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204 IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 HENRY ET AL.

Inserting a PIT tag into a Texas Horned Lizard (Phrynosoma cornutum) allows for individual recognition during the entire study.

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The genus Phrynosoma contains 13 species inhabiting the
grasslands and deserts of the central and southwestern

United States and much of Mexico. Horned lizards are charac-
terized by a flattened body and enlarged spines surrounding the
head. Their shortened legs and broad body leave them ill suited
for speed over longer distances. As protection against potential
predators, they depend primarily on their cryptic coloration.
When threatened, a horned lizard’s first line of defense is to flat-
ten itself against the substrate. If pressed further, it will run a
short distance and then “freeze” to reestablish its camouflage or
seek cover under nearby vegetation. Horned lizards are well
known for another defense mechanism employed primarily
against canids, the ability to squirt blood from the eye socket.
They are considered dietary specialists, with ants comprising
50% or more of their diet and the remainder of other small
arthropods (Sherbrooke 2003). 

The suite of characteristics mentioned above makes horned
lizards unique among North American lizards. That many of
these species have experienced declines in recent years (Fisher et
al. 2002, Mathies and Martin 2008, Wone and Beauchamp
2003) is disturbing. The Texas Horned Lizard (Phrynosoma cor-
nutum) is listed as threatened in the state of Texas and as a
species of special concern in both Oklahoma and Colorado.
Several studies have shown a reduction in both its range and

numbers over the last 40–50 years. Habitat conversion for agri-
culture and urbanization appears to be the leading cause
(Donaldson et al. 1994). Activities associated with farming, such
as plowing and pesticide use, also may harm horned lizards. The
destruction and fragmentation of habitat for urbanization seems
to have a negative effect, but lizards are consistently found in
suburban areas and even remnant habitats within urban areas
(Stark 2000, Endriss et al. 2007, Moody et al. 2007). The intro-
duction and spread of imported Red Fire Ants (Solenopsis
invicta) also has been linked to the decline of the Texas Horned
Lizard (Price 1990). Eradication efforts aimed at the invasive fire
ant have included widespread use of insecticides. Although this
is not likely to produce direct mortality in horned lizards, it does
harm populations of Harvester Ants (Pogonomyrmex spp.) and
other insects on which the lizards feed. Moreover, the invader
effectively competes with native ants. Lastly, collection for the
pet industry may have contributed in the past to the Texas
Horned Lizards’ dwindling numbers (Price 1990).

Since 2005, we have been part of an effort to understand
the ecology of this species, determine the proximate causes of its
overall decline, and develop management strategies to enhance
its long-term survival. As a part of this effort, we have been
studying two populations of P. cornutum in Texas, one near Post
and the other outside of Brownwood.

The Texas Horned Lizard 
in Central and Western Texas

Emily Henry, Jason Brewer, Krista Mougey, and Gad Perry

Department of Natural Resource Management, Box 42125, Texas Tech University, Lubbock, TX 79409-2125, USA

Photographs by Emily Henry except where noted.

IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 205TEXAS HORNED LIZARD

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Although horned lizards usually rely on crypsis to avoid detection by a predator, if discovered, they fill their bodies with air to look as large and
fierce as possible.



Study Sites
A portion of this study was conducted on the Beach Ranch, a
3,636-ha private ranch located 21 km east of Post, in Garza
County, Texas. Garza County averages 48 cm of rain annually,
most of which falls in thunderstorms during May and June. The
average minimum temperature in January is just below freezing,
and the average maximum in July is 35.0 °C. Soils at the site are
primarily clay and fine sandy loams (Richardson et al. 1975).
The major vegetation communities are mesquite grasslands and

desert scrub. The dominant woody species is Mesquite (Prosopsis
spp.). Grasses such as Buffalograss (Buchloe dactyloides) and
Sideoats Grama (Bouteloua curtipendula) and cactus species such
as Prickly Pear (Opuntia spp.) and Cholla (Opuntia imbricata)
are common. The rolling grasslands are fragmented by rocky val-
leys, artificial stock ponds, and a fork of the Brazos River. The
Beach family leases this land for cattle grazing, but also works
hard to restore and maintain it as good habitat for wildlife. No
controlled burns have been conducted on the site in recent years,
but a recent wildfire burned a large swath of the ranch. 

An additional study site was located at Camp Bowie, a mili-
tary training facility outside of Brownwood, Brown County, Texas.
Precipitation in the area follows a bimodal pattern with a large peak
in May–June and a smaller peak in September–October. Annual
precipitation for Brownwood averages 72 cm (NOAA 2008). The
climate is characterized by hot summers and cool, dry winters. Soils
on the site are mostly fine sandy loams with clay subsoils (NRCS
Soil Data Mart). Camp Bowie is situated in a transition zone
between the Western Cross Timbers and Rolling Plains ecoregions
(Omernik 1995). As a result, the vegetation varies from wooded
areas of Post Oak (Quercus stellata) and Blackjack Oak (Quercus
marilandica) with an understory of Greenbrier (Smilax spp.) and
grasses to grassland areas with Mesquite (Prosopis glandulosa) scrub.
The base is managed primarily for National Guard training activ-
ities, but some hunting and fishing is allowed by permit. Livestock
grazing is currently suspended, but was historically a common
practice.

206 IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 HENRY ET AL.

Jason Brewer measuring the snout-vent length (SVL) of an adult
horned lizard. 

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Horned lizard habitat at the Beach Ranch, Post, Texas (top and mid-
dle) and Camp Bowie, Brownwood, Texas (bottom).

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IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 207TEXAS HORNED LIZARD

Materials and Methods
Each field season began shortly after horned lizards emerged from
hibernation in April or early May. Throughout the activity season,
we captured lizards by hand and recorded data on collection loca-
tion, morphology, and weather conditions. Lizards over 20 g were
implanted with a passive integrated transponder (PIT tag) for

Attaching radio-transmitters to horned lizards: To prevent the radio
transmitter from becoming separated from the lizard during shedding,
a plastic zip tie collar was put around the neck of the lizard and fish-
ing line was used to attach the transmitter to the collar (top); Krista
Mougey attaching the radio transmitter to the collar using fishing line
(middle); Kelly Bollin sealing a transmitter with silicone (bottom).

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After transmitters were attached (top), the transmitter was covered with
silicone pressed into dirt to act as camouflage (bottom).

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Recording data on a juvenile Texas Horned Lizard.



future identification. These tags work somewhat like a barcode;
each has a unique number that can be read with a specialized scan-
ner. We fitted each lizard over 40 g with a radio transmitter,
attached between the shoulder blades using silicone adhesive, so
that it could be located again over the rest of the season. The
lizards were re-located at least three times per week during the
spring and summer and once or twice per week during the fall,
when activity declined greatly. We repeated the morphological

measurements once a week to keep track of growth and identify
when a female had deposited a clutch. We plotted the location
information onto satellite images of the study sites and calculated
home range size for each animal using the minimum convex poly-
gon method. In 2008, we also characterized habitat use at both
study locations, using digital photographs to quantify the avail-
ability of bare ground, rocks, grass, and other plants.

Results and Discussion
We observed a total of 442 lizards at the Beach Ranch from
2005–2008 and 26 at Camp Bowie during 2007–2008. As
expected, females were significantly larger than males in both
populations. Montgomery et al. (2003) suggested that this
species follows the reverse of Bergmann’s rule and decreases in
body size with an increase in latitude (contrary to what is typi-
cal of mammals and birds), presumably because the shorter
warm season does not allow animals to grow as large. Compared
to the mean sizes found in other studies (Stark 2000, Henke
2003, Montgomery and MacKessy 2003, Moeller et al. 2005,
Endriss et al. 2007), the lizards at the Post site were somewhat
larger than expected and those at Camp Bowie were somewhat

208 IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 HENRY ET AL.

Because the effect of radio-transmitters on lizards has been debated in
previous studies, during the 2008 season at the Beach Ranch, we fit-
ted lizards with transmitters of varying sizes and weights (top: small;
bottom: large). Rocks were added to some for additional weight.
Calculations of home range sizes of these lizards will be used to evalu-
ate the impact of different weights.

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Once the transmitters were firmly attached (top), animals were released
(middle). Emily Henry uses a handheld receiver and antenna to track
a lizard (bottom).

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IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 209TEXAS HORNED LIZARD

smaller than expected. We suspect that the larger adult size
observed on the Beach Ranch indicates superior habitat;
although grazing occurs, the prairie remains mostly in its natu-
ral state. A substantial Harvester Ant population has been
observed, perhaps because the site is not treated with pesticides.
Sex ratios at the two sites were similar, with females slightly out-
numbering males at both locations. Previous studies suggest that
this may be caused by differences in capture rates, rather than
true differences in numbers between the sexes (Sherbrooke
2002). According to capture numbers, adults outnumbered
juveniles at both sites. Once again, however, we suspect that this
is at least partially a result of differences in detectability; hatch-
lings are very small, highly cryptic, and very difficult to see in
the field.

Mating was observed at both study sites during May and
June, with oviposition dates from mid-May to late July. We
repeatedly observed double clutching (an instance of a female
laying two clutches over a single summer) at the Post location.
Hatchlings were generally first seen in early August, but, in
2008, we observed hatchlings at Post in early July. Survivorship
at Post ranged from 19–53%, and similar values were noted at
Camp Bowie. We have noted mortality resulting from predation
by birds (especially Roadrunners and shrikes), mammals (espe-
cially coyotes and rodents), and snakes (especially Coachwhips
and Diamondback Rattlesnakes). In some cases, death was
caused by management actions such as road maintenance dur-
ing the winter; for others the cause of mortality could not be
determined.

Horned lizards were most active when temperatures were
between 27 and 35 °C. As in previous studies, activity remained
high from April through July and then tapered off, although we

We characterized habitat by using digital photographs to quantify the
availability of bare ground, rocks, grass, and other plants.

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Radio-transmitter-equipped lizards among native grasses and forbs.



noted minor variations between the sexes and among years
(Henke and Montemayor 1998, Fair and Henke 1999,
Montgomery and MacKessy 2003). Based on movement levels,
previous studies have noted that lizards remained active through-
out the day during the spring and then exhibited a strong
bimodal pattern during the summer, with peaks occurring in the
cooler hours of morning and evening (Montgomery and
MacKessy 2003). We prefer a broader definition of activity, and
classified lizards as active if they were engaged in behavior such
as moving or foraging, similar to previous researchers, but also
if they were stationary but alert, with open eyes and clearly
responsive to the environment. Using that definition, lizards on
our study sites were indeed active all day during spring, but their
activity did not decline much during the summer.

Based on Stark’s findings that male lizards moved consider-
ably longer distances than females during the mating season

(Stark 2000, Stark et al. 2005), males would be expected to have
correspondingly larger home range sizes. Our results do not sup-
port this hypothesis; male and female horned lizards had home
ranges of similar sizes at both of our study sites. The variation
among individual lizards was much greater than that between the
sexes, years, or even between sites. Texas Horned Lizards at Beach
Ranch were often found in close proximity to each other and
had overlapping home ranges. At both study sites, lizards fre-
quently used dirt roads, cattle trails, and adjacent areas. In 2007,
lizards at both sites were almost exclusively found on or in the
immediate vicinity of roads, presumably because this was an
unusually wet year and the vegetation was particularly dense. In
years with low summer rainfall, however, we noticed lower over-
all activity and found considerably fewer lizards along roadsides.
As observed at other locations (Burrow et al. 2001, Fair and
Henke 1998) lizards at both of our study sites made use of all

210 IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 HENRY ET AL.

When mating, the male holds the female’s horn in his mouth and
wraps his tail around hers.

In addition to casting shade, forbs and grasses provide lizards with a
way to elevate their bellies off the hot substrate.

A female horned lizard emerging from her overnight burrow.



IGUANA  •  VOLUME 15, NUMBER 4  •  DECEMBER 2008 211TEXAS HORNED LIZARD

habitat categories except embedded rock. Open areas allow the
animals to increase their body temperatures and also facilitate
movements. Litter and vegetation shelter them during the hottest
parts of the day and provide protection from predators. This
mosaic is critical in providing suitable habitat for this species.

Conclusions
Studies carried out on native range habitat such as the Beach
Ranch, even in remnant pockets within urban areas (Stark 2000,
Endriss et al. 2007, Moody et al. 2007), have shown healthy and
stable populations of Texas Horned Lizards. This suggests that
loss of habitat is the most important factor in this species’
decline, and other factors may have more localized effects. This
makes preservation and proper management of remaining habi-
tat especially critical. Common land management practices can
have both beneficial and harmful effects on horned lizards. Low
to moderate levels of grazing appear to improve the habitat for
horned lizards, perhaps by increasing open space at ground level.
Fire is likely to have a similarly positive effect on habitat. An
increase in mortality from vehicles and maintenance activities
can result because lizards often frequent roads and other dis-
turbed areas, but this was minimal at our low-traffic sites. 

Acknowledgments
We thank the Beach Family, the National Guard, and the Taylor
family for allowing us to conduct this work on their land. Our
gratitude goes out to the colleagues and assistants who helped in
the field: Jacob Goldfarb, Juan Carlos Diaz, Kelly Bollin, Alan

Sosa, and Jacqueline Forbus. Funding for this project came from
the Texas National Guard, the Horned Lizard Conservation
Society, and Prairie Biotic Research. This is manuscript T-9-
1151 of the College of Agricultural Sciences and Natural
Resources, Texas Tech University.

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Even with transmitters providing information, well-camouflaged
lizards were sometimes hard to find.

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