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Eastern Short-horned Lizard

Horned lizards are small, well-camouflaged reptiles, which are common in arid regions of western North America. The subspecies found in Alberta is the Eastern Short-horned Lizard (Phrynosoma douglassii brevirostre), which, at the northern extreme of its distribution, ranges into the southeastern corner of the province. In the past, little was known about Eastern Short-horned Lizards in Alberta. Earlier status reviews had described the species as threatened or had placed it on the provincial Red List. The most recent provincial review of the status of Alberta wildlife has assigned the Eastern Short-horned Lizard to the Blue List of species that may be at risk in the province.

As lizards go, horned lizards are very atypical. Rather than running, or seeking refuge in burrows from predators, horned lizards generally refrain from movement, even when approached. These lizards use Camouflage to avoid and escape their predators, and their mottled, sandy colouration and spiny skin blends in very well with the dry substrates they inhabit.

Phrynosomes are sit-and-wait predators, dashing out and nabbing their prey as it wanders within range. Most phrynosomes consume primarily ants, and have specialized teeth and a large stomach capacity for the digestion of this highly chitinous food item. Horned lizards are round and flat in body shape, with sharp, spiked skin and short legs. This unusual lizard body form renders them rather slow and cumbersome in movement, which likely helped gain them the name "horned toad".

Eastern Short-horned lizards in Alberta are relatively small creatures, with the average adult female weighing about 18 grams and having an average snout-vent length of approximately 70 millimeters. Adult males are considerably smaller, with an average snout-vent length of around 50 millimeters and weight of approximately 10 grams. Short-horned Lizards in Alberta are generally active from emergence in mid- to late April or early May until around mid-September. However, observations of emergence as early as April have been documented, with activity being sustained although much reduced until late October or early November. Female Short-horned Lizards in Alberta are thought to live approximately five years, but the lifespan of males remains unclear.

Horned lizards move between heat sources and heat sinks to control their internal body temperature within a preferred range, a characteristic behaviour among lizards known as shuttling. These lizards often bask in direct sunlight in order to increase body temperature -- a process known as heliothermy. They may flatten the body (by extending the ribs), adjust their position, orientation, and tilt to increase surface area exposed to incident radiation. Thigmothermy, or the gaining of heat from an object (such as a rock) has also been observed in these lizards. To cool down, individuals may seek shade, alter their orientation, narrow the body profile, and may even burrow in the substrate.

The overwintering dens of Eastern Short-horned Lizards were initially hypothesized to be within the deep crevasses of the rock comprising the Bearpaw formation, which underlies much of the habitat of these lizards in Alberta. However, radiotelemetry has shown that individuals simply excavate a relatively shallow burrow (approximately 10 centimeters beneath the surface) I n the loose soil of a south-facing slope, perhaps relying on snow cover for insulation. Overwintering mortality and mortality due to early and late seasonal extremes have been suggested as possible reasons for the low population densities of these lizards in Alberta.

Reprinted from Alberta Wildlife Status Report No. 5 (1997), with permission from Alberta Sustainable Resource Development.

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