The desert tortoise complex consists of three species: Agassiz’s, or the Mojave desert tortoise (Gopherus agassizii), Morafka’s, or the Sonoran desert tortoise (G. morafkai), and the newly recognized Goode’s thornscrub, or the Sinaloan desert tortoise (G. evgoodei). The southernmost species in the complex, Goode’s thornscrub tortoise is found only in the Sinaloan thornscrub and the tropical deciduous broadleaf forest ecosystems in the states of Sonora, Sinaloa, and Chihuahua in northwestern Mexico.
The Desert Tortoise in Mexico
An extensive amount of research has been conducted on the desert tortoise in the United States. However, until recently, a limited amount of study has been devoted to the desert tortoise in Mexico. The following section describes some of the early research conducted with regard to the desert tortoise in Mexico.
In 1922 California herpetologist John Van Denburgh (1872-1924) provided the earliest report of desert tortoises in Mexico from sightings near Álamos in southwestern Sonora. His observations were published in an article titled “The reptiles of western North America, Vol. I, Snakes and turtles” in Occasional Papers of the California Academy of Science. The next mention of the desert tortoise in Mexico in the scientific literature occurred in 1945 when Charles M. Bogert and James A. Oliver published the article “A preliminary analysis of the herpetofauna of Sonora” in the Bulletin of the American Museum of Natural History (Bury et al., 2002).
Neither Van Denburgh nor Bogert and Oliver described the habitat in which the observations of the Mexican desert tortoises occurred. Bogert and Oliver were the first scientists to identify “the distinct morphology 1 of the tortoises at Álamos,” but they could not “quantify it due to the small sample sizes” (Edwards et al., 2016). In 1964 Richard Loomis and Julius Geest published the first report of desert tortoises in Sinaloa in an article in Herpetologica entitled “The Desert Tortoise Gopherus agassizii in Sinaloa, México “ (Bury et al., 2002).
In 1969 biologist Walter Auffenberg furnished “the first substantial field observations” of desert tortoises in Sonora in his book Tortoise Behavior and Survival (Bury et al., 2002). In 1994 Thomas Fritts and Randy Jennings coauthored a report on the distribution of the desert tortoise in Mexico for the publication Biology of North American Tortoises. The authors documented 74 localities within Sonora and Sinaloa at which they had observed desert tortoises. In 2002, Kristin Berry, David Morafka and Robert Murphy coauthored an article for Chelonian Conservation and Biology entitled “Defining the Desert Tortoise(s): Our First Priority for a Coherent Conservation Strategy.” The authors emphasized the need for systematic studies of “morphological, ecological, behavioral, and physiological distinctions among populations“ of desert tortoises. The authors also suggested several possible taxonomic arrangements, one of which presents the Mojave, Sonoran and Sinaloan populations as three separate species (Berry, et al., 2002).
In 2002 Trip Lamb and Ann McLuckie coauthored a chapter published in The Sonoran Desert Tortoise entitled “Genetic Differences among Geographic Races of Desert Tortoises.” They reported on the analysis of desert tortoise genetic data. Their research indicated that a special form of DNA (mitochondrial DNA) is “a high resolution genetic marker for assessing geographic variation” in tortoise populations based on earlier studies. Their investigation revealed distinct genetic lineages within the Mojave, Sonoran and Sinaloan desert tortoise populations (Lamb and McLuckie, 2002).
In 2011 Dr. Robert Murphy and an international team of scientists from Canada, the United States and Mexico published a paper in the online journal Zookeys presenting a description of a separate species of desert tortoise native to the Sonoran Desert, Gopherus morafkai, Morafka’s desert tortoise. G. morafkai was named in honor of the late herpetologist Dr. David J. Morafka. Prior to the publication of this paper, all desert tortoises in the southwestern United States and northwestern Mexico were assigned to a single species, Gopherus agassizii. The ground-breaking research and DNA analysis of Dr. Murphy and his colleagues added a second species to the desert tortoise complex, the first “new” desert tortoise species in 150 years.
The Sinaloan “sister-species”
Dr. Taylor Edwards, an evolutionary biologist and conservationist from the University of Arizona, led an international team of researchers from Canada, the United States and Mexico that conducted a six-year study of the desert tortoise in the southern Sonora and northern Sinaloa in Mexico. The team took blood samples and measured as many tortoises as they could find during their fieldwork.
The study commenced in 2010 and culminated in February 2016 with the publication of the new species description entitled “The desert tortoise trichotomy: Mexico hosts a third, new sister-species of tortoise in the Gopherus morafkai–G. agassizii group“ in the online journal Zookeys. Dr. Edwards and his team utilized data management services provided by a system from the National Science Foundation called CyVerse to analyze the vast amounts of field-collected data and allowed the sharing of these data with colleagues (Littin, 2016).
Dr. Edwards proceeded to suggest a novel way to name the new tortoise species while protecting a portion of its habitat, setting up, in essence, a “trust fund” for newly-recognized desert tortoise species.
For decades prior to Dr. Edwards’ study, researchers had observed striking differences in the physical characteristics of Mexico’s northerly population of desert tortoises as compared with the southerly population.
The northerly population was recognized in 2011 as Morafka’s desert tortoise, G. morafkai, which ranges from western and central Arizona in the United States south to the northern Sonoran desertscrub habitat in Mexico.
Dr. Edwards and his team developed a set of 37 variable physical characteristics in order to systematically measure the tortoises in the tropical deciduous forest and the Sinaloan thornscrub and compare those tortoises’ characteristics to the other species in the desert tortoise complex, G. agassizii and G. morafkai.
All the desert tortoises share certain characteristics: 11 marginal scutes on the left and right edges of the carapace, five toenails on each forefoot, and four toenails on each hind foot. All desert tortoise species display sexual dimorphism: adult males have a somewhat longer tail, a concave plastron, a larger gular horn, and prominent chin glands (Edwards et al., 2016).
G. evgoodei has several distinctive characteristics compared to its sister-species: a noticeably flatter carapace, protruding scales (“spurs”) on the upper forelimbs, a relatively bulbous snout, and rounded pads on the hind feet. Both sexes have noticeably shorter tails than the other desert tortoise species. The female’s tail is miniscule at 2–8 millimeters (0.08–0.3 inch) and the male’s tail is less than 13 millimeters (0.5 inch) in length (Edwards et al., 2016).
The carapace coloration of both Agassiz’s desert tortoise and Morafka’s desert tortoise is medium to dark brown or dark gray and the skin is brownish-gray to dark gray. The carapace coloration of Goode’s thornscrub tortoise is brown to gray with subtle orange mottling or spotting, and its skin is dark tan to medium brown with an “orange cast” (Edwards et al., 2016).
Goode’s thornscrub tortoise is native to the thornscrub and tropical deciduous forest ecosystems of northwestern Mexico. It inhabits southern Sonora, northern Sinaloa, and western Chihuahua. See the map to the right for regions and species information. G. evgoodei inhabits the smallest geographic range of the three species in the desert tortoise complex, an area of approximately 24,000 square kilometers (9,300 square miles) (Edwards et al., 2016). As a result, the newly-recognized tortoise species is already imperiled by the conversion of portions of its habitat to buffelgrass pasture for livestock grazing. See the section in this article titled “Threats” for more information.
Diet in the Tropical Deciduous Forest
Thomas Van Devender and a team of researchers collaborated on a project designed to study the foods consumed by desert tortoises in their various habitats. Their research findings were published in a chapter included in The Sonoran Desert Tortoise.
Of particular relevance to the current article is the section of that chapter titled “Diet in Tropical Deciduous Forest in Southern Sonora.” The researchers collected desert tortoise scat from five burrows in Las Piedras Canyon, located three kilometers (1.9 miles) south of Álamos, Sonora. They analyzed the scat samples to determine the types of plant materials that the local tortoises were consuming” (Van Devender et al., 2002).
The researchers separated the forty-four collected fecal pellets into their component parts in order to identify the plant fragments contained in the pellets. Although some plant fragments were unidentifiable, researchers were able to recognize 25 plant species (Van Devender et al., 2002).
The combined totals for all samples were summarized by quantity consumed as follows: herbaceous perennials, 28%; trees and shrubs, 20%; grasses, 16% with 12% being annual grasses; annuals, 16%; perennial/annual herbs, 8%; a liana [ed. note: a woody climbing plant], 4%; a cactus (Opuntia spp.), 4%; and a subshrub, 4% (Van Devender et al., 2002).
According to the fragment analysis of the tortoise scat, the plants most commonly consumed included the following species:
- Trans-Pecos ayenia, Ayenia filiformis
- bristlegrass, Setaria liebmannii;
- Natal grass, Melinus [Rhynchelytrum] repens;
- panicgrass, Panicum trichoides;
- Sida alamosana (no common name, a Mallow family member).