Sceloporus occidentalis: Some notes on the common Oregon Western Fence Lizard

Marya Brookshire

Introduction

The Western Fence Lizard (Sceloporus occidentalis) is a quite common iguanid lizard occupying a variety of habitats in the western United States. It is a common lizard in Oregon, but not so big as to be considered "important" or interesting by the public as a whole. In Oregon there are two sub species of the Western Fence Lizard: The Northwestern Fence Lizard (Sceloporus occidentalis occidentalis) and the Great Basin Fence Lizard (Sceloporus occidentalis longipes) which occurs throughout eastern Oregon (Brown et. al. 1995). S. occidentalis covers most of Oregon in its geographical range except for the north coast range above Coos County and the North-Central part of the Cascades (Brown et. al. 1995). The following description of the behavior and biological attributes of S. occidentalis will hopefully lend a better understanding of the complexity of this species and perhaps a deeper respect for the lizard population as a whole.

Description

Western Fence Lizards, at least in Oregon, are gray to black in color and have blue-green patches on the abdomen, back and tail in the males. Bright blue to greenish patches on each side of the belly and a throat patch are present. Young males can be distinguished from females by their deeper shade of blue-green color that their ventrical patches carry. Also, the blue-green patches are lighter in females (Camp 1916; Brown et. al. 1995; Fitch 1938). Oregon S. occidentalis lizards measure up to 3.2 inches from snout to vent and 6.6 inches in total length (including the tail). S. occidentalis in eastern Oregon or S. occidentalis longipes can get up to 3.9 inches from snout to vent and 8.4 inches in total length (Brown et. al. 1995). "Most of the growth in S. occidentalis takes place in the first year of life" (Fitch 1938). Lizards (S. occidentalis) become full adults by the end of their second hibernation and then become a major part of the breeding population. Yet, even though they have reached adult status, at this age "none has attained the maximum size and most are well under 70mm" (Fitch 1938) in snout to vent length. Adults apparently undergo molting only once every year (Fitch 1938).

Distribution

S. occidentalis is known in central Washington, Oregon, through California into Baja California, southwestern Idaho and western Utah. In Oregon their range continues from the south coast and are widespread east of the cascades. They occur in all parts of eastern Oregon within the Great Basin range. They have been found up to at least 5000 feet in southeastern Oregon (Brown et. al, 1995; Stebbins 1985).

Habitat Preferences

In eastern Oregon S. occidentalis can be found mostly and frequently on large boulders along mountain or hill sides. It seems they prefer higher elevations or at least slopes that increase in altitude. They are rarely, if ever, seen on valley floors or flat desert playas. Generally though, S. occidentalis can occupy a wide variety of habitats (Stebbins 1985). Fence Lizards in California tend to be arboreal and prefer "wooded or rocky areas, old buildings, wood piles and fences" (Marcellini and Mackey 1970). Its habitat in the wild seems to consist of low bushes, widely spaced rocks, logs, or clumps of brush. there isn't any specific or well-defined habitat area for S. occidentalis, though they seem to prefer "objects upon which they could climb....Individuals were often seen basking or displaying on such objects" (Marcellini and Mackey 1970). It seems that Fence Lizards in general prefer elevated areas, whether in trees in north-central California or on rocky slopes in eastern Oregon.

Behavioral Features

Territorial behaviors are only seen in male S. occidentalis. This behavior includes "defending sites suitable for thermoregulation, protecting a local food supply, or defending potential mates" (Eisen and Schall 1997). Young males who don't yet have a designated territory or rock will still display signs of territorial defense, for example, young males will dart away when approached by an adult, but will often ventrically flatten its body to "display the bright ventral coloring" (Fitch 1938). "Pushups" or rapid up and down movements made by the front limbs are also displays of territorial defense but is also done to attract mates. Fitch (1938) describes a typical territorial behavior:

In the spring each male spends much time at some elevated site within his territory and attacks any adult males which may venture into the vicinity. Hence there is usually but one male to each boulder, stump, or log where conditions are favorable.

However, in the Steens Mountains area of southeastern Oregon more than one male S. occidentalis has been seen on one boulder(personal observation).

Mating activity usually begins in early March after the adults have emerged from hibernation. Adult males are found perched on high rocks, fence posts or logs and spends most of his time there. Females tend to stay closer to the ground or undercover making it difficult for the observer to find them (Fitch 1938).

In terms of courtship the male will usually stand high on his perch with his body compressed vertically and at the same time "raises and lowers himself on his front legs in a jerky, rhythmical fashion (Fitch 1938). He will also ventrically flatten himself to better display his blue side patches. The female usually stays away from the male, even when he is trying to court, and she usually will not allow advances until she is ready to mate (Fitch 1938). When actual copulation occurs, the male becomes brilliantly colored, blue patches appear on all his dorsal scales (Fitch 1938).

Egg laying usually occurs anywhere from May to June. Damp, moveable soil is usually chosen for the place of egg burying, or sometimes within a log or under a rock (Brown et. al. 1995). An interesting scenario was observed in 1928 at the west base of Spencer's Butte in Lane County, Oregon:

Several females were found digging burrows for their eggs in damp soil along a nearly dry creek Evidently they had wandered outside their usual foraging areas for this purpose, since no adult males or Juveniles were seen along the creek bed (Fitch 1938).

Thermoregulation

S. occidentalis lizards maintain their body temperature within a relatively narrow range. The normal preferred mean body temperature for S. occidentalis is 35.0 degrees Celsius (McGinnis1966). "Seasonal mean body temperatures of lizards captured above ground in the field exhibit a wide range (30.4 to 35.9 degrees Celsius), the most striking deviation occurs in the winter (McGinnis1966). In the winter the cloacal temperature was found below 28.0 degrees Celsius, yet in the other three seasons no cloacal temperatures were found below this point (McGinnis1966), so "only with the advent of warmer spring temperatures may the preferred body temperature range be readily attained and maintained (Ibid). The daily mean or fluctuating body temperature of S. occidentalis is from 32.5 to 36.4 degrees Celsius (McGinnis 1970).

The frequency of sun basking and sun orientation have considerable affect on radiant heat absorption. At higher elevations S. occidentalis will be perched on lower objects, basking in the sun more often and orient themselves perpendicular to solar radiation, all to increase heat absorption given the lower temperatures at higher elevations and the frequent wind (Adolph 1990).

S. occidentalis also tends to emerge later in the day on a cool day than it would on a warmer day. On cooler days, "when the ground surface temperatures in the sun at midday never exceed 30 degrees Celsius emergence was delayed until early afternoon" (McGinnis 1970). Hours of activity were also more on hot days than on cool days (McGinnis 1970).

Diet

S. occidentalis lizards have a diet consisting of small invertebrates, mostly insects and spiders. All prey ranged from 1mm to 34mm in length. Immature S. occidentalis lizards capture similar prey, only smaller: 1mm to 13 mm in length (Rose 1976; Stebbins 1985).

Fence Lizards are also the prey of some snakes, birds and mammals. The Racer snake (Coluber constrictor) is one of the Fence Lizards most dangerous predators. To escape from being preyed upon, Fence Lizards usually run and hide (Finch 1938).

Conclusions

The lizard Sceloporus occidentalis is widespread in Oregon and it seems to do well in the variety of habitats that Oregon provides. Its ability to live in a number of habitats and its large populations makes this species stand out in a natural selection sense. It has made use of its environment extremely well and should be viewed as unique and important within the family of Iguanids.

References

Adolph, Stephen C. 1990 Influence of Behavioral Thermoregulation on Microhabitat use by Two Sceloporus Lizards. Ecology, Vol. 71: 315-327

Brown, Herbert A., R.Bruce Bury, David M. Darda, Lowell V. Diller, Charles R. Peterson and Robert M. Storm. 1995 Reptiles of Washington and Oregon Seattle Audobon Society

Camp, Charles Lewis 1916 The Subspecies of Sceloporus occidentalis . Zoology vol.17, no. 7:63-74

Eisen, Rebecca J. and Jos. J. Schall 1997. Comparing Foraging Success in Submissive Malaria-Infected and Territorial Non-Infected Fence Lizards. Journal of Herpetology, vol.31: 147-149

Fitch, Henry S. 1938 Field Study of the Growth and Behavior of the Fence Lizard. University of California Publications in Zoology.

Marcellini, Dale and James P. Mackey 1970. Habitat Preferences of the Lizards Sceloporus occidentalis and S. graciosus. Herpetologica, vol. 26: 51-56

McGinnis, Samuel M. 1966. Sceloporus occidentalis: Preferred Body Temperature of the Fence Lizard. Science, vol. 152:1090-1091

McGinnis, Samuel M. 1970. Flexibility of Thermoregulatory Behavior in the Western Fence Lizard. Herpetologica, vol. 26: 70-76

Rose, Barbara R. 1976. Habitat and Prey Selection of Sceloporus occidentalis and S. graciosus. Ecology 57: 531-541

Stebbins, Robert C. 1985. Western Reptiles and Amphibians; Peterson Field Guides. Roger Tory Peterson, New York.