Eastern Fence Lizard

A wary Eastern Fence Lizard ponders flight or fright as a defensive measure. The tail can be sacrificed if necessary to escape

Common Name: Eastern Fence Lizard – The eastern geographic and fence habitat descriptions are used to identify the lizard according to where it may be found. There are many subspecies named according to different regions and forage preferences including both Northern and Southern Fence, Prairie, and Plateau in addition to the more specific White Sands Prairie, and Red-lipped Prairie. Lizard is derived from  lacerta, Latin for “upper arm” or lizard. It is not clear if there is any association between the two but it is suggested that it may derive from the notion of a serpent with legs i.e. arm-like  appendages.

Scientific Name: Sceloporus undulatus – The genus is from the Greek skelos meaning leg and poros meaning pore or passage. This refers to the anatomical arrangement of femoral (the femur is the upper leg bone) pores which extend along the inner side of the thighs that are believed to be used for pheromonal chemicals used for territorial marking. Undulate means “to form or move in waves” from the Latin undula, referring here to the wavy dorsal patterns of the reptilian scales.

Potpourri:  The Eastern Fence Lizard is the most prototypical of all reptiles, fleetingly seen darting across the rocks or up a tree trunk on four splayed legs covered with scales from the  head to the end of a long, narrow tail.  Lizards are relict reminders of the passage of geologic time from the dominant  “terrible lizard” dinosaurs of the Mesozoic Era to the modern Holocene Epoch that has become the Anthropocene of Homo sapiens everywhere. Even though the birds are the closest dinosaur relatives, it is the lizards and snakes of the Order Squamata named for there scale-covered bodies that evoke some sense of what the earth might have been like with tank-sized predators as the dominant life form. Reptiles evolved from amphibians in the late Carboniferous Era in part due to  the availability  of high quality protein from the burgeoning insect populations as terrestrial, lunged carnivores reproducing with amniotic eggs. What is left of this once apex class of animals is now divided into the clade Archcosauromorpha (first lizard body) comprised of crocodilians, turtles, and birds and the Lepidosauromorpha (scaly lizard body) which includes snakes, lizards and tuataras. [1]

While a scaly, slender tetrapod like S. undulatus is what comes to mind when a lizard is imagined, the Lacertilian suborder is globally quite diverse and expansive.  Unlike their snake cousins in the Squamate order all of which slither, most lizards come with legs though some are legless and some can run on just two. Some are massive and some are miniscule and many sport a variety of spines, crests, horns, and expanding plates that mostly evolved for defensive purposes. Lizards are also distinguished from snakes in having movable eyelids and external ear openings. Because they are cold-blooded, most of the roughly 3,000 lizard species live in  warmer temperate and tropical climates where the necessary external thermoregulation is   sufficient. For the most part, they still live on insects and run away or hide from nearly everything else. Eastern Fence Lizards were originally in the overly large Iguanid family with 14 genera and 44 species in North America and fifteen times that many when the tropics of Central and South America were included. As a part of the DNA biological revolution, Iguania is now an infraorder with 14 families of which Phrynosomatidae, meaning “toad body”  is the most recent, but probably not the last, taxonomic host family of the genus Sceloporus. [2]

The complicated relationships of the various lizards that live on trees (or fences), plateaus, and prairies across the United States and into northern Mexico provide an illustrative case study of the limits of phylogeny, the lines of descent or evolutionary development of all living things. Based on a mitochondrial DNA dataset of 56 populations of the “wide ranging and geographically variable” S. undulatus, it was concluded that the species was really a group that was monophyletic (single ancestor) comprised of at least four separate lineages each recognizable as an evolutionary species. It was further concluded that the variability among S. undulatus populations was “remarkable” with stark contrasts in “behavior, morphology and color pattern, sexual dimorphism, life history, demography, reproductive ecology, and chromosome structure.” But this is only the beginning of what promises to be an even more granular breakdown of species according to their DNA. It is not unreasonable to conclude that any classification apparatus intended to provide a better human understanding of the complexities of living things but instead offers only a death spiral of over-speciation is in sore need of rethinking. A general geographic  tripartite division of the ten-odd subspecies of Sceloporus undulatus has been proposed that lumps them into  eastern woodland, central grasslands, and western canyon types to address the most prominent differences to impose some sense of order. [3] This is likely to be a more enduring outcome.

The life cycle of the Eastern Fence Lizard and its cohorts lends itself to the genetic diversity and dispersion that DNA changes chronicle. Two physiological factors stand out. One is that cold-blooded carnivore reptiles can subsist on a fraction of the food that a warm-blooded bird or mammal requires. This has its downside in geographic restrictions to warmer climates where freezing is not a risk factor and the need to locate and capture other animals that are just as adept at not being eaten as any other extant surviving species. The second factor  is the ability to excrete uric acid directly without the copious amounts of water needed by most other animals to expel nitrogenous wastes in the form of urine. This allows for the exploitation of desiccated terrestrial habitats like rocky outcrops, arid wastelands, and scruffy canyons that are anathema to mammals and amphibians. Taken together, a behavioral pattern that sustains population divergence and adaptation emerges with individuals spreading radially in search of food and seeking shelter in literal niches between the rocks. Specific adaptation is not uncommon among lizards. The anole populations of a number of small Caribbean islands consist of as many as ten different species each specializing in particular prey and habitats within the severely constrained ecological confines.

Sex is the engine of  speciation which would otherwise result only from random mutation. Any reptilian diaspora must therefore account for the essential meetup. The colocation of a male and a female of the same species is one of evolution’s most interesting and enduring outcomes. This is especially true in the case of widely dispersed carnivores hunting for food with only individual satiation in mind like lizards.  Territory and smell are the key factors. Male Sceloporus lizards establish boundaries to their domains using a series of aggressive displays that rarely involve physical aggression. Each species has a somewhat different variant that generally includes some combination of tail waving, head bobbing, and gaping. [4] There is a tradeoff between the establishment of territory and exposure to lurking predators like birds for whom such movements are an invitation. Those males that successfully survive while maintaining larger areas of control will have access to more females who are attracted by the pheromones that are distributed around the perimeter through the leg (skelos)  pores (porus) for which they are scientifically named. Evolutionary success favors those males that balance the hazards of exposure with the benefits of reproductive monopoly which would make them genetically fittest to survive. Field studies have found that male body size is the only parameter that correlates to home-range area and that the size and the shape of the area is determined by the location and number of females.  [5] Life then imitates nature as it is demonstrably true that jocks get the most dates.

Squamates (snakes and lizards) are endowed with forked male genitalia called hemipenes that are held internally, everted and erected when needed for copulation. The redundancy is neither a matter of improved fecundity nor a matter of multiple insertion but in all probability a matter of genetic happenstance. Amphibians have no need for a sexual insertion device since everything happens in the water. The hypothesis is that terrestrial reptiles required some new “hardware” and the parsimony of genetic creativity repurposed the leg forming genes to that end … two new legs became two proto-penises. By the same logic, the mammalian penis evolved from the tail forming genes and therefore retained its singularity. When a successful lizard male lures an interested female into his territory, one of the hemipenes is inserted into the cloaca with the ultimate result of the fertilization and deposition of about half a dozen eggs. A fertilized egg surrounded and protected by the amniotic sac is a second terrestrial adaptation necessary for nonaqueous survival. Amphibian eggs are suspended in water and don’t risk desiccation; on land it is a different story.  The penis and egg innovations coevolved so that the embryotic union would occur before the release of the fertilized egg with the protective, amniotic coating. [6] Live young came later on the way to us.

The realities of the natural food chain, as brutal as they frequently are, are no less a matter of evolutionary fitness. Cinematic excess from Jaws to Jurassic Park exploit the grim result. Just as lizards seek a meaty meal they may become one. Cryptic coloration is one of the main defenses employed to confuse the offense. It is also effective as an agent of stealth in duping nonobservant prey into a false sense of security ― nothing there does not always mean that nothing is there. Lizards are masters of the palette, notably chameleons  and anoles. Pigment cells called melanophores contain granules that are able to migrate based on a complex combination of hormones, temperature, and nerve signaling. The many lizards of the woodlands, grasslands, and canyons that are genetically related but distinct display a variety of bars, hues, and splotches that distinguish and protect them. Running and hiding under a rock, down a burrow, or into a tree knothole is a backup option if the disguise doesn’t work. The Gadarene leap would of course be headfirst leaving the tail as the last thing to disappear… which makes it a particularly vulnerable appendage. To a lunging fox or grasping bird the tail is a perfect target. Which is why most lizards have evolved with a tail that is not only detachable, but even wriggles suggestively as a decoy. Special muscles constrict at the connection point to minimize blood loss and a new tail grows back in about three months. [7] The would-be predator is at least left with something, albeit a small, bony snack.

Since lizards are masters of water management and prefer warmer temperatures, they are prolific in desert wastelands. One would think that the increasing temperatures of climate change would therefore be a boon and leaping lizards more than a dated Orphan Annie epithet. Maybe yes and maybe no. Nest temperature is correlated to the sex ratio of the hatchlings. Male lizards are more cold tolerant than females so cooler temperatures produce more males according to evolutionary pressures. Conversely,  a warmer climate would gradually shift the ratio toward more females, which would mean more eggs and more lizards. A recent study of Australian skinks found that the average nest temperature had increased by 1.5 degrees Centigrade between 1997 and 2006. The study also showed that lizards incubated in warmer nests were smarter based on laboratory testing of escape route detection. [8] So there should eventually be a surplus of  smarter females and fewer dumb males and the world will presumably be a better place according to a certain mindset. However, this does not seem to be the case, as lizard populations are declining. Since 1975, Mexico has experienced a 12 percent decline and reductions in lizard sightings have been reported elsewhere. Experiments have revealed that the temperature in the desert areas where lizards are no longer found is significantly hotter than areas where they remain. The hypothesis is that foraging time becomes restricted by excessive temperatures to the extent that survival is threatened. Extrapolations to 2080 result in a 20 percent lizard population decline.[9] By then, perhaps another meteor will crash into the earth near Mexico like the last one and all biological clocks will be reset, perhaps making the reptiles top dog again.



  1. Starr, C. and Taggart, R. Biology, The Unity and Diversity of Life 5th Edition, Wadsworth Publishing Company, Belmont, California, 1989, pp 683-687.
  2. Behler, J. and King, F. National Audubon Society Field Guide to North American Reptiles and Amphibians, Alfred A. Knopf, New York, 1979, pp 497-538.
  3. Leaché A., Reeder T. “Molecular systematics of the eastern fence lizard (Sceloporus undulatus): A comparison of parsimony, likelihood, and Bayesian approaches”. Systematic Biology. 2002, Volume 51 (1) pp 44–68.
  4. Gorman, G. “Sauria” Encyclopedia Britannica Macropedia, William Benton, Publisher, Chicago, 1974, Volume 16 pp. 282-288.
  5. Haenel, G. et al “Home-Range analysis in Sceloporus undulatus (eastern fence lizard). I. Spacing patterns and the context of territorial behavior”. Copeia. 2003 pp 99–112.
  6. Laslo, M. “When two become one: the evolution and development of external genitalia on land” 8 April 2015, https://evobites.com/2015/04/08/laslo_external_genitalia/
  7. Missouri Department of Conservation https://web.archive.org/web/20120702004844/http://mdc.mo.gov/discover-nature/outdoor-recreation/nature-viewing/amphibians-and-reptiles/lizards
  8. Youngsteadt, E. “Could Climate Change Alter Lizard Learning?” Science 10 January 2012.
  9. Price, M. “Climate Change Causing Lizards to ‘Wink out of existence’.” Science, 13 May 2010
Brown hues enhance survival on tree trunks (lichens don’t help)
The wavy scale pattern of S. undulatus is cryptic