Pygmy short-horned lizard COSEWIC assessment and status report: chapter 6

Biology

A substantial amount of anecdotal biological information has been collected and numerous studies in the southern United Stateshave been conducted on many of the species of horned lizards, including populations that were formally considered to be the same species as the Pygmy Short-horned Lizard, but are now recognized as P. hernandesi. Excellent observations are summarized in Amphibians and Reptiles of the Pacific Northwest (Nussbaum et al. 1983) and Reptiles of Washington and Oregon (Brown et al. 1995). In addition to these texts, a doctoral study was conducted on lizard life histories with specific information on the ecology and behaviour of Pygmy Short-horned Lizards (Zamudio 1996). Another study was conducted in Idaho, focusing on the growth rates and densities of Pygmy Short-horned Lizards (Guyer and Linder 1985) and another on the ecology of a population in Kittitas County, Washington (Lahti 2005). A brief study of the relationship between land management activities and Pygmy Short-horned Lizard abundances was also conducted in Idaho (Reynolds 1979). Beyond this, it is necessary to speculate about their biology, based on what is known of other horned lizards, in particular, the extensive studies that have been done on the Greater Short-horned Lizard in Alberta at the same latitude (e.g., Powell and Russell 1985, 1991a,b, 1993, 1994, 1996, 1998, James 1997, 2002, 2004, Powell et al. 1998). Consequently, great attention has been paid in this report to identifying whether the following information is specific to Pygmy Short-horned Lizards, other species of horned lizard, or just horned lizards in general.

Life cycle and reproduction

Short-horned Lizards are the most cold tolerant of the horned lizards but must nonetheless seek winter refugia during the cold winters typical of northerly latitudes or high elevations (Heath 1964, 1965). During hibernation, metabolism is greatly reduced as a function of body temperature. Hibernacula have been observed for the Greater Short-horned Lizard in Alberta where lizards burrow only to about 10 cm, and it is thought that snow cover provides additional insulation against freezing (e.g., James 1997). This strategy poses a dilemma in areas such as the Okanagan and Similkameen valleys that often have little or no snow cover during cold periods in the winter. These lizards must find deeper refugia, employ cryonics, or suffer widespread mortalities. Pygmy Short-horned Lizards emerge from winter hibernacula in late March to early June, depending on local climate (Brown et al. 1995). In the Okanagan, the Western Skink emerges from hibernacula in late March to early April (Sarell unpubl. data).

Mating occurs shortly after winter emergence (Brown et al. 1995), which is typical for most horned lizards (Pianka and Parker 1975, Montanucci and Baur 1982, Sherbrooke 2003). During the mating period, adult male horned lizards defend territories by performing head bobbing and pushup displays when another male is encountered, while adult females roam among males’ territories until mating occurs (Sherbrooke 2003).

The embryos are retained in the mother throughout gestation and young are born live. Viviparity (retention of eggs until live birthing) is typical of all short-horned lizard species and two other species of high elevation horned lizards in Mexico. Live bearing is an adaptive strategy employed by many reptiles in cooler climates or where seasons of activity are shorter (e.g., Zamudio and Parra-Olea. 2000). Clutch sizes for other species of short-horned lizards have exceeded 30 (e.g., Goldberg 1971), but in the Pacific Northwest clutch sizes appear to range from 3 to 15 (Nussbaum et al. 1983, Brown et al. 1995, Stebbins 2003), with a tendency for larger and presumably older, female horned lizards give birth to larger numbers of young (Sherbrooke 2003). Pygmy Short-horned Lizards are born in August to mid-September in eastern Washington and Oregon (Brown et al. 1995); however, the first neonate observation in Kittitas County was on the 16th of July (Lahti 2005). Phrynosoma hernandesi in Alberta is reported to have a single brood each year with parturition occurring in late July (Powell and Russell 1991a), similar to that found in Kittitas County. Two high elevation species in Mexico extend embryonic development over winter and young are born in the following summer (Zamudio and Parra-Olea 2000). Maternal care for newborns has not been reported.

One mark-recapture study in Idaho surmised that male Pygmy Short-horned Lizards cease to grow after one year whereas females continue to grow throughout their second year, attaining larger adult sizes than males (Guyer and Linder 1985). Growth rates for P. hernandesi in Alberta have been observed to be the same for both sexes until they reach approximately 270 active days old, at which point males cease to grow and female growth continues (Powell and Russell 1991a). Sexual maturity in males is thought to occur in their second year, as has been documented for P. hernandesi in Alberta (Powell and Russell 1991a). Consequently, their first breeding experience could not occur until the following spring. Females of the same species in Alberta become mature in their second year and would also have to wait until the following spring to breed (Powell and Russell 1991a).

Longevity of short-horned lizards is not well documented. Growth studies of the Greater Short-horned Lizard (P. hernandesi) indicate that minimum ages of adults are two for males and three for females and estimated maximum longevity exceeds five years (Powell and Russell 1991a). Guyer and Linder (1985) refer to Pygmy Short-horned Lizards in Idaho as being “long-lived iteroparous” (capable of multiple, separate breeding events). This statement is based on the low mortality rate associated with adults (35%) as compared to neonates (90%). However, it should be noted that mortality rates could be lower if emigration had gone undetected. In the wild, the Desert Horned Lizard (P. platyrhinos) is known to live at least eight years and horned lizards are capable of living 10 years in captivity (Sherbrooke 2003). A generation time of five years for the Pygmy Short-horned Lizard is assumed.

Foraging

All horned lizards consume terrestrial invertebrates, with at least 50% of their diet consisting of ants (Sherbrooke 2003). The most sought after are harvester ants (Pogonomyrmex spp.). Horned lizards are unaffected by the stings of these ants (Schmidt et al. 1989) and are able to consume some 70+ ants per day, and perhaps as many as 200 (Whitford and Bryant 1979, Sherbrooke 1995). Worker ants are usually taken at the entrance to the nest or along foraging trails. Harvester ants can be locally plentiful and are eaten by very few other predators and subsequently are easily exploited, even by young horned lizards with small gapes. Ants, however, have a very low nutrient to exoskeleton ratio so many must be eaten to fulfill a lizard’s nutritional demands, and hence the advantage of a very large stomach.

Juvenile Short-horned Lizards almost exclusively eat ants, which constitute up to 90% of their diet (Lahti 2005). Adult horned lizards, especially females, have large gapes and are capable of consuming larger insects (e.g. Orthoptera, Coleoptera) and other invertebrates (Powell and Russell 1983), including snails (Stebbins 2003). In Kittitas County adult diets were almost 70% ants, and beetles were the next most commonly eaten prey (Lahti 2005), which is consistent with prey studies in northern California (Zamudio 1996). This diversity may permit some partitioning of prey resources between different ages and sexes within the same general area.

It is believed that horned lizards do not require free-standing water. It is assumed that their water requirements are met by licking dew, taking advantage of precipitation by “rain harvesting” (translocation of water along scutes to the mouth) and from the prey they consume (Sherbrooke 2003).

Defences and predation

Horned lizards avoid predation initially by flattening their bodies onto the ground and remaining motionless. When immobility is coupled with their cryptic appearance, lizards often go unnoticed by potential predators. Smaller, fast-moving predators that detect horned lizards are met with a bluff consisting of inflating the body and sometimes lunging at the predator (Sherbrooke 2003). Fleeing is attempted when slow-moving predators approach. Attempts to bite the lizard are usually painful because of the horns and attempting to swallow the lizard can cause death to predators such as snakes and hawks when the lizard becomes lodged in the throat (Sherbrooke 2003). This is less effective for P. douglasii as its horns are relatively small, possibly a result of having few reptilian predators which swallow their prey whole (e.g., leopard lizards, whiptails) throughout their range (Dumas 1964). Some horned lizards are capable of squirting blood derived from a sinus below their eyes, especially during encounters with canids (e.g., coyotes), which find the liquid distasteful. In humans, the blood causes eye irritation (Sherbrooke 1983, 2001, 2003) and hence these two species do not see eye to eye. Squirting blood has not been observed in Pygmy Short-horned Lizards (Sherbrooke 2001).

The most effective predators of Short-horned Lizards are birds, especially Loggerhead Shrikes (Pianka and Parker 1975). This shrike is considered a rare visitor in western Canada so it is unlikely to be a significant predator of the Canadian population of Pygmy Short-horned Lizards. The Northern Shrike is a common overwintering resident within the range of these lizards, but most of the birds do not arrive until October and leave by April (Cannings et al. 1987), during which time the lizards are hibernating. Other avian predators include hawks, such as the American Kestrel (Sherbrooke 2003), which can be relatively abundant in the Okanagan and Similkameen Valleys. Ravens, Burrowing Owls and Swainson’s Hawks have also been reported to prey upon horned lizards (Duncan et al. 1994, Whitford and Bryant 1979).   Wild felids have not been identified as predators but the domestic cat has been cited as a significant predator (e.g., Monaster 1997).

Dispersal/migration

Virtually nothing is known of dispersal or migration behaviour of the Pygmy Short-horned Lizard.  Hibernacula could be located within their active home range or could be located some distance away. The Greater Short-horned Lizard was detected moving 100 m during the breeding season and 266 m over a one week period prior to hibernation (James 2004). One Greater Short-horned Lizard in Utah was translocated and returned 400 m to its place of capture (Pianka and Parker 1975), indicating a capability of homing behaviour.

Adaptability

Horned lizards are not very adaptable to changes in their environments. They have evolved physiology and behaviours that bond them to arid environments. Even within these arid ecosystems their occurrence is not ubiquitous (James 2004). Even subtle changes such as colonization by invasive plants or agronomic species can make habitat unsuitable (e.g., Reynolds 1979). In addition, horned lizards are dietary specialists, feeding primarily on one species of ant.

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