Horned Lizard Research and Observations

Phrynosoma modestum, JCM
The Phrynosomatidae, is a group of iguanian lizards that range from Canada to Panama and contains some of the most familiar saurians in North America, including: the spiny lizards (Sceloporus), horned lizards (Phrynosoma), and side-blotched lizards (Uta). The family contains nine genera (ten genera if the genus Sator is recognized separately from Sceloporus) and more than 136 species. Weins et al. (2010) used molecular techniques to show that phrynosomatids are divided into two major clades the Phrynosomatinae and Sceloporinae. Phrynosomatinae contains the horned lizard clade Phrynosoma and the sand lizard clade (Callisaurus, Cophosaurus, Holbrookia, and Uma).

Skull of a Phrynosoma with spiny processes.JCM
Horned Lizards of the genus Phrynosoma are remarkably cryptic. Their dorso-laterally flattened bodies, tan and brown coloration and spine covered heads and bodies make them exceptionally difficult to find. The spine covered heads and bodies of these lizards undoubtedly serve as a deterrent to predators, but horned lizards also eat ants and store the noxious formic acid from the ants in their blood. Yet some predators are able to deal with the spines and chemicals. O’Connor, et al. (2010) report finding an adult female Great Basin Nightsnake (Hypsiglena chlorophaea deserticola) in Kittitas County, Washington that regurgitated a half-digested adult male Pygmy Short-horned Lizard (Phrynosoma douglasii) estimated to be about 41 mm SVL and 6 grams. The snake's mass was estimated to be just over 12 g. so the predator-prey mass ratio was about 0.50.

Lahti, et al. (2010) found Phrynosoma (Tapaja) douglasii inhabiting 3 distinct microhabitat types (lithosol, loamy, and ecotone) within the shrub-steppe of central Washington’s Quilomene Wildlife Area. The study site had been used for grazing until 1979, and fires were minimal in the last 30 years. June and July were the peak activity months for lizards. Most lizards were encountered in lithosol (61%), a habitat with sparse vegetative cover and weathered fragments of rock; followed by ecotone (31%); and loamy (8%) microhabitats. Lizards, particularly those inhabiting lithosol microhabitats, did not usually retreat to shrub cover until approached within 1 m. While horned lizards are considered low-density species relatively high population densities have been reported for Phrynosoma (Tapaja) douglasii (14.3 to 14.6 lizards/ha in eastern Idaho). However, the authors report a density of about 2 lizards per hectare at their study site, a density that is more characteristic of that reported for other Phrynosoma species. Neonates were almost always encountered closest to Thymeleaf Buckwheat and would often retreat toward the plant when approached. Thymeleaf Buckwheat has the smallest and most compact growth form of any perennial plant at Quilomene Wildlife Area. In contrast, adults would usually retreat to either Stiff Sage or Rock Buckwheat, both of which are larger. The authors conclude that Phrynosoma (Tapaja) douglasii occurs at low densities in the shrub-steppe of Washington where females are larger and more abundant than males, neonates are rare, and reproductive output is low. Younger lizards maintain activity into hotter periods and remain active later in the activity season than do adults, a trait likely related to the importance of garnering sufficient energy to emerge in good condition after a long winter. While this species is most commonly encountered in shrub-steppe habitats, it shows considerable spatial and seasonal variation in the use of microhabitats.

Hellgren et al. (2010) describe the effects of rotational livestock grazing and prescribed winter burning on the resources and survival of the Texas Horned Lizard (Phrynosoma cornutum) in southern Texas. Winter burning provided an increase in food resources and led to increased survival rate in the second growing season after fire. However, grazing-induced changes in vegetation cover reduced survival, probably by increasing lizard vulnerability to predation. Fire and grazing reduced litter, increasing open ground and forb cover but did not alter woody vegetation. Ant activity was greater in burned sites and varied with grazing level, season, and year. Higher survival observed on burned sites in the second year after burning. Survival rates were ordered from highest in un-grazed sites to lowest in heavily grazed sites.

In three papers Cooper and Sherbrooke (2010a,b,c) investigated anti-predator behavior in the Round-tailed Horned Lizard (Phrynosoma modestum) and the Texas horned lizard (Phrynosoma cornutum). They (Cooper and Sherbrooke, 2010a) investigated the effects of repeated attacks by a predator on the Texas Horned Lizard, P. cornutum, and the opportunity cost of fleeing during a social encounter in P. modestum. The results suggest flight initiation distance was greater the second time a predator approached and probability of fleeing decreased as the distance between the predator and prey increased, but was greater when the predator turned toward than away from a lizard. The flight initiation distance was shorter during social encounters than when lizards were solitary. It appears that risk assessment by horned lizards conforms to the predictions of escape theory and is similar to that in other prey despite their specialized defenses. The results suggest that escape theory based on costs and benefits applies very generally, even to highly cryptic prey with specialized defense mechanisms.

In a second paper, Cooper and Sherbrooke (2010b) note that Phrynosoma modestum is eucryptic in that it resembles small stones and the authors predicted that flight initiation distance by P. modestum is shorter among stones than on uniform sand and that flight initiation distance is greater after movement and when standing than when still and lying on the ground.  Movement and upright posture disrupt crypsis in this lizard. The authors measured running speed and flight initiation distance to determine relationships among body temperature, speed, and escape decisions. Running speed and flight initiation distance were reduced at lower body temperature, suggesting that crypsis reinforced by immobility is more advantageous than longer flight initiation distance for cool, slow lizards. Thus, the Round-tailed Horned lizard adjusts its escape decisions to the current effectiveness of crypsis and escape ability.

Cooper and Sherbrooke (2010c) found that Texas horned lizards (Phrynosoma cornutum) would take flight sooner when approached rapidly rather than slowly and when approached directly rather than indirectly. They also found  P. cornutum were much more likely to move and jump when a model predatory bird passed overhead and cast a direct shadow on them as opposed to casting a shadow near the lizard. They suggest P. cornutum assess themselves as being in immediate peril when suddenly covered by a shadow. So, while the Texas Horned Lizard relies heavily on crypsis, they make escape decisions based on the degree of predation risk.

Cooper, W. E. and W. C. Sherbrooke. 2010a. Plesiomorphic Escape Decisions in Cryptic Horned Lizards (Phrynosoma) Having Highly Derived Antipredatory Defenses. Ethology, 116: 920–928. doi: 10.1111/j.1439-0310.2010.01805.x
Cooper, W. E. and W. C. Sherbrooke. 2010b. Crypsis influences escape decisions in the Round-tailed Horned Lizard (Phrynosoma modestum). Canadian Journal of Zoology, 88:1003-1010.
Cooper, W. E. and W. C. Sherbrooke. 2010c. Initiation of Escape Behavior by the Texas Horned Lizard (Phrynosoma cornutum). Herpetologica 66:23-30.
Hellgren, E. C., A. L. Burrow, R. T. Kazmaier, and D. C. Ruthven. 2010. The Effects of Winter Burning and Grazing on Resources and Survival of Texas Horned Lizards in a Thornscrub Ecosystem. Journal of Wildlife Management 74(2):300-309.
Lahti, M. E., D. D. Beck, and T. R. Cottrell. 2010. Ecology of the Pygmy Short-Horned Lizard [Phrynosoma (Tapaja) douglasii] in Washington. Northwestern Naturalist 91(2):134-144.
Montanucci, R.R. 2004. Geographic variation in Phrynosoma coronatum (Lacertilia, Phrynosomatidae): further evidence for a peninsular archipelago. Herpetologica 60 (1): 117-139
O'Connor, A. P., J. L. Wallace, R. E. Weaver, and M. P. Hayes. 2010. Pygmy Short-Horned Lizard (Phrynosoma douglasii): Unrecorded Prey for the Great Basin Nightsnake (Hypsiglena chlorophaea deserticola).  Northwestern Naturalist 91(1):79-81.
Wiens , J. A., C. A. Kuczynski, S. Arif , and T. W. Reeder. 2010. Phylogenetic relationships of phrynosomatid lizards based on nuclear and mitochondrial data, and a revised phylogeny for Sceloporus. Molecular Phylogenetics and Evolution 54:150–161

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