The Mexican Tortoise Project

The Tucson Herpetological Society is a Partner in Investigating the Desert Tortoise (Gopherus agassizii) in Mexico to aid in Conservation and Management of the Species.

MexTort06A team of Mexican, Canadian and American collaborators has begun an ambitious effort to study Desert Tortoises throughout their range in Mexico.

The primary purpose of this study is to investigate the status of the desert tortoise south of the border to aid in conservation of the species throughout its geographic range. This project is a multi-faceted, multinational, and cooperative effort to focus on crucial aspects of desert tortoise health, genetics, general biology and ecology in Mexico.

 

 

TortoisRangeThe Desert Tortoise, Gopherus agassizii, occurs primarily in desertscrub, thornscrub and dry tropical deciduous forest (TDF) environments in the southwestern United States and northwestern Mexico. The species is defined as having distinct Mojave and Sonoran populations separated geographically mostly by the Colorado River, but no new taxa have yet been described.

Although approximately 40-45% of the desert tortoise’s geographic range is in Mexico, the distribution, ecology, and taxonomic status of the tortoise in Mexico are poorly understood. Over the past three decades some work has been done to fill in these knowledge gaps, but much more is needed relative to conservation status of the species particularly in the southern portions of its distribution.

 

 

MexTort08Across its large geographic range, the desert tortoise occupies a variety of habitats, from arid Mojave Desertscrub in California, Nevada, and Utah, south and east through Sonoran desertscrub and thornscrub in Arizona and Sonora, and finally into the semiarid TDF (sometimes termed ‘thornforest’) of southern Sonora and northern Sinaloa.

Behavioral patterns and ecological challenges for tortoises in these different environments can also vary greatly, and must be understood to evaluate the status of populations. In particular, we currently know very little about behavior of tortoises in thornscrub and thornforest biomes. In the initial phase of this project have focused on desert tortoises in the TDF near Alamos, Sonora, where its habitat is most different from that in the tortoise’s range outside Mexico. We are conducting focal studies of individual tortoises using radiotelemetry to compare movements, behavior, and feeding of tortoises in the TDF to similar studies in other habitats. We have affixed radiotelemetry units to 19 tortoises at Alamos and have tracked them monthly since November 2005.

We are also gathering data on genetics and health of Desert Tortoise populations in Mexico. Since 2004, we have collected samples from three main geographic regions in Mexico representing three different vegetation types; 22 samples from two sites near Alamos, Sonora (TDF), 19 samples from two sites near Ciudad Obrégon (foothill thornscrub), and 14 samples from two sites north of Hermosillo (Sonoran Desertscrub). In 2008, we extended further south and obtained 6 samples from the state of Sinaloa.

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At the University of Arizona, we are examining the population genetic structure of Mexican tortoises. In our analysis, we have also incorporated samples previously collected in Arizona. Thus far, we have found a continuum of genetic similarity from central Sonora northward spanning 850 km of Sonoran Desertscrub, from Hermosillo, Sonora to Kingman, Arizona. However, distinct mtDNA haplotypes at the southern edge of the species range suggest a more complex story in this vegetative transition zone. In foothill thornscrub and tropical deciduous forest in southern Sonora, we found fixed differences in STR alleles, autosomal STR motifs, and mtDNA haplotypes that clearly distinguish a unique, “Sinaloan” Gopherus. We estimate this Sinaloan type diverged 5-6 million years ago from a common ancestor with the Sonoran and Mojave lineages.

Defining these taxonomic distinctions are necessary for effective conservation of tortoises outside the U.S. Our goal is to assist in the designation of management units for the entire range of the desert tortoise, including Mexico, so that conservation efforts can be applied with specificity to each area. We can also use genetic data to determine the extent to which sampled populations have experienced population declines and therefore which populations are most vulnerable to the combined effects of barriers to gene flow and loss of genetic variability.

This multinational, cooperative effort has brought together over 40 collaborators from Mexico, Canada, and the United States, including the Tucson Herpetological Society. The project has received funding from the Desert Tortoise Council and private individuals, but has been operating on a shoestring budget. Its accomplishments have primarily been fueled by participants donating their own time, equipment and expertise. We need your help to keep this project going! For more information, please contact Dr. Phil Rosen, pcrosen@cox.net.

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