Greater short-horned lizard COSEWIC assessment and status report: chapter 6
Biology
Short-horned Lizards have been relatively well studied in Canada (Powell 1982; Powell and Russell 1984; 1985a; 1985b; 1991a; 1991b; 1992a,b; 1993a; 1993b; 1994; 1996; 1998; Powell et al. 1998; James 1997, 2002, 2003; ASRD 2004). Although the majority of the observations herein are from Alberta populations, they are presumed to be generally applicable to the Saskatchewan populations as well.
Diet
Horned lizards are insectivorous ‘sit-and-wait’ predators that take advantage of their cryptic colouration to ambush prey, which is comprised mainly of ants (Pianka and Parker 1975). The diet of Short-horned Lizards in Alberta was determined to be somewhat more general in nature than that of other horned lizard species (Powell and Russell 1984). Ants predominate the diet in terms of numbers of items eaten, but in terms of the dry mass of prey ingested, Coleopterans (beetles) and Orthopterans (crickets and grasshoppers) combined were more significant (Powell and Russell 1984). Despite the smaller number of grasshoppers, crickets and beetles eaten, their greater combined mass is likely a better indicator of the relative importance of these prey items. Once they have captured large prey, such as crickets or beetles, lizards have been observed to smash them against the ground in an attempt to stun them before they are swallowed head first (Laird and Leech 1980). Not surprisingly, larger lizards are able to consume larger prey items (Powell and Russell 1984).
Life cycle and reproduction
In Albertan populations of Greater Short-horned Lizards, spring emergence generally occurs around mid- to late April or early May (Laird and Leech 1980), with courtship and mating observed to occur in mid- to late May (James 1997). Powell and Russell (1996) recorded emergence as early as April 1 for males. After emergence, females move to their individual feeding areas and become somewhat sedentary, with intermittent shifts amongst relatively small feeding areas (Powell and Russell 1993b; 1994; James 1997). Males, on the other hand, may roam relatively widely to seek receptive females (Henke and Montemayor 1998). The marked sexual dimorphism in the species is most evident at this point in the life cycle, with the comparatively small males seeking and courting the notably larger females. The receptive period for mating is relatively short, perhaps from a few days to a week or so, at which point females become unreceptive to further advances (Montanucci and Baur 1982; James 1997).
Females are thought to become sexually mature following their second hibernation, while males are thought to be capable of breeding in the summer following their first hibernation (Powell and Russell 1985a; 1991b). It is estimated that females live up to five years in Alberta populations, although male longevity remains unclear (Powell and Russell 1985a). An estimate of generation time, or the average age of parents of current newborns, would therefore fall between the two and five year age cohorts for females, and could be between one and five years for males, based upon age estimates put forth by Powell and Russell (1985a). Approximate age categories may be determined by the snout-vent length (SVL) of individuals, with young-of-the-year (late July onward) ranging between 22-37 mm SVL, yearlings 31-46 mm, and adult lizards having SVLs of >46 mm (Powell and Russell 1985a).
Greater Short-horned Lizards are one of six species within Phrynosoma that are viviparous (Stebbins 2003). Parturition generally occurs annually in late July or early August (Powell and Russell 1991b; James 1997). The reproductive toll on females is significant (Powell and Russell 1991b; James 1997). Following parturition, females weigh roughly half of their pre-parturition body mass, and appear gaunt (Powell 1993b; 1994; James 1997). However, females seem to recuperate body mass prior to hibernation (Powell and Russell 1994; 1996). Sex ratios of neonates are approximately even, with recorded clutch sizes in Alberta varying from 6-13 (Powell and Russell 1991b; 1998; UAMZ record #131). Sex ratios at birth are approximately even (Powell and Russell 1991b). Neonates weigh approximately 0.7 g and are about 24 mm SVL (Powell and Russell 1991b). Neonate survival is best described as low; Powell and Russell (1991b) found about 7% were recaptured the following year, compared with 9% of older lizards. Following parturition, females ignore neonates and may move a considerable distance from the natal area (Powell and Russell 1991b; James 1997). The active season generally begins to wind down around mid-September (Laird and Leech 1980; Powell and Russell 1991a; 1992a; 1993a) although given appropriate weather conditions, activity has been observed as late as November (Powell and Russell 1994; 1996).
Predation
As Greater Short-horned Lizards are small, relatively slow, and lack any truly aggressive defences, such as poisonous glands or sharp fangs, they are quite easy to capture, once located, and are likely susceptible to a wide range of generalist predators. Their low abundance and cryptic nature has probably ensured they are not the sole food source for any specific predator species. They are probably most vulnerable in the first days and weeks of life, when relative inexperience makes them more likely to move when approached, and therefore expose themselves.
The higher mobility of males, especially during the mate-seeking season, may expose them to additional risks, such as predation or road mortality (Sherbrooke 2002). In a recent survey of Short-horned Lizard populations in Alberta (James 2002), males were the least commonly encountered (n=36), when compared with the numbers of both females (n=49) and neonates (n=40), which would seem to support this suggestion. Adult males, or those with an SVL >46 mm, were even less common, numbering only nine of the 125 captures made in 2001 (James 2002; pers. data).
Opportunistic snakes, birds, and mammals comprise the majority of potential predators. A number of different snake species have been recorded to ingest horned lizards (Grant and Alberts 2001; LaBonte 2001; James 1997). Birds, such as loggerhead shrikes (Lanius ludovicianus; Young et al. 2004), magpies (Pica pica), crows (Corvus brachyrhynchos), ravens (Corvus corax), hawks (e.g. Ferruginous hawks Buteo regalis; Cartron et al. 2004), or harriers (Circus cyaneus), are also prospective predators. Raccoons (Procyon lotor), coyotes (Canis latrans), foxes (Vulpes vulpes or Vulpes velox), grasshopper mice (Onychomys torridus; Sherbrooke 1991), ground squirrels (Spermophilus richarsonii, Powell and Russell 1996), and other mammals may also attempt to attack a horned lizard if the occasion presents itself.
For Greater Short-horned Lizards, the primary means of protection from predators is their highly effective camouflage colouration and their remarkable ability to remain motionless in order to avoid detection. When a potential predator notices a lizard, the lizard has a number of defensive measures that it may undertake to ward off harm. For example, they may exhibit a number of threat displays, such as opening their mouths, hissing, jumping on all four legs, and inflating themselves to appear larger and more bristly (Sherbrooke et al. 2002; Sherbrooke and Greenfield 2002). Famously, many species of horned lizards may squirt blood from their eyes if stressed to an extreme, even as juveniles (Sherbrooke and Middendorf 2001); however, this behaviour has not been reported for P. hernandesi (T. Russell, L Powell, pers. comm. 2007). Physically, they have their parietal horns to protect the neck area and discourage potential predators from swallowing them, and are covered with rough skin that is difficult to puncture.
Physiology
For ectothermic animals, Short-horned Lizards may be active at unexpectedly low temperatures, given adequate solar radiation, or even after sunset if latent heat is adequate (J. James pers. obs.). Morning emergence is considered temperature-independent, whereas the pattern of diurnal activity is in response to temperature; otherwise known as behavioural thermoregulation (Heath 1962; 1965). Horned lizards that have buried themselves overnight may protrude their heads to facilitate considerable warming of the brain prior to full emergence (Heath 1964). Short-horned Lizards have been documented to have mean preferred body temperatures of around 35 °C (Prieto and Whitford 1971), while a mean body temperature of 35.8 °C was documented for reproductive females in Alberta (James 1997). Diurnal body temperature is maintained by shuttling between sun and shade, and when no direct sunlight is available, by absorption of heat from a warm surface, a practice known as thigmothermy (Heath 1964; 1965). Alberta populations have been documented to endure a broad range of body temperatures, a condition known as eurythermy (Powell and Russell 1985b; James 1997). As their active season begins prior to the date of last spring frost and continues well past the first fall frost, it is likely they are capable of withstanding some degree of freezing or, perhaps, are adept at avoiding freezing conditions (Powell and Russell 1991a).
Following parturition, females generally regain mass prior to hibernation (Powell and Russell 1991b). Generally, appetites diminish prior to the onset of hibernation (Powell and Russell 1993b). During their fall activity, lizards locate shallow burrows, which they may or may not dig themselves (Powell and Russell 1993b; 1994; 1996). Powell and Russell‘s (1996) investigation of overwintering behaviour described the onset of hibernation in detail for a number of individual lizards. They found that hibernacula were often located in areas lower down on protected slopes, where drifting snow may accumulate and provide insulation from extremely low winter air temperatures. Although there may be more than one lizard per burrow, this is not usual, and they are not considered to be communal hibernators like most co-occurring snake species in the region. The burrows themselves are relatively narrow, around 2-3 cm wide, and shallow; generally less than 10 cm below the surface. Although some lizards enter hibernation as early as September, others may remain active until mid-November, which was unimagined prior to this study (Powell and Russell 1996). The remarkable eurythermy of these animals was therefore previously underestimated (e.g. Powell 1982; Powell and Russell 1991a; 1993a).
Dispersal/migration
Short-horned Lizards are relatively small, slow-moving animals. Physically, their dispersal is probably inhibited most by thick vegetation, bodies of water, and their own somewhat sedentary tendencies. There is some evidence that males roam more widely than do females (Henke and Montemayor 1998), although Powell and Russell (1996) found no significant difference between male and female home ranges. The longest recorded movement of a female lizard over the entire active period, in Alberta, is approximately 700 m (J. James pers. data). Generally, lizards in Alberta seem to have large home ranges within which they shift among smaller centres of activity over the active season (Powell and Russell 1994; 1996). There is evidence of seasonal shifts from summer home range areas to areas of hibernation (Powell and Russell 1994). This movement to and from hibernation areas may constitute the greatest distance travelled within their seasonal activity (Powell and Russell 1996; ASRD 2004). Very little is known of the dispersal patterns of newborns or juveniles owing to their small size and the resultant difficulty in tracking them.
Three of the four main Albertan populations of Short-horned Lizards are considered to be completely isolated from all other populations. The populations along the border, that is, those along the Milk River and tributaries, in both Alberta and Saskatchewan may have some likelihood of repopulation from southern populations should they become extirpated.
Interspecific interactions
The Greater Short-horned Lizard, unlike some other species of Phrynosoma, is not entirely reliant on ants as a food source. In Alberta, Short-horned Lizards have been documented as relatively generalist insectivores that do not rely exclusively upon any single species of insect but rather eat an array of available arthropod prey (Powell and Russell 1984).
Adaptability
As ectotherms, the Greater Short-horned Lizard populations in Canada are remarkable for their persistence in an area with a relatively short active season and with considerable climatic variation. These populations of Short-horned Lizards exhibit an extreme capacity for eurythermy that has enabled them to survive in such northern locations (Powell and Russell 1996). However, Powell and Russell (1994; 1996) suggested that notable levels of mortality in late fall and over winter might be one of the determining factors in the altitudinal and latitudinal limits for this species in Canada. These populations are probably at the limit of their physiological capacity within the Canadian range.
Short-horned Lizards are not considered to be territorial and their home ranges do not generally overlap (Powell and Russell 1996). The estimated median home range area of adult Short-horned lizards, from minimum convex polygon estimates, was calculated to be 601.09 m², with data from 1994 ranging from 18.05 – 4200 m² (Powell and Russell 1996). Overall, it was much more common for individuals to have smaller home ranges than larger ones (Powell and Russell 1996). However, as this information was not gleaned from continuous monitoring over the course of an entire season, but rather from a combination of shorter observations taken in mid-summer and fall, the total home ranges for individual lizards, over an entire active season, could be considerably larger (Powell and Russell 1996).
Like all ectotherms, Short-horned Lizards are incapable of sustained activity in cold, overcast weather. They may be vulnerable to sudden temperature drops or to predators just prior to winter hibernation, due to their tendency to remain above surface while attempting to take advantage of warm, late fall weather (Powell and Russell 1996). The practice of using shallow burrows to hibernate may contribute to low over-winter survival rates (Powell and Russell 1996).
Behaviourally, Short-horned Lizards are likely susceptible to road kill, or to being run over by off-road vehicles, due to their tendency to remain motionless as potential threats approach. Lizards will flush from cover when approached closely at walking speeds, but the more rapid approach of vehicles may not provide them with adequate response time.
Prior to legal protection in the United States earlier in the last century, horned lizards were exploited for the pet trade and as novelties that were sold to tourists and through mail-order, both live and stuffed (Sherbrooke 1981). As horned lizards are generally difficult to maintain in captivity these unfortunate individuals inevitably perished (Sherbrooke 2003). Phrynosoma hernandesi have been maintained in captivity for research purposes (Monanucci and Baur 1982; Montanucci 1983). This species of lizard has been observed to mate in captivity (Monanucci and Baur 1982). The minimal maternal care afforded newborn Short-horned Lizards (Powell and Russell 1991b) would imply that if successful breeding were to occur in captivity, release of captive-reared young into suitable habitat should not have any negative implications for the offspring.
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