United States Department of Transportation
i
Use of simulated highway underpass crossing structures by flat-tailed horned lizards (Phrynosoma mcallii).
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2008-05-01
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Edition:Final report; June 2005-Feb. 2007.
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Abstract:The flat-tailed horned lizard (Phrynosoma mcallii) occupies a restricted range in the Lower Sonoran Desert of
southwest Arizona, southeast California, and adjacent land in Mexico. Because they exhibit behavior patterns
that include basking and remaining motionless when danger approaches, flat-tailed horned lizards are
particularly susceptible to mortality on roads. Therefore, roads and new road construction are recognized as
threats influencing the long-term persistence of this species. The propensity for flat-tailed horned lizards to
use culverts as road crossing structures to avoid vehicle-caused mortality is unknown. From 2005-2006 we
studied flat-tailed horned lizard use of a variety of simulated road crossing structures. The study objectives
were to 1) determine if flat-tailed horned lizards will pass through culverts of sizes commonly used in road
construction, and 2) compare and describe the characteristics of culverts used by flat-tailed horned lizards to
those not used. We built a testing facility with six culverts of three dimensions and two interior lighting
options. All culverts were 40 feet long; the three types included 24-inch diameter steel culverts, 36-inch
diameter steel culverts, and 4-foot tall by 8-foot wide box culverts. One of each type of culvert was lit with
skylights, and one of each type of culvert had only natural light from the ends. Light and temperature
conditions in the culverts were evaluated during the study. Out of 54 flat-tailed horned lizards placed in the
testing facility, we observed 12 complete crossings. The 36-inch diameter culvert without skylights was used
five times. The 24-inch diameter culvert with skylights was not used, and other culvert designs were each
used once or twice. Results indicated that flat-tailed horned lizards can use culverts as road crossing
structures, but the evidence did not reveal a strong selection for or against any culvert type.
Recommendations for employing appropriate road crossing structures are discussed.
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