San Francisco State University
Department of Geography      
                    
Geography 316:  Biogeography                       Source: Jeremy Bailey 1991

In progress 11/28/2001

               The Biogeography of  Sceloporus occidentalis

                                    by, Jeremy D Bailey  student in Geography 316, Fall 2001

   Sceloporus occidentalis, commonly    known blue-bellied lizard is a familiar fixture in the Western United States. Anyone who hikes and enjoys the outdoors knows this lizard by sight. How many of these lizards have we almost stepped on, caught or saved from the claws of our cats over the period of our lives here in the Western United States?  Let me now introduce you to the Western Fence Lizard, (Sceloporus occidentalis)

  Sceloporus occidentalis posing for the camera

                      (Patricia A. Michaels 2001)

TAXONOMY:                                        Photo: John Sullivan
Kingdom: Animal
Phylum: Craniata
Class:Reptilia                                                 

Order: Squamata
Family: Phrynosomatidae      
Genus: Sceloporus  
Species: Sceloporus Occidentalis

Description of Species:

   Sceloporus occidentalis has coloration on the underside of its body, males have blue patches on their underarms, throats and on the undersides of their abdomens, females have much less of this coloration (Figs 2 and 3).  Sceloporus occidentalis ranges in size from two and a quarter to three and one half inches in snout-vent length (Stebbins,1954).  Its dorsal scales range in color from brown, tan, gray and sometimes black (Schwenkmeyer,2001).  Their scales overlap each other and appear pointy and rough, although not shiny in appearance (Fig 1). These lizards like all reptiles are ectothermic (cold blooded) and need to sun themselves at every opportunity in prominent places like rocks and fence posts making them easy prey for birds and snakes; as a result the lizards have developed fast reflexes. Sceloporus occidentalis  lives up to five or six years, but because of predators they typically live only one year. (USGS,2001)

Figure1: Scale Comparison (Sceloporus occidentalis,S.Undulatus)
Source: Stebbins 1954
 

Photos: USGS

                     
Figure 2: Sceloporus occidentalis, Male coloration        Figure 3: Sceloporus occidentalis, Female coloration     


  
Habitat/Distribution: Sceloporus occidentalis belongs to the highly successful cosmopolitan Phrynosomatidae  family (Stebbins,1954)  This species is eurytopic and dispersed in great numbers across the Western United States.  Its distribution continuously ranges geographically from Southwestern Canada, Oregon, Washington, Western Idaho, Nevada, Utah, California to Northwestern Baja California (Figs 4 and 5).  In these regions the lizard ranges from the coastal areas to the mountains up to 6,000ft.  Their closest relative the eastern fence lizard, (Sceloporus undulatus) is almost identical to Sceloporus occidentalis with the exception of a slightly different pattern, number of scales and range of distribution (Fig5).  Sceloporus undulatus can exist in mountainous areas while Sceloporus occidentalis does not; with the Rocky Mountain Range being the major barrier between them (Stebbins,1954), (Fig5). The blue bellies wide array of habitats include moderately open coniferous forests, rocky canyons, slopes and woodlands.  It also enjoys sagebrush and grassland habitats, but excludes desert environments.  Their wide distribution gives them access to abundant food sources.  These lizards feed on insects like beetles, flies, caterpillars and ants as well as arthropods(Brookshire,2001).  The blue belly can typically be found near the ground or on fence rows and on the branches of bushes and shrubs. They typically make their homes in old tree trunks, under rocks and in wood piles.  Due to their ectothermic nature the blue bellies are diurnal; the seasonality of their habitats causes the lizards to hibernate or go into periods of inactivity in the cooler winter months.  They then reemerge around late winter early spring, March (Karr,1999)

Maps of Distribution:        (CAS 1995)                                                                                    (Stebbins 1954)

                                    

     Figure 4: Sceloporus occidentalis, California Distribution 

            (Yellow Green=Sceloporus occideantalis range)

                      Figure 5: Distribution N. America,                 

  (Sceloporus occidentalis, S. undulatus Distribution 1-4)

                 

Breeding: During the spring Sceloporus occidentalis starts to reestablish its home territory in the same area year after year, which is usually around .01 hectars (Giorni,1996).  A females territory is almost always two thirds the size of a males (Sheldahl,2000).  Both defend their hibernation area, food supplies and home range throughout the year. During this time both genders will establish dominance over their territory from other lizards by posturing and posting chemical cues or cent marks.  As a result there is usually only one lizard per sunny boulder or tree trunk.    

    During the breeding season males will sit atop their territory to both fend off other males and to attract females.  The lizards begin to mate their second year, the males will do what looks like a rhythmic set of pushups to attract mates.  Females are usually closer to the ground and harder to spot than males.  Once ready to mate she will appear and the male will vertically flatten his body to display his brilliant blue colors (Schwenkmeyer,2001). He then holds the females neck in his jaws while mating commences.  If the female changes her mind during copulation she turns on her back and kicks the male off with all four legs (Angilletta,2001).  During mating the normally tan to brown dorsal scales on the male will turn a brilliant blue (Brookshire,2001). At present it is unknown if the couple is monogamous during the breeding season.     

    To assure species success the female will have two to three clutches per breeding season. She will expend more energy in the present season in case of her death before the next.   Her first clutch will have the largest egg size and the final the smallest. To compensate for the difference in egg size the female will expend more energy on the care of the last clutch than the first, to maximize offspring survival (Angilletta,2001).  Once the eggs are laid they can range in size from six to fourteen millimeters, she buries them under shallow moderately moist soil (Angilletta,2001).  If consistent with similar species of reptiles the female will bury and care for the eggs without assistance from the male. The eggs usually hatch after two months in late April to June or July.  Clutch sizes can range from three to seventeen and appear to increase with higher latitudes; larger females typically have more offspring (Schwenkmeyer,2001).  After a couple of months the infants emerge at around twenty six millimeters in snout-vent length.  Most of their growth will occur during their first year of life.

Evolution: This species breeding and territorial behavior are a result of millions of years of evolution.  The first reptiles evolved from amphibians around 250 million years ago during the Carboniferous period when reptile species diversified, and multiplied, to become the dominate large animal on the planet.  In the Jurassic Period, around 65 million years ago, there was a planet wide mass extinction and reptile dominance ceased.  All that remain of the reptiles are the 6,000 known species that survived the mass extinction (Cambell 1993).

        Presently, these 6,000 species are divided by herpetologists into taxonomic levels to assist the classification of their evolutionary patterns, through morphological classifications (Cambell 1993).  This process is also applied to the 3,000 known species of lizards.  Through morphological examination and comparison herpetologists have traced the evolution of specific families and genus of lizards (Stebbins 1954).          

        The Phrynosomatidae family is cosmopolitain and differs little in morphology, habitat and reproduction preferences throughout the world. Evidence suggests the Phrynosomatidae family originated in one unknown area and spread with continental drift.  Several genus and species evolved from this family due to the isolation of phenotypes and challenges of climate and predation.  These genus are further differentiated by morphological characteristics (Stebbins 1954), (Figure 6).  

         Sceloporus occidentalis is distinguished by several unique morphological traits, Stebbins differentiated the species by morphological uniqueness by stating that, "Scales on the back of the thigh are larger, pointed and keeled more than sceloporus undulatus, dorsal scales usually larger-about 35 to 55 (average 41), between  interparietal plate and line connecting posterior surfaces of thighs, adults usually over two and three fifiths inches and usually without rust on the lower sides and a single blue patch on the throat (Stebbins 1954)."  These characteristics can be traced to an originating species in the genus as well as the family (Figure 6).
Figure 6: Morphological Tree  (Stebbins 1954)

 Other interesting facts: A protein discovered in the blood of Sceloporus occidentalis and S. undulatus kills the bacterium that cause lyme disease.  Ticks are hosts to the bacteria, Borrelia birgdorferi and transfer the disease through biting.  It has been discovered that ticks who have bitten Sceloporus occidentalis were disinfected of the bacterium.  The species has evolved an immune response that kills Borrelia, not only in themselves but in parasites as well (Karr 1999). I assume the eastern fence lizard, Sceloporus undulatus developed this resisitance first because, lyme disease developed in the eastern United States.  The resistance remained when the Rocky Mountains isolated the eastern and western fence lizards. Who would have assumed that one of the most recognizable species of lizard in the American west would hold a cure for lyme disease, this is true natural selection at work.   
 
 
 
Bibliography and Websites:  

Angilletta, M.J., M.W. Sears and S.R. Winters.  2001. "Seasonal variation in reproductive effort and its effect on offspring size in the lizard 
	Sceloporus undulatus." Herpetologica 57(3): 365-375.
 
 
Brookshire, Marya. Western Fence Lizard Natural History.
	http://www.uoregon.edu/~titus/herp/occidentalishistory.htm#Introduction.10/06/01.

 
Cambell, Neil. 1993. Biology. Redwood City, C.A. The Benjamin/ Cummings Publishing Company, Inc.

 
Giorni, Christopher. 1996. Character Displacement and Resource Use in Two Species of California Lizards. Biology Thesis, 
	San Francisco State University.
 
 
Karr, Jennifer. California Academy of Sciences: 1999. Lizards that fight Lyme Disease.
	 http://www.calacademy.org/science_now/archive/wild_lives/fence_lizards_050601.html 10/ 06/01.

 
 Rhodin, Miyata. 1983. Advances in Herpetology and evolutionary biology: essays in honor of Ernest E. Williams/ 
	edited by Anders G.J. Cambridge, Mass. Museum of Comparitive Zoology.
 
 
Schwenkmeyer, Dick. San Diego Natural History Museum-Western Fence Lizard.
	http://www.sdnhm.org/fieldguide/herps/scel-occ.html. 10/06/01.

 
Sheldahl, L., E.P. Martins.  2000. "The territorial behavior of the western fence lizard, Sceloporus occidentalis" Herpetologica 56(4): 469-479.
 
 
Stebbins, Robert C. 1954. Amphibians and Reptiles of Western North America. New York, NY. McGraw-Hill Book Company, Inc.

 
--------. 1985. A Field Guide to Western Reptiles and Amphibians. Boston,Mass. Houghton Mifflin Company.

 
USGS: Jan 2001. Sceloporous Occidentalis, The Western Fence Lizard http://www.werc.usgs.gov/fieldguide/scoc.htm  10/06/01.

 
Wiens, J., Reeder, T. 1997. “Phylogeny of the Spiny Lizards (Sceloporus) based on molecular and morphological evidence.” 
	Herpetological Monographs 11:1-101.
 
 
 
send comments to bholzman@sfsu.edu
 

Geog 316 homepage     Back to Geography home page        Back to SFSU homepage