Sonoran Lizards

LOTS OF LIZARDS

In the Sonoran Desert, lizards are particularly conspicuous to the casual observer because so many are diurnally active.

Most species are carnivorous and the majority are important consumers of insects. A few, like the common collared lizard (Crotaphytus collaris) eat other vertebrates, including their own young.

Like the desert spiny lizard (Sceloporus magister), they are occasionally omnivorous, eating blossoms and buds in the spring.

Few are primarily plant eaters like the common chuckwalla (Sauromalus obesus) and desert iguana (Dipsosaurus dorsalis). In both species, the young are well-known to eat insects while the adults are chiefly or exclusively herbivorous.

The well-known Gila monster (Heloderma suspectum) is one of two living venomous lizards.

It is a predatory scavenger specializing in the eggs and helpless young of reptiles, birds, and small mammals.

Its brief surface sojourns from subterranean retreats occur mainly in April and May, the best seasonal window for activity requiring moderate daytime temperatures and reaping an energy bonanza from the spring bounty of eggs and nestlings.

Gila monsters spend about 98% of their lives underground.

Gila monsters consume most of their annual caloric intake during this period and spend most of the remaining time, about 98% of their lives, in climatically-sheltered dormancy.

The Gila monster beats a dignified retreat when approached, but halting if pressed, and hissing loudly with mouth agape.

The lizard can move with surprising quickness when in defensive mode; the jaws holding tenaciously as venom permeates the wound.

The evolutionary origin and subsequent radiation of the Gila monster is unclear.

While most authorities agree it evolved from a now-extinct helodermatid ancestor in tropical dry forest communities, when and where this occurred is open to debate.

It seems likely that the Gila monster evolved in the southwestern desert region as the tropical deciduous forests which once covered much of western North America receded southward.

Selective climatic factors caused it to become increasingly fossorial and limited to narrow surface activity periods by temperature and humidity extremes.

Natural selection refined water and energy conservation mechanisms enabling it to survive primarily in the biseasonal rainfall Sonoran Desert and marginally in the Mohave Desert where it is restricted to areas receiving some summer rain.

It then radiated southward into the Sinaloan Thornscrub where it is now sympatric with the closely-related beaded lizard (Heloderma horridum).

Interestingly, the two geographic subspecies of the Gila monster may reflect the morphological results of this radiation.

The banded Gila monster (H. s. cinctum), the northwestern-most race, is characterized by a distinctly banded pattern dominated by a light pink ground color.

The reticulated Gila monster (H. s. suspectum) is strongly banded during embryonic development but develops the characteristically mottled pattern, more or less dominated by black, as it matures.

These facts raise some interesting questions regarding phylogeny and center of evolutionary radiation in the Gila monster.

Ontogeny recapitulates phylogeny: This is a biological axiom stating that the evolutionary history of an organism is repeated in its embryonic and early postnatal development.

Wwe can therefore speculate that because the northernmost modern Gila monster populations are more similar to the banded morph through which all Gila monsters pass in their development...

... then the northernmost populations must be least derived and therefore closest in appearance to the Gila monster ancestor.

As eons passed, it became increasingly refined in morphology, physiology and behavior to deal with the changing climate.

It may be that more open habitat conditions in the Mohave Desert serve to retain the light ground color dominance...

...while the more shrubby understory of the Arizona Upland and Sinaloan Thornscrub habitats selects for darker pattern to improve camouflage among the light and shadows common there.

Sonoran Desert lizards employ various forms of thermoregulatory behavior to achieve and sustain optimal metabolic body temperature without overheating.

The majority depend primarily on the sun as a source of radiant heat to raise body temperatures to levels necessary for metabolic activity (heliothermia).

When necessary, they can achieve body temperatures substantially higher than the surrounding air temperature.

Many heliothermic lizards initially expose only their heads as they emerge. The smaller head mass achieves higher temperature more quickly than would the unexposed body.

The jugularis muscle constricts the jugular vein, slowing blood flow back to the body. Blood is thus warmed in the head and transferred to the body.

The lizard is thus able to raise the body temperature to a metabolically efficient level with less risk of exposure to predators. When the head-body temperature reaches equilibrium, emergent activity begins.

When fully basking, lizards first align their bodies at a right angle to the sun and gradually shift their position to face it as their body temperature rises.

Many also rely to a greater or lesser degree on warmth conducted from the substrate, often flattening their bodies tightly to increase heat uptake.

Most desert lizards have long legs not only for speed but also to elevate the body to reduce heat absorption.

At their thermal peak, many species extend their legs and stand as high as possible, often elevating the toes and tail and supporting their weight entirely on their heels, further reducing conduction from the hot surface.

These traits provide desert lizards with distinct advantages over desert snakes, most of which must wait for cooler times to be active.

Like most other reptiles, lizards retreat to shade, crevices, or underground when temperatures exceed their tolerance limits, resulting in distinctive morning and afternoon activity periods during the hottest months.

This "siesta" wisely avoids work during the hottest part of the desert day! Many species even pant as a means of ameliorating body heat by evaporative cooling.

While all Arizona Upland reptiles hibernate during the short winter, some are "light sleepers" and often emerge briefly during winter warm periods. Most, however, are primarily active from April through October.

A few, like the tree lizard (Urosaurus ornatus) and side-blotched lizard (Uta stansburiana) are able to achieve their preferred body temperatures through the entire year, and are often seen during winter.

A few, like the banded gecko (Coleonyx variegatus) are rarely found afield in daylight, relying instead on soil and surface warmth to achieve metabolic temperatures.

These strictly terrestrial lizards often hibernate beneath surface cover. They are more primitive than the more-familiar "modern" geckos, lacking adhesive toepads and lidless eyes.

The distribution of the regal horned lizard (Phrynosoma solare) closely conforms to that of the Arizona Upland, occasionally extending into desert-grassland edges and open oak-juniper woodland, and ranging southward into Sinaloan Thornscrub in northern Sinaloa.

It seems to be most common on the rich bajadas where it shifts its activity time according to temperature, even becoming crepuscular when the morning temperatures reach 100 degrees F.

Like other desert-dwelling horned lizards, it is oviparous, producing up to 28 eggs per clutch. Their frequently-encountered scats are composed of about about 90% ants.

The ubiquitous western whiptail (Cnemidophorus tigris) is often seen in the early morning hunting for insects, digging erratically as it progresses in its irregular, jerky gait . When pursued, it is among the fastest of desert lizards, eluding all but the most adept lizard predators like the red coachwhip (Masticophis flagellum), western patchnosed snake (Salvadora hexilepis), and roadrunner (Geococcyx californianus).

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