Rigor Vitae: Life Unyielding

Monday, May 08, 2006


The horned lizards (Phrynosoma spp.) comprise a genus of fifteen or so well-known lizards distributed through most of western North America, from southern Alberta to Guatemala. The entire genus is a pretty uniform lot: small (the giant among them is a Mexican brute of 8 inches), flattened and sprinkled with spine-like scales. The most unusual of the American horned lizards is the Flat-tailed Horned Lizard (P. mcallii), a comparatively slender species that is faster and more active than its congeners, having adapted to survive in the harsh sand dune habitat of the lower Colorado River. This area, where the American states of California and Arizona meet the Mexican states of Baja California Norte and Sonora, is known as the Salton Trough, the lowest and driest portion of the Sonoran Desert. It is the former sea bed of a Pliocene extension of the Gulf of California, and as recently as 500 years ago much of it was occupied by the ancient Lake Cahuilla. For the past century, the lowest part of the lake bed has been inadvertently filled with irrigation runoff, forming the Salton Sea, a 376 square-mile lake that formed the basis for a number of small cities, including Salton City, Salton Sea Beach, Bombay Beach and Desert Shores.

The development of the Salton Trough has diminished the habitat for the few animals that are endemic there, most notably P. mcallii and two species of Fringe-toed Lizard, Uma inornata and U. notata. The last species is listed as a threatened species, the first two are considered species of concern. Today, the range of the Flat-tailed Horned Lizard consists of four disjunct populations, divided by the Colorado River, and by human development around the Salton Sea and in the Imperial Valley. Where the Arizona population abuts the range of the widespread Desert Horned Lizard (P. platyrhinos), individuals that appear to be morphological intermediates occur.

Daniel Mulcahy and colleagues from Utah State University have been sampling mitochondrial DNA from Desert and Flat-tailed Horned Lizards of the Salton Trough region in an attempt to learn not only how long the Flattail populations have been fragmented (and to what degree it was caused by human development), but whether the “intermediates” are hybrids or a discreet lineage. Their results thus far appear in a paper in the latest edition of Molecular Ecology. Although their conclusions will require further sampling to verify, they suggest that the ancestral range of P. mcallii was in the southeast end of the Trough, where genetic diversity is the richest. Two major clades were identified, one on either side of the Colorado River, although at least one unique haplotype was found to occur in both populations. The authors theorize that the lizards entered the region that is now west of the Colorado River during the Pleistocene when the river changed its course and moved east to its current course, leaving Lake Cahuilla to dry. The reptiles then dispersed up the western edge of Lake Cahuilla. The authors believe that the Imperial Valley has been a genetic barrier for the lizards since Lake Cahuilla went dry, but that the populations on either side of the Salton Sea were connected until fairly recently, although the case for this looks less strong to me.The most interesting evidence came from their analysis of Desert Horned Lizard samples. The “intermediate” lizards on the southeastern edge of the Trough appear to be hybrids between P. mcallii and the subspecies P. platyrhinos goodei. It seems from this study, that P. p. goodei, however, represents a lineage distinct from the Desert Horned Lizards, and should probably be regarded as a new species, the Sonoran Horned Lizard (P. goodei). The authors theorize that P. goodei originally derived from hybrids of P. mcallii and P. platyrhinos. This is an interesting example of the utility of comparing your study species with closely related taxa. I would like to see a similar survey done of the Colorado Desert and Coachella Valley Fringe-toed Lizards (Uma notata and U. inornata), which are also endemic to the Salton Trough. It would be interesting to compare the two studies; the two fringetoe species are separated by the same Salton Sea expanse that separates the two westernmost Flat-tailed Horned Lizard populations.
upper: DUST DEVILS--DESERT HORNED LIZARD (2005) acrylic 12" x 4"
lower: COAST HORNED LIZARD (1998) acrylic 20" x 30"


Anonymous Anonymous said...

now you've made me homesick :) I have such fond memories of these lizards and grew up with 'horned toads' as we called them, and I discovered that if you rub them gently between their horns, they will go to sleep. I've read horror stories where they held 'round ups' in Texas, where they were turned into belt buckles and other trinkets. So now they're endangered in those areas. Duh.
As always, the art adds so much to your post, I feel as if I can reach out and stroke them to sleep :)

9:45 PM  
Anonymous Anonymous said...

Fascinating post! It's so refreshing to see examples of speciation occurring that don't involve dirty moths. I hope someone takes your suggestion and looks into the fringe-toed lizard populations. Something tells me that understanding the ways an ecosystem adapts to abrupt changes might be rather important. ;)

10:23 AM  
Blogger Carel Brest van Kempen said...

Cindy: They are charming little things, alright. One of the many things I like about living in the west. I understand the invasion of fire ants is having a real detrimental effect on horned lizards in many parts of the southwest.
Karmen: Thanks, I thought you might like that. Where I live, the Salt Lake Valley forms a barrier that separates Desert Horned Lizards from Short-horned Lizards (P. douglasii).

9:39 AM  
Anonymous Anonymous said...

Cool stuff! I have spent lots of time in the Coachella Valley and in the Anza Borrego Desert State Park, but unfortunately never got to see a flat-tailed horned lizard. I did my master's research on the phylogeography of the California treefrog (Pseudacris cadaverina), which has populations in the mountains surrounding those desert areas. I have wondered about the effects of Lake Cahuilla in shaping the evolutionary history of P. cadaverina. I will have to read the original paper on the horned lizards. Your paintings are absolutely amazing by the way!

1:38 PM  

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