Friday, April 13, 2018

Chuckwallas and Other Miniature Dinosaurs of the Colorado Desert

Perhaps, just perhaps, there is a place where dinosaurs walk among us. 
Common Chuckwalla, peering out from beneath a shrub in Borrego Palm Canyon
Perhaps the Colorado Desert of California is another "Isla Nublar" and Jurassic Park could also take place here, amongst ocotillos and chollas... rather than Isla Sorna's Coast Redwoods that would never be found growing on tropical Costa Rican islands in the present day, never grew there in the past, and were not even around during the reign of the dinosaurs at all. 
(This is what happens when botanists watch Hollywood films...  Though I realize Coast Redwoods are ancient trees that grew during the warm, wet climate of the past, they flourished across the northern hemisphere during the Tertiary Period (65-1.6 million years ago), which is known as the beginning of the age of mammals and took place after the extinction of the dinosaurs.  I've hiked Fern Canyon and I suppose redwoods and ferns do look like they could have been growing during the reign of the dinosaurs' and must have seemed appropriate enough for the habitat that Dr. Hammond was trying to create for his dinosaurs.  Clearly, he should have consulted Dr. Ellie Sattler, the paleobotanist (and my favorite character).  The real problem I have is that the islands in the book/movie were in the tropics, where fully-mature several-hundred-year-old Coast Redwoods just wouldn't be found.  But I digress.) 
All of that to say, we certainly encountered an abundance of reptilian life last week in the desert.  Perhaps the Colorado Desert, and other deserts of the American Southwest, remain as great strongholds of living dinosaurs, mini versions of their long-extinct kin.
Desert Spiny Lizard near the Salton Sea

Together, the Mojave and Colorado deserts of Southern California are home to 80% of California's reptile species.  There are even a few surprising amphibians that make their livings in harsh desert environments.  Most are anurans (the taxonomic order including frogs and toads) that have adapted to arid conditions, like spadefoot toads.  Other surprising desert anurans include the California Treefrog, a species that requires the permanent water of desert oases.  It was an unexpected experience to hear a chorus of treefrogs while camped beside a creosote bush, within sight of several cholla cacti, at the mouth of an arid canyon!  (The frogs inhabited a small nearby palm oasis.)

A larva (tadpole) of what I presume to be a California Treefrog, in Borrego Palm Canyon, San Diego County.
(Herpetologists, please correct me!)

Most reptiles found in the Colorado and Mojave deserts are squamates - scaled members of the order Squamata, which includes all lizards and snakes.  There are more lizards in the desert than any other bioregion in California, and almost all of California's nearly 40 lizard species have ranges that extend into desert regions.  Likewise, 26 of California's 37 snake species also make their home in the desert (which includes five species of rattlesnake). 

Desert Iguana

Desert lizards range in size from the tiny Desert Night Lizard (1.5-2.75 inches long) to the whopping Common Chuckwalla (Sauromalus ater).  The Chuckwalla reaches a snout-to-vent length of 9 inches, a measurement that doesn't include its substantial tail, which is nearly as long as its body.  (Total lengths can be up to 20 inches!)  This two-pound reptile is the only species of Californian lizard that is nearly entirely herbivorous as an adult.  In the United States, it is the second-largest lizard; only the 14-inch, five-pound Gila Monster is larger - and also venomous.  (Conjuring up Jurassic Park memories yet?)
Chuckwallas are active during the day, from spring through fall.  From late fall through winter, they remain in an inactive state similar to hibernation (called brumation).  They may also enter into estivation (summer or warm weather hibernation) during dry years when food is scarce.  As cold-blooded animals, or ectotherms, they require external energy from the sun to warm their bodies.  In the morning, they emerge from rock crevices to bask on sun-warmed rocks until their body temperature raises sufficiently to forage. 
A basking Chuckwalla in Borrego Palm Canyon
The coloration of individual Chuckwallas varies (from light to dark or even reddish) with age, gender and region (based on the colors found in the surrounding habitat).  Incredibly, their color also lightens as they warm up.  In the mornings when they emerge from rock crevices, they are dark in color, which allows them to absorb the most heat.  As their bodies warm up, they lighten in color, thus reflecting heat during the hottest part of the day. 
In addition to requiring rocks for basking, Chuckwallas need deep, rocky crevices for protection from predators as well as from the elements.  The only defense of the Chuckwalla is to wedge itself firmly and deeply inside protective rock crevices.  When threatened, it puffs itself up by inflating its lungs to prevent being pulled out by a predator.
The large rocky areas inhabited by Chuckwallas have one more surprising benefit for these almost exclusively herbivorous reptiles: the rocks water the Chuckwallas' gardens.  When rain falls on rocks, it runs off, soaking into the ground or even pooling at the base of the rock.  In this way, the rocks "collect" a surprising amount of water.  This creates a microenvironment at their base perfect for growing a lush crop of annual plants, like the grasses and forbs favored by Chuckwallas, that far surpasses the amount of plant growth found in surrounding areas.  Conveniently, the rocks provide shelter, basking sites and even food for the Chuckwalla, all in one location.
Five Chuckwallas, enjoying the morning sun on a favorite basking rock. 
(Also note the presence of inviting, protective crevices!) 
While Desert Night Lizards are classic habitat specialists, restricted to the leaf litter beneath Joshua trees, the habitat of Chuckwallas is almost just as restricted.  These large lizards require rocky terrain, including canyons, hillsides and isolated outcrops of rock.  My Field Guide to Amphibians and Reptiles of California states: "A typical habitat consists of a group of massive angular, fractured rocks that provide elevated outlooks, deep crevices, and platforms for basking." (Stebbins, 2012)  And this describes the habitat we found them in exactly (see photo above). 
Borrego Palm Canyon, with its combination of rocky canyon walls and outcrops, stretches of sandy substrate and wash bottom habitat, is an excellent place to see not only Chuckwallas, but a variety of other lizards as well.  With so many species sharing similar food preferences (insects), lizards practice niche partitioning, illustrated below.

This illustration shows how so many species of lizards are able to successfully coexist within a limited area, such as
Borrego Palm Canyon.  Illustration from A Natural History of California (Schoenherr, 1992) 

For further reading about California's diverse array of reptiles and amphibians, I direct you to an excellent book in the California Natural History Guides series, Field Guide to Amphibians and Reptiles of California,(Stebbins and McGinnis, 2012). 
For more information on the natural history of California's deserts in general, Allan A. Schoenherr's comprehensive A Natural History of California has a nice chapter on deserts; the entirety of The California Deserts: An Ecological Rediscovery, by Bruce M. Pavlik, is also an excellent resource!
And, of course, you can read about my previous adventures in desert herping (searching for reptiles)by following this link.

Desert Horned Lizard, Joshua Tree NP, April 2016

P.S.  I know there are a few problems with my Jurassic Park dinosaurs - Colorado Desert lizards comparison.  For one, California's deserts would have pretty much been inundated by water during the age of the dinosaurs; dinosaur fossils are more common on the Colorado Plateau, particularly in the deserts of Arizona and Utah. 

The second problem, of course, is that dinosaurs are probably more closely related to birds and crocodiles than modern lizards.  But they sure do bear a striking resemblance! 

So maybe if we talked about the desert's Greater Roadrunners instead...  Perhaps they are the real living dinosaurs of the Colorado Desert!!

A dinosaur-esque Greater Roadrunner, appropriately crossing the road. Death Valley NP, April 2016

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