Phrynosoma cornutumTexas Horned Lizard

Geographic Range

The distribution of the Texas Horned Lizard ranges from Kansas to Northern Mexico and from Arizona to Louisiana (Bockstanz 1998).

Habitat

The Texas Horned Lizard lives mainly in sandy areas where it often inhabits abandoned animal burrows (Bockstanz 1998). The habitat of the Texas Horned Lizard is usually in close proximity to the nests of harvester ants (Seymour 1996).

Biomes: Desert, grasslands, prairies, scrubland (Bartlett and Bartlett 1999)

Physical Description

The Texas Horned Lizard has a dorsolaterally flattened, toad-like body (Seymour and Royo 1996). Due to their unusual body shape body and short legs, they have been mistaken for an amphibian, hence the name "Texas Horned Toad."

There are spines located on the head, along the sides of the body and down the tail (Bockstanz 1998). These spines are modified epidermal scales. At the back of the head, there are two elongated spines that look like horns (Seymour and Royo 1996). There are also spines located along the dorsal surface of the lizard (Bockstanz 1998).

The ventral surface of the lizard is either gray or tan (Seymour and Royo 1996). The dorsal surface of the lizard is tan or gray with white and red or yellow highlights (Bockstanz 1998). There is a pattern of dark spots on the dorsal surface of the lizard, which correspond to the location of the dorsal spines (Bockstanz 1998).

The length of an average Texas Horned Lizard is 69mm snout-vent length (Munger 1986), however the upper boundary for males is 94mm and for females it is 114mm (Munger 1984).

Reproduction

The breeding season begins in late April and continues into July (Seymour 1996). These lizards are oviparous, and will lay their eggs in moist, sandy areas (Bartlett 1999). The eggs have a flexible, white shell, which measures 1-and1/2 inches in diameter (Seymour 1996). The incubation period for the eggs is 45-55 days (Bartlett 1999). The hatchlings are approximately 1.25 inches long, and are relatively smooth. However, the hatchlings do have the spines around their heads. There is no evidence of parental care for the young, so they must find food and defend themselves against predators immediately after hatching. The age of reproductive maturity is not known, however they are full-grown adults at three years of age (Seymour 1996).

Behavior

Phrynosoma cornutum is a diurnal lizard (Bockstanz 1998). The daily activities of the Texas Horned Lizard are a response to temperature changes in the environment (and related activity of its prey, the harvester ants).

The main methods of behavioral thermoregulation used by the Texas Horned Lizard are basking and burrowing. Throughout the morning hours, the lizard angles itself to maximize the amount of heat received when basking in the sun (Heath 1964). In order to keep cool, Texas Horned Lizards will burrow in the sand or hide in the shade. The burrowing process involves pushing the pointed snout into the sand and moving it from side to side. While continuing this movement, the body is inflated and is moved in the same way until the entire body is covered with sand (Heath 1965). The burrowing process is an important behavior in thermoregulation, since it can protect the lizard from heat or cold depending on the temperature of the soil in which the animal is buried (Potter and Glass 1931).

Hibernation is much like the daily burrowing activities of the lizard. However, during hibernation the animal will slow down its metabolism and can persist for long periods of time without food or water (Potter and Glass 1931). The hibernation season lasts from late summer to late spring(Bockstanz 1998). When they emerge from hibernation, the breeding season begins(Bockstanz 1998).

There are other unique behaviors of the Texas Horned Lizard, which aid in its survival. For example when an enemy approaches the lizard it will inflate itself, and if it is sufficiently frightened it can squirt up 1/3 of its volume of blood out of a pore near the eye (Middendorf and Sherbrooke 1992). One behavior that may explain how it can persist in arid habitats is the process of "rain-harvesting." During heavy rains the lizard will stand high on its feet, spread the body out flat, and lower the head so that falling rain will be funneled to the mouth through interscalar channels (Sherbrooke 1990).

Food Habits

The Texas Horned Lizard eats mainly Harvester ants, Pogonomyrmex spp., but will also eat grasshoppers, isopods, beetles and beetle larvae. In order to obtain enough energy, adult Texas Horned Lizards must forage from several colonies of Harvester ants. The Texas Horned Lizards' daily activities are planned around the times of highest ant activity. (Donaldson, et al 1994)

Conservation Status

Phrynosoma cornutum is listed as a threatened species in Texas and Okalahoma. The numbers of the Texas Horned Lizard have declined for several reasons: the collection of the lizard as a pet, the invasion of the imported fire ant, Solenopsis invicta, and loss of habitat (Donaldson, et al 1994). The change in land use has taken away homes from the lizards, and the use of pesticides on the harvester ants has depleted their main food source (Donaldson, et al 1994). Texas Parks and Wildlife along with other conservation groups are studying aspects of the Texas Horned Lizard as to protect it from any further negative human impact.

Contributors

Rebecca Todd (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.

Glossary

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5? N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

References

Bartlett, P., R. Bartlett. 1999. A Field guide to Texas Reptiles and Amphibians. Houston Texas: Gulf Publishing Company.

Bockstanz, L. 1998. Accessed September 22, 1999 at http://www.zo.utexas.edu/research/txherps/lizards/phrynosoma.cornutum.html.

Donaldson, W., A. Price, J. Morse. 1994. The Current Status and Future Prospects of the Texas Horned Lizard (*Phrynosoma cornutum*) in Texas. The Texas Journal of Science, 46: 97-113.

Heath, J. 1965. Temperature Regulation and Diurnal Activity in Horned Lizards. Los Angeles: University of California Press.

Heath, J. 1964. Head Body Temperature Differences in Horned Lizards. Physioological Zoology, 37: 273-279.

Middendorf, G., W. Sherbrooke. 1992. Canid Elicitation of Blood-squirting in a Horned Lizard. Copeia: 519-527.

Munger, J. 1984. Home Ranges of Horned Lizards (*Phrynosoma*) Circumscribed and Elusive?. Oecologica, 62: 351-360.

Munger, J. 1986. Rate of Death Due to Predation for two Species of Horned Lizard, Phrynosoma cornutum and Phrynosoma modestum. Copeia, 3: 820-824.

Potter, G., H. Glass. 1931. A Study of Respiration in Hibernating Horned Lizards, Phrynosoma cornutum. Copeia: 128-131.

Seymour, G., A. Royo. 1996. "Desert USA" (On-line). Accessed September 22, 19999 at http://www.desertusa.com/april96/du_hliz.html.

Sherbrooke, W. 1990. Rain-Harvesting in the Lizard, Phrynosoma cornutum: Behavior and Integumental Morphology. Jornal of Herpetology, 24: 302.