Blunt-nosed Leopard Lizard
Blunt-nosed Leopard Lizard
|Listed||March 11, 1967|
|Description||Leopard lizard with short, broad skull and blunt snout; has prominent markings.|
|Habitat||Sparsely vegetated plains and grasslands.|
|Reproduction||Normally one clutch of two or three eggs per season.|
|Threats||Loss of habitat; fragmented distribution.|
The blunt-nosed leopard lizard (Gambelia silus ) is a relatively large lizard of the family Iguanidae. It has a long, regenerative tail; long, powerful hind limbs; and a short, blunt snout. Adult males are larger than adult females, ranging in size from 3.4-4.7 in (8.6-11.9 cm) in snout-vent length. From snout to vent, females are about 3.4-4.4 in (8.6-11.2 cm). Adult males weigh 1.3-1.5 oz (36.9-42.5 g) and adult females weigh 0.8-1.2 oz (22.7-34 g). Males are distinguished from females by their enlarged post-anal tails, femoral pores (visible pores on the underside of the thigh), temporal and mandibular muscles (muscles on the skull that close the jaws), and tail base.
Although blunt-nosed leopard lizards are darker than other leopard lizards, they exhibit tremendous variation in color and pattern on the back. Background color ranges from yellowish or light gray-brown to dark brown, depending on the surrounding soil color and vegetation association. The undersurface is uniformly white.
The color pattern on the back consists of longitudinal rows of dark spots interrupted by a series of seven to 10 white, cream-colored, or yellow transverse bands. In the blunt-nosed leopard lizard, the cross bands are much broader and more distinct than in other leopard lizards and extend from the lateral folds on each side to the middle of the back, where they meet or alternate along the midline of the back. With increasing age, the cross bands may fade and the spots may become smaller and more numerous, particularly in males. Similarly colored bands or rows of transverse spots produce a banded appearance to the tail. Juveniles have blood-red spots on the back that darken with age, becoming brown when sexual maturity is reached, although a few adults retain reddish centers to the spots.
Except for the throat, undersides are uniformly white to yellow in immature lizards and prenuptial females. Nuptial females have bright red-orange markings on the sides of the head and body and the undersides of the thighs and tail. This color fades to pink or light orange by late July. Males in many populations develop a nuptial color during the breeding season that spreads over the entire underside of the body and limbs. This salmon to bright rusty-red color may be maintained indefinitely.
The blunt-nosed leopard lizard can be distinguished from the long-nosed leopard lizard by its color pattern, truncated snout, and short but broad triangular head. The blunt-nosed leopard lizard has dark blotches on the throat instead of parallel streaks like the long-nosed leopard lizard. Other distinguishing characteristics are a significantly smaller number of maxillary and premaxillary teeth (this may be related to the shortened snout) and a smaller variation in the number of femoral pores. In general, blunt-nosed leopard lizards can be distinguished from all other leopard lizards by their retention into adulthood of the primitive color pattern shared by all young leopard lizards: the absence of ornamentation around the dorsal spots; retention of wide, distinct cross bands; the presence of gular (on the throat) blotches; and fewer spots arranged in longitudinal rows.
Blunt-nosed leopard lizards feed primarily on insects (mostly grasshoppers, crickets, and moths) and other lizards; some plant material is eaten rarely or is perhaps consumed unintentionally with animal prey. This species of lizard appears to feed opportunistically on animals, eating whatever is available in the size range it can overcome and swallow. Which lizards are eaten is largely determined by the size and behavior of the prey. Lizard species taken as prey include side-blotched lizards, coast homed lizards, California whiptails, and spiny lizards. Young of its own species also are eaten. Because they have similar diets, interspecific competition probably occurs between the blunt-nosed leopard lizard and the California whiptail.
Breeding activity begins within a month of emergence from dormancy and lasts from the end of April through the beginning of June (in some years to near the end of June). During this period and for a month or more afterward, the adults often are seen in pairs and frequently occupy the same burrow systems. Male territories may overlap those of several females, and a given male may mate with several females. Copulation may occur as late as June.
Two to six eggs are laid in June and July, and their numbers are correlated with the size of the female. Females usually produce only one clutch of eggs per year, but some may produce three or more under favorable environmental conditions. Under adverse conditions, egg-laying may be delayed one or two months, or reproduction may not occur at all. Eggs are laid in a chamber either excavated specifically for a nest or already existing within the burrow system. After about two months of incubation, the young hatch; this typically occurs from July through early August, rarely to September. Before their first winter, young leopard lizards may grow to 3.5 in (8.9 cm) in snout-vent length.
Sexual maturity is reached at nine to 21 months, depending on the sex and environmental conditions. Females tend to become sexually mature earlier than males, breeding for the first time after the second dormancy, while males usually do not breed until later.
Social behavior is more highly developed in the blunt-nosed leopard lizard than in the long-nosed leopard lizard. For example, territorial defense and related behavioral activity are completely absent in the long-nosed leopard lizard, whereas blunt-nosed leopard lizards are highly combative in establishing and maintaining territories.
Leopard lizards use small rodent burrows for shelter from predators and temperature extremes. Burrows are usually abandoned ground squirrel tunnels or even occupied or abandoned kangaroo rat tunnels. Each lizard uses several burrows without preference but will avoid those occupied by predators or other leopard lizards. In areas of low mammal burrow density, lizards will construct shallow, simple tunnels in earth berms or under rocks. While foraging, immature lizards also take cover under shrubs and rocks.
Potential predators of blunt-nosed leopard lizards include whipsnakes, gopher snakes, glossy snakes, western long-nosed snakes, common king snakes, western rattlesnakes, loggerhead shrikes, American kestrels, burrowing owls, greater road-runners, golden eagles, hawks, California ground squirrels, spotted skunks, striped skunks, American badgers, coyotes, and San Joaquin kit foxes. Blunt-nosed leopard lizards are hosts to endoparasites such as nematodes (worms) and ectoparasites such as mites and harvest mites.
Seasonal aboveground activity is correlated with weather conditions, primarily temperature. Optimal activity occurs when air temperatures are 74-104°F (23.3-40°C) and ground temperatures are 72-97°F (22.2-36.1°C). Some activity has been observed at temperatures as high as 122°F (50°C). Body temperatures range from 90 to 108°F (32.2 to 42.2°C). Because diurnal activity is temperature dependent, blunt-nosed leopard lizards are most likely to be observed in the morning and late afternoon during the hotter days. Smaller lizards and young have a wider activity range than the adults. This results in the smaller, subadult lizards emerging from hibernation earlier than adults, remaining active later in the year, and being active during the day earlier and later than adults. Adults are active aboveground in the spring months from about March or April through June or July, with the amount of activity decreasing so that by the end of June or July almost all sightings are of subadult and hatchling leopard lizards.
Blunt-nosed leopard lizards inhabit open, sparsely vegetated areas of low relief on the San Joaquin Valley floor and in the surrounding foothills. On the valley floor, they are most commonly found in the non-native grassland and valley sink scrub communities. The valley sink scrub is dominated by low, alkali-tolerant shrubs of the family Chenopodiaceae (such as iodine bush). The soils are saline and alkaline lake bed or playa clays that often form a white salty crust and are occasionally covered by introduced annual grasses. Valley needlegrass grassland also provides suitable habitat for the lizard on the valley floor.
Valley needlegrass grassland is dominated by native perennial bunchgrasses, including purple needlegrass and alkali sacaton. Associated with the perennial grasses are native and introduced annual plants. Blunt-nosed leopard lizards likewise inhabit valley saltbush scrub, a low shrubland with an annual grassland understory; it occurs on the gently sloping alluvial fans of the foothills of the southern San Joaquin Valley and adjacent Carrizo Plain. This community is dominated by the chenopod shrubs, common saltbush, and spiny saltbush, and it is associated with nonalkaline sandy or loamy soils. In general, leopard lizards are absent from areas of steep slope, dense vegetation, or areas subject to seasonal flooding.
Fifteen to 30% groundcover was optimal for leopard lizard habitat; greater than 50% is unsuitable. On the Elkhorn Plain Ecological Reserve, high percentages of groundcover (nearly 100% in 1991-1993, 1995) may not have provided optimum habitat conditions, but grasshoppers and large moths and other prey for leopard lizards were abundant under these conditions. Blunt-nosed leopard lizards survived such conditions in similar proportions in grazed and nongrazed areas in years of both low and high plant productivity.
The blunt-nosed leopard lizard is endemic to the San Joaquin Valley. It is not found above 2,600 ft (792.5 m) in elevation. Although the blunt-nosed leopard lizard has been listed as endangered for 30 years, there has never been a comprehensive survey of its entire historical range. The known occupied range in the middle to late 1990s was across scattered parcels of undeveloped land on the San Joaquin Valley floor and in the foothills of the Coast Range.
In the southern San Joaquin Valley, extant populations are known to occur on the Pixley National Wildlife Refuge, Liberty Farms, Allensworth, Kern National Wildlife Refuge, Antelope Plain, Button-willow, Elk Hills, and Tupman Essential Habitat Areas. Remaining undeveloped lands farther north support some blunt-nosed leopard lizard populations. The species is presumed to be present in the upper Cuyama Valley, though no recent inventory is known for that area.
There are no overall population size estimates for the species. The 1985 densities ranged from 0.1-4.2 individuals per acre (per 0.4 hectare) for a population on the Pixley National Wildlife Refuge. In 1991, after three previous years of severe drought, two 20-acre (8.1-hectare) plots had estimated densities of 2.7-2.8 per acre (per 0.4 hectare) on the Pixley Refuge.
Since the 1870s and the advent of irrigated agriculture in the San Joaquin Valley, more than 95% of the original natural communities have been destroyed. This dramatic loss of natural habitat was the result of (1) cultivation, modification, and alteration of existing communities for petroleum and mineral extraction, (2) pesticide applications, (3) off-road vehicle use, and (4) construction of transportation, communications, and irrigation infrastructures. These processes collectively have caused the reduction and fragmentation of populations and the decline of blunt-nosed leopard lizards.
Farming began in the valley as a direct response to increased demands for local food supplies created by the migration of settlers to California during the 1849 gold rush. Land conversion was accelerated in the 1920s with the advent of reliable electrical groundwater pumps and in the 1950s and 1960s with importation of water via federal and state water projects. By 1985, 94% of wildlands on the valley floor had been lost to agricultural, urban, petroleum, mineral, or other development.
Habitat disturbance, destruction, and fragmentation continue to be the greatest threats to blunt-nosed leopard lizard populations. Construction of facilities related to oil and natural gas production (well pads, wells, storage tanks, sumps, pipelines, and their associated service roads) degrade habitat and cause direct mortality to leopard lizards, as do leakage of oil from pumps, transport pipes, and storage facilities. Dumping of waste oil and highly saline wastewater into natural drainage systems also degrades habitat and causes direct mortality, but these activities are no longer permitted. Lizards displaced by degraded or lost habitat may be unable to survive in adjacent habitat if it is already occupied or unsuitable for colonization. Direct mortality occurs when animals are killed or buried in their burrows during construction, killed by vehicular traffic on access roads, drowned or mired in pools of oil and uncovered oil cellars, or trapped in excavated areas from which they are unable to escape.
Although lizards occur in areas of light petroleum development and recolonize oil fields that have been abandoned, their population densities decrease as oil activity increases. Eighty-three percent of the blunt-nosed leopard lizard population on Elk Hills Naval Petroleum Reserves in California inhabited areas where little or no petroleum-related activity had occurred.
Livestock grazing can result in removal of herbaceous vegetation and shrubcover, destruction of rodent burrows used by lizards for shelter, and associated soil erosion if the stocking rate is too high or if animals are left on the range too long after annual plants have died.
The use of pesticides may directly and indirectly affect blunt-nosed leopard lizards. The insecticide malathion has been used since 1969 to control the beet leafhopper. The California Department of Food and Agriculture treats areas on the west side of the San Joaquin Valley from Merced to San Luis Obispo Counties up to three times a year, depending on the seasonal densities of the sugar beet leafhopper and whether or not it is carrying the curly-top virus. The most significant effects of malathion on the blunt-nosed leopard lizard may be those associated with the reduction of insect prey populations.
Blunt-nosed leopard lizard mortality is known to occur as a result of regular automobile traffic and off-road vehicle use. Typically, roads surround and often bisect remaining fragments of habitat, increasing the lizards' chances of being hit by traffic.
Conservation and Recovery
The three most important factors in recovering the blunt-nosed leopard lizard are determining appropriate habitat management and compatible land; protecting additional habitat for them in key portions of their range; and gathering additional data on population responses to environmental variation.
Several important populations are isolated on fragmented natural land on the valley floor and along its southern and western perimeter. Determining viable population size, genetic variation, and methods to enhance population movements and restore habitat on retired farmlands are needed to ensure recovery.
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
U.S. Fish and Wildlife Service. 1998. "Recovery Plan for Upland Species of the San Joaquin Valley, California." Region 1, Portland, Oregon. 319 pp.