Pygmy short-horned lizard COSEWIC assessment and status report: chapter 3
Species Information
Name and classification
Horned lizards (Family: Phrynosomatidae), colloquially known as horny or horned toads, belong to a single genus (Phrynosoma). All species inhabit portions of the arid and semi-arid ecoregions of North and Central America. Currently there are 13 recognized species within the genus, eight of which occur in North America (Crother 2000). Two species extend their ranges into Canada. The Greater Short-horned Lizard (P. hernandesi) extends from the Great Plains into southern Alberta and Saskatchewan and the Pygmy Short-horned Lizard (P. douglasii) historically extended from the Great Basin into the southernmost part of central British Columbia. Previously, as many as six subspecies of short-horned lizards were recognized (Nussbaum et al. 1983). Recent genetic assessments have resulted in the Pygmy Short-horned Lizard being recognized as a distinct species (Zamudio et al. 1997). More recent work suggests that P. hernandesi is more closely related to the Rock Horned Lizard (P. ditmars) than to P. douglasii (Hodges and Zamudio 2004). No subspecies of P. douglasii are currently recognized.
The spellings of both the common name and the species name have been in contention. Pygmy has also been spelled Pigmy, recently Short has been dropped (e.g., Sherbrooke 2003), and douglasii has been spelled with two “s” and one “i”. This report adopts the updated spelling presented in the Scientific and Standard English names of Amphibians and Reptiles of North American (Crother et al. 2003).
Morphological description
All horned lizards are relatively small with rounded and flattened trunks and short legs. This squat form makes it difficult for them to move quickly or with agility in a cluttered environment, but does allow them to have large stomachs, accommodate a large number of embryos, and create a large dorsal surface area for heliothermy (basking in the sun). Equally as distinctive are elaborate parietal and squamosal horn-like protuberances on their heads and bodies. Each species has its own unique array of horns. The tail is short and incapable of regrowth if lost.
Males have an enlarged tail base that houses the hemipenes and a pair of elongated post-cloacal scales (Nussbaum et al. 1983). Males also have enlarged femoral pores in single rows along the lower back edge of the thigh that may function as scent glands (Sherbrooke 1981). Female Pygmy Short-horned Lizards are slightly larger than males, averaging 7 mm longer from snout to vent (SVL) (Nussbaum et al. 1983).
The Pygmy Short-horned Lizard is the smallest of the horned lizards, usually about 4.5 cm long from snout to vent (SVL) (Lahti 2005) with a maximum size of 6.5 cm SVL (Nussbaum et al. 1983). The horns on the rear crest of the head are shorter than in other horned lizards (same width at their base as they are long) and a distinct notch separates these horns from either side of the head. A single row of horns forms a fringe along the margins of the body (Figure 1).
Colour and pattern are variable within the species. Overall, the colour matches the substrate so that when they are motionless, the lizards are very difficult to detect. The dorsum is typically grey with about 12 dark blotches, usually arranged in transverse rows of 2 to 4 blotches (Nussbaum et al. 1983) with no blotches being centred above the vertebrae (Brown et al. 1995). Each blotch is paler on its posterior edge. When horned lizards are cool they tend to be darker, which allows greater absorption of solar heat, and they become paler as they warm (Sherbrooke 2003). The tail has dark transverse bands and the ventral surface is pale.
Newborns resemble adult lizards but they are a mere 2.2 cm (Nussbaum et al. 1983, Lahti 2005) to 3.2 cm SVL (Brown et al. 1995). Newborns appear to lack horns, which develop as they mature (Brown et al. 1995).
Figure 1. Photographs of an adult and a juvenile Pygmy Short-horned Lizard from the nearest known population in WashingtonState.
Photos by Scott Fitkin of the Washington Department of Fish and Wildlife
Two other lizard species are sympatric with the Pygmy Short-horned Lizard in Canada; the Northern Alligator Lizard (Elgaria coerulea) and the Western Skink (Eumeces (Plestiodon) skiltonianus). Neither of these species resembles horned lizards.
Genetic description
There has been an assumption that the Canadian population of Pygmy Short-horned Lizards was disjunct from populations to the south in Washington State. This is unlikely, though, as there is a continuum of suitable habitat to remaining populations in Washington State, suggesting that populations were continuous, at least until historic times. The limited vagility of the species, especially in a heterogeneous landscape, may account for great differences in life histories of neighbouring populations (Nussbaum et al. 1983, Marcot 1997). This effect seems evident in the Oregon Cascade Mountain population, which appears to be significantly different than other populations in the Pacific Northwest (Brown et al. 1995). The most intensive genetic analysis has been conducted by Hodges and Zamudio (2004) who examined 38 populations of Pygmy Short-horned Lizards and confirmed that P. douglasii was a distinct clade, although this finding is not supported by morphological data. No genetic analysis has been conducted to determine whether the Canadian population was distinct in any way from nearby populations in Washington State or elsewhere.