Desert Tortoises (Gopherus agassizii) from Arizona's Black Mountains

by Donald L. Blanchard

This article was originally published in The Cold Blooded News, the newsletter of the Colorado Herpetological Society, Vol 26, #5, May, 1999.

The Colorado River, which only assumed its present course some time within the last 10 million years, is generally regarded as forming an isolating barrier between the two populations. However, as reported in the March, 1999, issue of the Journal of Herpetology, an anomalous population of Desert Tortoises has been found in the Black Mountains of northwestern Arizona, north and west of Kingman, and on the Sonoran side of the Colorado River. Like the Mojave populations, these tortoises tend to prefer open country over rocky slopes.

Eleven tortoises from the Black Mountain study area were collected and blood samples collected, after which the turtles were returned to their original location. Mitochondrial (mt) DNA from the blood of these individuals was then compared with that of 17 Mojave tortoises from 4 locations and 18 Sonoran varieties, also from 4 locations. Ten of the eleven Black Mountain specimens showed mtDNA signatures characteristic of the Mojave form.

Additionally, a number of morphological traits were also accumulated and criteria established to morphologically distinguish Sonoran from Mojave tortoises. Similar data was collected for a sample of 37 Black Mountain tortoises, and the criteria were used to categorize them as Mojave (24) or Sonoran (13). However, some of the measurements also allow an incomplete degree of differentiation between Black Mountain and Mojave specimens. The indications suggest that the Black Mountain tortoises represent an isolated remnant population descended from the Mojave lineage, but apparently slightly differing in some morphological traits.

Correct identification of the Black Mountain population has a degree of political importance, as the Mojave population is federally listed as threatened under the Endangered Species Act of 1990, while the Sonoran population is only listed as of special concern. This listing has considerable bearing on land management policies, and clearly the Black Mountain tortoises are in need of further study. They are currently listed as part of the Sonoran population, and hence not entitled to the same level of protection as their more closely related Mojave morphs.

Whether the Black Mountain tortoise population was established from transport across the Colorado River by Native Americans, or by stream meanders in the river before its current channels became established, remains speculative.

Editor's speculation:
It has long been known that at some time in the geological past, before the uplift of the Colorado Plateau, the upper Colorado River flowed from Colorado west across Utah, to vanish somewhere in western Utah or central Nevada (although its exact course remains unknown -- at least to this author). Within the last few million years, a stream discharging into the newly opening rift that became the Gulf of California eroded its channel north, then east and northeast across the Kaibab Plateau, ultimately capturing the Colorado River and diverting its discharge. The added water flow from the upper Colorado allowed this new stream to erode its way through solid rock at the same pace that the Colorado Plateau was being uplifted. Thus was born the Grand Canyon; its sediments were transported to a delta in the middle of the Gulf of California, eventually closing its northern part, which once extended to Palm Springs, California, off from the sea.

Using standard DNA dating methodology, the Journal authors estimated that the Mojave and Sonoran Desert Tortoises were isolated 5 to 6 million years ago. This corresponds nicely with the probable date for the opening of the Gulf of California and the formation of the stream that later captured the Colorado. After the capture, and during the formation of the Grand Canyon, stream flows were probably sufficient to preclude migration of tortoises across the river. However, the erosion of the Grand Canyon was not a smooth and continuous process. Volcanic eruptions along the rim of the canyon periodically spewed their lavas down into the canyon, forming natural dams across the river channel, and temporarily blocking its flow. Conceivably, these dams could have held the river's waters back for many years, allowing Mojave tortoises plenty of time to migrate across the (temporarily) dry river channel. Migration across the river farther downstream could have been prevented by discharge from tributary streams which enter the river south of Kingman.

References:
McLuckie, Ann M., Trip Lamb, Cecil R. Schwalbe, and Robert D. McCord. Genetic and Morphometric Assessment of an Unusual Tortoise (Gopherus agassizii) Population in the Black Mountains of Arizona. Journal of Herpetology. Vol.33, No.1, March, 1999.

Halka Chronic. Roadside Geology of Arizona. Mountain Press Publishing Co. Missoula Montana, 1993.