Western fence lizard

Sceloporus occidentalis

C.Michael Hogan PhD

August 30, 2008

S. occidentalis is the most common lizard in its range, with chief observed food competition with the Sagebrush lizard. Its widespread presence in the western USA is attributable to its catholic diet and adaptation to a wide dietary palette. Colorful patterning of the male is used in courtship, which process begins in spring. When alarmed this spiny lizard can sprint with considerable velocity over distances of approximately three metres. This reptile is thought to be useful in insect control and its occurrence is associated with suppression of Lyme Disease.

DISTRIBUTION AND SUBSPECIES. This species occurs in parts of the western USA from Washington, eastern Oregon, California, Nevada, southwest Idaho, western Utah and northwestern Baja California. At least five subspecies are recognized: S.o. becki, California Channel Islands; S.o.biseriatus: San Joaquin Valley from the Tehachapi Mountains north to Merced County; S.o.bocourti: California coastal region Sonoma to Santa Barbara County; S.o.occidentalis, Washington; S.o.longipes, Great Basin.

MORPHOLOGY. This spiny lizard's body is approximately 8.5 cm long, with the tail adding another 60 percent. Color varies from light gray to black with dark dorsal patterning. These spectral characteristics have been demonstrated to match closely that of burnt chemise, a widespread chaparral plant whose propagation depends on periodic fire; this is deemed to be an example of co-evolution. (Schoenherr) Males present electric blue or green undersides, with yellow leg underparts. Females are duller, but scales of both sexes manifest a spiny, keeled texture.

HABITAT AND DIET. The species occurs in diverse habitats, especially chaparral and open woodlands..It is absent from true desert and from dense closed canopy forest. It has not been observed above 3000 m in elevation. S. occidentalis is mostly terrestrial, sighted on rocks and open patches of ground where sunlight is present on the ground. This lizard enjoys perching on rocks, fence posts and other elevated features to look for prey and for courtship display. When alarmed, he seeks cover in rock crevices, in a burrow or under detritus; the more secure burrows also provide hibernation venues. This reptile does not require a surface water supply, but rather realizes sufficient hydration from its diet of small terrestrial invertebrates:chiefly insects but also ticks, spiders and scorpions.(Rose)

TERRITORY AND HOME RANGE. Home ranges vary considerably in size based upon resource abundance. Low values of 0.2 to 0.7 .males per hectare in Nevada scrub-land; prime habitat in California coastal ranges illustrate a total S.occidentalis density of 22 to 33 per hectare. (Ruth). Males defend territory vigorously in spring by posturing and combat, but less intensely in later seasons. My observations in the North Coast Ranges of California indicate locally higher densities of this species, particularly in mixed oak woodland and even in certain garden settings that offer extensive rock accumulations and other burrow opportunities. Highways and major rivers are impediments to migration and intrinsic separators to continuous range.

LIFE CYCLE. After inactivity in the autumn and winter, courtship begins in March and April. The male attracts a mate by "push-up" movements and head bobbing from a perch postion. (Ironically this prominent posturing exposes the male to higher predation from raptors and snakes.) Copulation may occur as early as May, with egg-laying occurring within 30 days; cluthes may contain up to ten eggs. Incubation requires a further 60 days. (Davis)

ECOLOGY AND CONSERVATION. The abundance of this species renders it as of least concern in regard to survival; however, in Utah the species is considered threatened. The real question is whether a species as durable as this may be propagating and gaining relative advantage over more specialized species, whose numbers are declining from habitat loss and climate change. A noteworthy interaction appears to be S. occidentalis' role in suppressing lime disease. (New York Times)

REFERENCES.
* Allan A. Schoenherr (1992) "A Natural History of California", University of California Press, ISBN 0520069226
* B.R. Rose (1976) "Dietary overlap of Sceloporus occidentalis and Sceloporus graciosus", Copeia: 818-20
* S.B.Ruth (1977) ʽ'A comparison of the demography and female reproduction in sympatric western fence lizards (S. occidentalis) and sagebrush lizards (S.graciosus) PhD thesis, UC Berkeley
* J.Davis (1980) "The times of mating and oviposition of the Western fence lizard S.o.occidentalis, J. Herpetol. 14:102
* New York Times, April 19, 1998.

(The foregoing is copyrighted original research of the author, who grants use to GlobalTwitcher)