Habitat
This lizard inhabits all sorts of desert shrublands, such as those dominated by sagebrush, shadscale, hopsage, creosotebush, or greasewood, on sandy flats, alluvial fans, washes, or brushy dunes or dune edges (Grismer 2002, St. John 2002, Stebbins 2003). It is most consistently found where areas of bare ground exist among openly spaced shrubs. It occurs where summers are hot and winters are cold or mild; winter temperatures generally are too cold for activity.
In Nevada, P. platyrhinos is common in sandy or gravelly valleys and flat areas throughout most of the state at elevations of 610-1,980 meters (mainly above 1,220 meters) (Linsdale 1940). In Arizona, P. platyrhinos inhabits Sonoran, Mohave, and Great Basin desert scrub communities and the lower reaches of interior chaparral and Great Basin conifer woodland, usually in relatively flat, open, areas with sandy or loamy soil, less frequently on rocky bajadas and foothills (Brennan and Holycross 2006). During periods of inactivity the lizards bury themselves in the soil or occupy existing burrows.
Ecology
Desert horned lizards derive their body heat from the environment, They require warm body temperatures for activity, feeding, digestion, and reproduction, but conditions on the surface can become too warm. Lizards attain suitable body temperatures by basking in the sun, moving within the sun-shade mosaic produced by plants, and by burying in the soil or entering a burrow.
Desert horned lizards avoid predators through crypsis (they are very difficult to see unless in motion) and by rapid running into vegetative cover (e.g., Linsdale 1938). The lizards' head spines may interfere with attempted ingestion (and also likely enhance crypsis). This species does not exhibit the defensive blood squirting mechanism present in some horned lizard species (Middendorf and Sherbrooke 1992, Sherbrooke and Middendorf 2004). Predators include: prairie falcons, loggerhead shrikes, longnose leopard lizards and striped whipsnakes.
Drought
Drought may affect populations of horned lizards and other insectivorous reptiles by causing changes in body condition and survival. For example, Texas horned lizards (Phrynosoma cornutum) appear to be sensitive to climate-associated variations in food supply, and drought may reduce food availability and result in lizard weight losses (Whitford and Bryant (1979). In tree lizards (Urosaurus ornatus), reduced growth rate, body condition, and juvenile survival were associated with drought (Tinkle and Dunham (1983). The snake Coluber constrictor, the diet of which includes many insects, exhibited decreased survival during drought conditions in Utah, and juvenile growth was best in years with relatively high rainfall (Brown and Parker 1984).
Drought may also result in reduced reproduction. Fat bodies in the abdominal cavity provide most of the nutrition for reptilian reproduction. Reduced food supplies may reduce reproductive output due to inadequate fat storage. Periods of drought and food shortage may result in smaller clutch sizes. In southern New Mexico, (Worthington 1982) found that drought may result in a one-egg reduction in the average clutch size of the side-blotched lizard (Uta stansburiana).
The effects of drought on survival and reproduction are manifested in reduced population density. For example, in Texas, tree lizard density declined greatly during periods of drought (Ballinger 1977, 1984), evidently due to effects of reduced food resources (Dunham 1981). In California, western whiptail (Aspidoscelis tigris) populations tended to increase with periods of increased arthropod abundance associated with increased precipitation (Anderson 1994). Similarly, A. tigris density varied with drought conditions in southwestern Texas (Milstead 1965).
Thus it is likely that drought results in reduced density of desert horned lizards through the following scenario: Drought reduces plant productivity (including seed production), which in turn reduces insect populations and horned lizard food resources. Reduced food resources result in reduced horned lizard survival and reproduction, which result in reduced population density. In southern Nevada, Medica et al. (1973) observed substantial variation in P. platyrhinos reproduction. Individual females produced one clutch per year in most years and multiple clutches in one year; no evidence of reproduction was observed in one year. The authors did not attempt to associate these variations with environmental parameters but simply speculated that "these deviations may be intimately associated with various density-dependent regulating mechanisms or with differences in net primary production and availability of food."
Reproduction
This lizard is an egg layer. Females bury eggs in the soil. In southern Nevada, egg deposition occurs April-July (apparently mainly early June). Clutch size averages about 7. Individual females produce one or two clutches per year. Incubation lasts about 50-60 days. Hatchlings appear from mid-July to August in southern Nevada, and as late as mid-September in some areas. Individuals become sexually mature in about 22 months (Tanner and Krogh 1973, Nussbaum et al. 1983). Studies in Nevada indicate that some individuals live 7-8 years, occasionally longer (Medica et al. 1973, Tanner and Krogh 1973).