Greater short-horned lizard COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

The taxonomic classification of Short-horned Lizards has been significantly reorganized since the last COSEWIC report (Powell and Russell, 1992b). The species found in Alberta and Saskatchewan, formerly known as the Eastern Short-horned Lizard, or Phrynosoma douglasii brevirostre (Russell and Bauer 1993a) has been reclassified in accordance with new information put forward by Zamudio et al. (1997). Previously, all Short-horned Lizards were classified as part of Phrynosoma douglasii. This widely distributed species was then subdivided into six subspecies primarily based upon morphological considerations (Reeve 1952). Two of these subspecies had distributions that ranged into Canada: Phrynosoma douglasii douglasi, the Pygmy Short-horned Lizard, in south-central British Columbia, and Phrynosoma douglasii brevirostre, the Eastern Short-horned Lizard, in Alberta and Saskatchewan.

Zamudio et al. (1997) found genetic, as well as morphological, evidence to support division of Phrynosoma douglasi (sensu lato) into two separate species. Those populations located in the Pacific Northwest, and previously designated as the subspecies P.d. douglasi, or the Pygmy Short-horned Lizard, were re-classified as Phrynosoma douglasi (sensu stricto). The rest of the subspecies within Phrynosoma douglasi in the remainder of the distribution; those of the Great Plains, Colorado Plateau and high altitude locations along the Rocky Mountains in the southern United States and Mexico, were together designated P. hernandezi (Zamudio et al. 1997). The spelling put forward by Zamudio et al. (1997) was based upon Hammerson and Smith (1991). Subsequently, Smith et al. (1999) reviewed and corrected a previous interpretation, resulting in general acceptance of the species epithet of ‘hernandesi’.

Zamudio et al. (1997) recognized the potential for paraphyly of the P. hernandesi group, given the complex climatic and geological shifts that have occurred in the central part of their range over the last 20 million years, but found no evidence to support any sub-specific designations. However, within the larger P. hernandesi grouping two main clades were evident: one based in the Colorado Plateau and Great Basin area, and the larger comprised of essentially the remainder of the distribution; south and east of the Rocky Mountains (Zamudio et al. 1997). Crother (2000) suggested sub-specific designations within P. hernandesi and Sherbrooke (2003) stated there are five subspecies, but these subspecies do not appear to be widely recognized. Crother (2000) suggested that the segment of the distribution that includes those populations in Alberta and Saskatchewan should be classified as Phrynosoma hernandesi hernandesi. Ultimately, it is suggested that the species in Canada be referred to as simply Phrynosoma hernandesi, based upon the findings of Zamudio et al. (1997).

Additionally, there remains a degree of discord about the proper common name of the former Eastern Short-horned Lizard. Russell and Bauer (2000) term Phrynosoma hernandesi as the Mountain Short-horned Lizard. The common name recognized by Sherbrooke (2003) and used by NatureServe (2005) is simply the Short-horned Lizard. Stebbins (2003) refers to the species as the Greater Short-horned Lizard while concurrently acknowledging the name Mountain Short-horned Lizard. However, it appears that the recognized authority of Stebbins (2003) has influenced the majority of references in the literature. Therefore it is suggested that the species be referred to as the Greater Short-horned Lizard (Crother 2000).

Morphological description

Horned lizards are small, well-camouflaged lizards with a rim of protective spikes around the posterior margin of the skull. The relative length and position of these horns, varies with each of the 17 species within Phrynosoma currently recognized (Leaché and McGuire 2006). All horned lizards are dorso-ventrally flattened, with a fringe of protruding scales along their sides. They have broad heads, wide bodies, and short tails, giving them a squat, stocky appearance. Due to their physical structure, their gait is somewhat waddling, and this, in combination with their stature, likely led to the slang terms “Horny toad” or “Horned toad” often used in popular texts. Of course, as reptiles, they have little in common with their amphibious namesakes, except perhaps the occasional items in their respective diets, and perhaps certain predators.

Short-horned Lizards (Phrynosoma hernandesi) are, as the common name suggests, characterized by a relatively short, though still noticeable, array of ‘horns’ and a deep hornless notch across the back of the head (Fig. 1; Sherbrooke 2003; Stebbins, 2003). They have a single row of fringe scales along their sides (Russell and Bauer 2000; Sherbrooke 2003; Stebbins 2003). Colouration varies by region but is always highly cryptic with local substrates. The back has a grey, beige, tan or even reddish base speckled with white and covered with blotches of darker brown, with a pair of darkest blotches behind the head (Russell and Bauer 2000; Stebbins 2003). Ventral surfaces are light coloured: beige or lighter, and sometimes tinged with yellow or orange (Russell and Bauer 2000; Stebbins 2003).

Field studies in Alberta (Powell and Russell 1985a) found the snout-vent length (SVL) of adult females to be approximately 69 mm while adult males were around 52 mm SVL. Males of all ages exhibit enlarged post-anal scales (Powell and Russell 1985a), and a noticeable swelling at the base of the tail during the breeding season (Stebbins 2003). In Alberta, Phrynosoma hernandesi exhibit marked sexual dimorphism, with females growing considerably larger than males (Powell and Russell 1984; 1985a). Both sexes grow at the same rate, but males cease growth and reach reproductive maturity earlier than females by at least one year (Powell and Russell 1985a). An adult female may attain a non-gravid mass of approximately 18 g, whereas an adult male will plateau at around 10 g (Powell and Russell 1985a).

Genetic description

Short-horned Lizards are the most widespread group within their genus and the species exhibits a notable degree of variation over its broad distribution. Zamudio et al. (1997) collected tissue samples and derived mtDNA sequences from five of the six subspecies of P. douglasii (sensu lato) from the across the distribution in the United States, and including samples from Alberta. This was a wide-scale, species-level investigation, and was not intended to investigate population-level variation at a more regional scale. From the phylogenetic tree derived in this study, the subspecific affiliation for those populations in Alberta was nearest to those populations in North and South Dakota, Montana, Wyoming, and Nevada (Zamudio et al. 1997). Therefore the species in Canada is expected to have only recently, in geologic terms, expanded its range from more southern refugia since the last glaciation (Zamudio et al. 1997).

In Canada, although there is presumed to be some degree of genetic exchange within each of the major populations, it is presumed unlikely that there is any exchange between the major populations, owing to the small size of the lizards and the considerable distances between the populations. Differences in colouration among populations are noticeable, which is thought to be reflective of genetic variation between populations (Powell 1982). As well as being widely separated, some of the major drainages that populations are associated with are not connected, further reducing the probability of interaction between populations through remaining habitat corridors. However, given the lack of genetic data to compare among populations, it was decided that there is only a single Designatable Unit for this species.