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8 IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 MOODY, ENDRISS, HELLGREN, AND FOX

A Texas Horned Lizard (Phrynosoma cornutum) in native grass at Tinker Air Force Base, Oklahoma.

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Agreat wealth of knowledge is beginning to unfold about the
Texas Horned Lizard (Phrynosoma cornutum) through part-

nerships and contributions from many agencies, universities,
and conservation groups. On the frontline of this research is the
United States Air Force!

Tinker Air Force Base (AFB) is the largest single-site
employer in the State of Oklahoma and home to large-bodied
aircraft such as the E-3 Sentry, E-6A, and the KC-135 refueling

Studying a Population of Texas Horned
Lizards (Phrynosoma cornutum) in an

Urban/Military Environment
Raymond W. Moody1, Debora A. Endriss, Eric C. Hellgren, and Stanley F. Fox

1 Biologist, Natural Resources Program
72nd ABW/CEVOE, 7701 Arnold St, RM 109

Tinker Air Force Base, Oklahoma, 73145-9100 USA
(raymond.moody@tinker.af.mil)

Photographs and graphics by the senior author.

Abstract.–University researchers from Southern Illinois (SIU), Oklahoma State (OSU), Virginia Tech (VT), and Federal agencies have
come together as partners to learn more about the Texas Horned Lizard (Phrynosoma cornutum) at Tinker Air Force Base (AFB) in
Oklahoma. Although listed as a state “Threatened” species in Texas, the lizard is a state “Species of Special Concern” in Oklahoma due
to uncertainty regarding its status and lack of population data, making it a species of considerable conservation interest. Researchers at
Tinker AFB are using tools such as handheld computers, passive integrated transponder (PIT) tags, global positioning systems (GPS),
radio-telemetry, and geographic information systems (GIS) for gathering and interpreting biological data on Texas Horned Lizards. Over
the last four years (2003–2006), 174 individual lizards have been captured and over 1700 capture locations and associated data have
been recorded in the GIS lizard-tracking database. These data are beginning to reveal aspects of morphology, distribution, habitat require-
ments, home ranges, and hibernation characteristics as well as details of its myrmecophagous (primarily ants) diet. Population sizes (± 1
SE) within Tinker AFB’s urban wildlife reserve are estimated to be 53 ± 11 individuals with a density of 5.0 ± 1.0 lizards/ha. Utilization
of these tools opens the door for different ways of studying horned lizard habitat requirements and population trends. They also provide
powerful insights for better managing a sensitive species on a busy military installation as well as in an ever-changing urban setting.

IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 9PHRYNOSOMA CORNUTUM

�

Military training on a busy Air Base. Both the lizard and the military
require access to the base’s resources.

Texas Horned Lizard (Phrynosoma cornutum), a Species of Special
Concern in Oklahoma.

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tanker. It is one of the world’s largest aircraft depot maintenance
facilities — but it is also home to the Texas Horned Lizard, an
Oklahoma State Species of Special Concern and a Threatened
species in Texas (ODWC 2006, TPW 2006).

The lizard is experiencing declines throughout its range,
particularly in Texas (Price 1990) but also in Oklahoma
(Carpenter et al. 1993). How then is it possible for a highly
industrialized air force base nestled within a major metropolitan
area to have a population of Texas Horned Lizards? A frequent
comment heard in Oklahoma is: “I used to see them all the
time.” Is this just a reflection of people not getting out these
days? That seems unlikely, however, considering the increasing
popularity of outdoor activities such as wildlife watching.

U.S. Fish and Wildlife Service (1999) surveys indicate that
wildlife-related activities doubled between 1980 and 1995. From
1991–2001, no significant statistical change occurred in these
types of activities (USFWS 2001). So, could habitat loss and
alteration from development be a more likely cause? Carpenter
et al. (1993) surveyed new landowners who reported seeing
horned lizards when they first moved into a new housing devel-
opment followed by quick disappearance of the lizards. Or is the
decline in populations a combination of other factors? Many
authors have suggested that insecticides (direct effects and prey
base alteration), agricultural practices, irrigation, mowing, till-

ing, over-collection for the pet trade, increased predation, and
roadkill (Price 1990, Carpenter et al. 1993, Donaldson et al.
1994) are reasons for declines. Interestingly, all of these factors
are connected to urban development, which seems to be the
common link.

These factors and the fact that Tinker AFB has a healthy
population of lizards raise many other questions for a natural
resources biologist managing land resources on a busy military
installation. That’s part of what the research at Tinker is about —
how can the lizard and the military mission coexist in harmony.

Study Site and Objectives
The 2,000-ha Tinker AFB is situated in a transition area
between Oklahoma City and rural settings, with major metro-
politan areas immediately north and suburban/rural areas to the
south. This also is reflected on the base, with industrial and com-
munity settings concentrated in the north fading into airfield
and fragments of developed and undeveloped land plots and rel-
atively undisturbed greenways to the south. The focal point of
the study is within and around a 15-ha Wildlife Reserve, part of
the base’s Urban Greenway.

The approximately 60-ha study site is characterized by
prairie, patches of riparian vegetation, shrubs, and transitional
woodlands. Dominant native species at the site include Little

10 IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 MOODY, ENDRISS, HELLGREN, AND FOX

Aerial view of Tinker Air Force Base and encroaching urban areas, a dark gray shaded area represents the 60-ha study site.

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IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 11PHRYNOSOMA CORNUTUM

Bluestem (Schizachyrium scoparium), Big Bluestem (Andropogon
gerardii), Indiangrass (Sorghastrum nutans), and Maximilian
Sunflower (Helianthus maximiliani). Other native species
include Side Oats Grama (Bouteloua curtipendula), Eastern Red
Cedar (Juniperus virginiana), and American Elm (Ulmus ameri-
cana). Prominent non-native species include Oldworld Bluestem

(Bothriochloa ischaemum) and Tall Fescue (Lolium sp.), with
some small patches of Chinese Lespedeza (Lespedeza cuneata)
usually associated with Maximilian Sunflower (Dorr et al. 2003).

The major focus of the study was to better understand the
ecology of the Texas Horned Lizard, particularly in fragmented
urban type habitat. Specific objectives of the study included: (1)
Determination of distribution, (2) determination of habitat
requirements, (3) understanding behavior and demography, and
(4) determination of population status and trends.

Materials and Methods
Horned lizards were captured by hand during fortuitous
encounters on trails, roads, along areas of bare ground and in the
brush. Morphological (sex, body mass, snout-vent length, and
total length) and various habitat measurements were taken and
if large enough, lizards were taken back to the lab for processing
with tags and radio transmitters. Passive integrated transponder
(PIT) tags were implanted for identification of individual lizards.
One of two different sizes of transmitters (Holohil Inc., Model:
BD-2, 1.8 and 1.3 g) was mounted on the lizard’s back with
superglue and an elastic collar (Hellgren et al. 2004). Over the
last year silicone was substituted for superglue which seems to
be less abrasive to the lizards and much easier to remove when
exchanging transmitters. Select lizards that were too small for
transmitters were tracked using fluorescent powder.

Measuring the total body length of a Texas Horned Lizard.

Texas Horned Lizard in hand showing an elastic collar and transmit-
ter mounted on its back.

Biologist Ray Moody with Debora Endriss locates a Texas Horned
Lizard within Wildlife Reserve 3 using radiotelemetry.

Debora Endriss records microhabitat data and GPS coordinates for
lizard locations.

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Movements of individual lizards were monitored through
radiotelemetry. Lizards were relocated 1–5 times per week dur-
ing the summer months and once per week or less during the
fall until they entered hibernation. Global Positioning System
(GPS) coordinates and microhabitat characteristics (%–grass,
forb, shrub, tree, bare ground, rock, litter) for each lizard loca-
tion were determined and entered into the GIS lizard tracking
database. Hibernation sites were also marked and data recorded
for site characteristics (e.g., depth, soil type, slope, aspect). Data
were recorded using hand-held computers (IPAQ and GeoXT)
equipped with GPS (Trimble GPS Pathfinder Pocket Receiver)
and customized ArcPad 6.02 software. Data were differentially
corrected and imported into ArcMap 9.1 (ArcGIS Version 9),
which was used to develop a GIS layer for the lizards at Tinker.
Once lizards entered and stayed in hibernation sites, wire mesh
cages were placed above them for capture and release with new
transmitters when they emerged from hibernation in the spring
(Endriss 2006).

Results and Discussion
Researchers collected data over four years starting in the sum-
mer of 2003, focusing on the active summer months and also
tracking lizards into the fall and start of the hibernation period.
Population sizes (± 1 SE) within Tinker AFB’s urban wildlife
reserve are estimated to be 53 ± 11 individuals with a density of
5.0 ± 1.0 lizards/ha. The lizard GIS layer has 174 individual
lizards with over 1700 capture/relocation points, which readily
portray macrohabitat use, home ranges, nesting and hibernation
sites on high-resolution digital ortho-quad color aerial photo-
graphs (9 cm/pixel resolution). 

Some observations that have potential management impli-
cations are starting to emerge. For instance, lizards at Tinker
were much smaller (average snout vent length for females was
68.4 mm; for males 59.4 mm) than those that have been stud-
ied farther south (Hellgren et al. 2004). These findings contra-
dict Bergmann’s rule, which proposes an increase in animal body
sizes with an increase in latitude (Ashton and Feldman 2003).

12 IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 MOODY, ENDRISS, HELLGREN, AND FOX

Expanded view of individual lizard locations at the study site; dots of different shades represent localities for individual lizards. Many lizard loca-
tions are stacked on top of one another at this scale, which maps over 1700 locations.

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IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 13PHRYNOSOMA CORNUTUM

Texas Horned Lizard cooling itself on gravel nature trail.

Texas Horned Lizard peering over Oldworld Bluestem. Texas Horned Lizard just prior to emergence from hibernation.

Texas Horned Lizard peering through native Bluestem.

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14 IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 MOODY, ENDRISS, HELLGREN, AND FOX

Texas Horned Lizard resting in small tree.

Five Texas Horned Lizards sluggish after removal from hibernacula on a cool spring morning.

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IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 15PHRYNOSOMA CORNUTUM

Stark (2000) and Burrows et al. (2001) found lizards to uti-
lize a mosaic of vegetation types from bare ground to areas
densely vegetated with forbs and grasses. Fair and Henke (1998)
and Burrows et al. (2001) indicated that Texas Horned Lizards
require a mixture of open and vegetated habitats to meet ther-
moregulatory requirements. This was apparent in our study,
with lizards using grasses, forbs, shrubs/small trees, and bare
ground. When lizard locations where compared with GIS vege-
tation layers, no preference was shown regarding vegetation type,
other than an apparent avoidance of Red Cedar shrubland. In
addition, some areas that were known to be more densely vege-
tated and isolated from bare ground and trails were lacking in
lizard locations. GIS layers also showed home ranges to be
closely associated with graveled nature trails and paved
walking/jogging trails (Endriss 2006). Vegetation structure and
availability of bare ground seems to be more important to lizards
than the vegetation species type (i.e., whether native or not).
Therefore, gravel and paved trails within the study site are likely
very important for thermoregulation and movement between
habitat resources.

Lizard home ranges were determined with the aid of the
GIS layer. They averaged 0.84 ha for adult females and 0.90 ha
for adult males (95% fixed kernel method, Arc View 3.3 exten-
sion). Sixteen lizards were tracked to hibernation sites, where
characteristics such as hibernation period (5.5–7.0 months),
slope aspects (facing south-southwest), and soil depths (2–12
mm) were recorded. Ten lizards were tracked to nesting loca-
tions, where nest depths of 5.0–7.5 cm were found (Endriss
2006). Characteristics such as these are important factors to con-
sider for planning land management activities such as prairie
restoration efforts (e.g., spraying, mowing, tilling, disking, and
prescribed burning).

Land managers often use prescribed burning for prairie management. Timing of these events is very important for Texas Horned Lizard management.

Texas Horned Lizard with an old-style transmitter and pack at nest
entrance.

Hatchling Texas Horned Lizards emerging from nest site.

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Capture locations and other documented historical sight-
ing locations were used to construct a distribution layer for
lizards. This was accomplished with the buffer tool in ArcMap,
creating 360-m (maximum home range length) buffers around

all lizard locations, and then eliminating known unfavorable
habitat. Knowing distribution and potential habitat areas for the
lizards allows for consideration in planning of base development,
as well as the ability to prescribe mitigation when conflicts arise.
Populations of Texas Horned Lizards often have a localized dis-
tribution, and thus can survive in protected areas within an
urban environment (Endriss 2006). For example, populations
of horned lizards often are found in small city parks and vacant
lots (Carpenter et al. 1993, Donaldson et al. 1994, Stark 2000).
Military bases often find themselves with islands of relatively
undisturbed habitat as urban development encroaches upon
them. Apparently, on Tinker, all the right natural resources,
including habitat and prey, are available to sustain a viable pop-
ulation of Texas Horned Lizards.

Learning more about horned lizards and other sensitive
species is important, and knowledge garnered is vital to proper
management of these species, especially in highly dynamic envi-
ronments such as an industrialized military installation. This
unfolding of Texas Horned Lizards life history and ecology at
Tinker AFB may be key to understanding the lizard in other

16 IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 MOODY, ENDRISS, HELLGREN, AND FOX

Spraying herbicide on turf to restore the area to native grass.

Texas Horned Lizard distribution on Tinker AFB showing historical and recent survey locations. Isolated sightings within potentially favorable
habitat have occurred outside the main study area.

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IGUANA  •  VOLUME 14, NUMBER 1  •  MARCH 2007 17PHRYNOSOMA CORNUTUM

environments. We hope that this knowledge will foster informed
management and decision-making regarding the species across
its range. Such management in return would promote more
robust and stable populations. This would lessen the potential
for federal or state listing as threatened or endangered species,
thereby precluding potential land use and other restrictions on
the military and private landowners. In this scenario, both
humans and nature win.

Acknowledgements
Work on this study could not have been accomplished without
the help of many partners. Special thanks go to colleagues who
have worked so hard in the field: Joe Hackler, Vic Bogosian, and
Matt Johnson. Jessica Dorr and Jennifer McCanne provided
GIS support and Clark Baker arranged field housing facilities.

Literature Cited
Burrow, A.L., R.T. Kazmaier, E.C. Hellgren, and D.C. Ruthven, III. 2002. The

effects of burning and grazing on survival, home range, and prey dynam-
ics of the Texas horned lizard in a thornscrub ecosystem, pp. 43–51. In:
W.M. Ford, K.R. Russell, and C.E. Moorma (eds.), The Role of Fire in
Nongame Wildlife Management and Community Restoration: Traditional
Uses and New Directions: Proceedings of a Special Workshop. USDA Forest
Service Northeastern Research Station General Technical Report NE-
288.Syracuse, New York.

Carpenter, C.C., R. St. Clair, and P. Gier. 1993. Determination of the distribu-
tion and abundance of the Texas horned lizard (Phrynosoma cornutum) in
Oklahoma. Final report, Federal Aid Project E-18, Oklahoma
Department of Wildlife Conservation, Oklahoma City, Oklahoma.

Donaldson, W., A.H. Price, and 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.

Dorr J.L., P. Swint, and V.R. Emrick. 2005. Tinker Air Force Base Vegetative
Communities Map and Vegetation Classification System. Conservation

Management Institute – Military Lands Division, College of Natural
Resources, Virginia Polytechnic Institute and State University. CMI-
MLD-2005-R-34. Blacksburg, Virginia.

Endriss D.A. 2006. Ecology of an Urban Population of the Texas Horned
Lizard (Phrynosoma cornutum) in Central Oklahoma. Unpublished M.S.
Thesis, Oklahoma State University, Stillwater.

Fair, W.S. and S.E. Henke. 1998. Habitat use of Texas Horned Lizards in
southern Texas. Texas Journal of Agriculture and Natural Resources
11:73–86.

Hellgren E.C., S.F. Fox, and D.A. Endriss. 2004. Ecology of the Texas Horned
Lizard on Tinker Air Force Base, Oklahoma. Annual Report 2004 for
Tinker AFB, Oklahoma State University, Stillwater.

Oklahoma Department of Wildlife Conservation (ODWC). 2006.
(www.wildlifedepartment.com/endanger2.htm).

Price, A.H. 1990. Phrynosoma cornutum. Catalogue of American Amphibians and
Reptiles (469):1–7.

Stark, R.C. 2000. Habitat use, daily movements, and body size of the Texas
Horned Lizard in an urban environment in north-central Oklahoma.
Unpublished M.S. Thesis, Oklahoma State University, Stillwater.

Texas Parks and Wildlife Department (TPWD). 2006. (www.tpwd.state.tx.us/
huntwild/wild/species/endang/animals/reptiles_amphibians/).

United States Fish and Wildlife Service (USFWS). 1999. Participation in
Fishing, Hunting, and Wildlife Watching National and Regional
Demographic Trends. USFWS Report, September 1999. Department of
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Fishing, Hunting, and Wildlife Associated Recreation – National Overview.
USFWS Report, May 2001. Department of the Interior, Washington,
D.C.

Educational signs alert humans to lizards on the base.

The white middorsal line of the Texas Horned Lizard is cryptic, pro-
viding excellent camouflage in a native grass environment where the
stripe mimics dead plant stems.

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