true horns (i.e. they have a bony core). Of the 22 species of horned lizards, 15 are native to the United States. The largest-bodied and most widely distributed of the US species is the Texas horned lizard.
Video Horned lizard
Defenses
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.
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. Ocular autohemorrhaging has also been documented in other lizards, 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.
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.
Maps Horned lizard
Population decline
A University of Texas publication notes that horned lizard populations continue to disappear throughout the southwest despite protective legislation. The Texas horned lizard 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
The following 22 species are recognized as being valid, three species of which have recognized subspecies.
- Giant horned lizard, Phrynosoma asio Cope, 1864
- Phrynosoma bauri Montanucci, 2015
- Phrynosoma blainvillii Gray, 1839
- Short-tailed horned lizard, Phrynosoma braconnieri A.H.A. Duméril, 1870
- Phrynosoma brevirostris (Girard, 1858)
- Cedros Island horned lizard, Phrynosoma cerroense Stejneger, 1893
- Texas horned lizard, Phrynosoma cornutum (Harlan, 1825)
- Coast horned lizard, Phrynosoma coronatum (Blainville, 1835)
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- 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
- Phrynosoma diminutum Montanucci, 2015
- Ditmars' horned lizard or rock horned lizard, Phrynosoma ditmarsi Stejneger, 1906
- Pygmy short-horned lizard, Phrynosoma douglasii (Bell, 1828)
- 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
- Mexican Plateau horned lizard or Chihuahua Desert horned lizard, Phrynosoma orbiculare (Linnaeus, 1758)
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- 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
- Phrynosoma ornatissimum (Girard, 1858)
- Desert horned lizard, Phrynosoma platyrhinos Girard, 1852
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- Southern desert horned lizard, P. p. calidiarum (Cope, 1896)
- Northern desert horned lizard, P. p. platyrhinos Girard, 1852
- Phrynosoma sherbrookei Nieto-Montes de Oca et al., 2014
- Regal horned lizard, Phrynosoma solare Gray, 1845
- Mexican horned lizard, Phrynosoma taurus Dugès, 1873
- Gulf Coast horned lizard, Phrynosoma wigginsi Montanucci, 2004
Nota bene: 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.
Symbol
The genus of horned lizards is the official state reptile of Wyoming.
Texas designated the Texas horned lizard (Phrynosoma cornutum) as the official state reptile in 1993 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.
References
External links
- 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
Source of the article : Wikipedia
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