The Mexican Tortoise Project

This story was originally published in The Tortoise Volume 1, Number 3

Mexican Tortoise Project

I awake in my tent to the sound of macaws passing overhead. I poke my head out and see a small highway of leafcutter ants carrying meticulously cut pieces of a nearby bush. A monstrous spiny-tailed iguana watches me from his perch on an adjacent strangler fig.

This is not exactly the scene you might expect to find yourself in on a quest for Desert Tortoises, but our search has led us here, to the tropical deciduous forest on the southern edge of the state of Sonora, Mexico. It bears little resemblance to the red cliffs of southwestern Utah or the sun-cooked creosote flats of the Mojave Desert in California. And although the columnar cactus and thorny acacias are a bit familiar if you live near Tucson, Arizona, the towering kapok trees and boa constrictors quickly overshadow any resemblance to home. Because Desert Tortoises occur in all of these places, it begs the question of how this came to be.

When friends ask me if it is dangerous to work in Mexico I have to say yes, but not at all for the reasons they assume.

In 2011, the Desert Tortoise was redefined as two separate species: Agassiz’s Desert Tortoise (Gopherus agassizii) and Morafka’s Desert Tortoise (Gopherus morafkai). Gopherus morafkai is found in Arizona and Mexico, in the Sonoran Desert. Although the two species are rather cryptic in their morphology, meaning they appear very similar to the untrained eye, they are behaviorally, morphologically, ecologically, and genetically distinct. The U.S. Fish and Wildlife Service now considers G. morafkai a candidate for listing under the Endangered Species Act. In the Sonoran Desert approximately two thirds of the range of the Desert Tortoise is in Mexico but virtually no research had been conducted there. Therefore, to truly understand the conservation status of tortoises in the Sonoran Desert we needed to go south of the border. 

Driven by a dearth of knowledge on the status of desert tortoises south of the U.S. border, a team of Mexican, Canadian, and American collaborators began to study them in Mexico in 2005. Like all great adventures, strong and determined personalities drive ours. Mercy Vaughn and Alice Karl, two indefatigable tortoise experts, fuel the project. This cooperative effort focuses on crucial aspects of Desert Tortoise health, genetics, general biology, and ecology, and has included radio-telemetry, focal and behavior studies, morphometrics, and blood sample collection. For my part, I am investigating the genetic diversity of tortoises in Mexico and their relationship to U.S. populations.

The biggest resource was, and still is, volunteers. Collecting trips were (and still are) the two-week vacation you look forward to the rest of the year. Since the project’s inception in 2005, we have had over 75 volunteers join us in the field in Mexico. They bring their vehicles, their gear, and their blood, sweat, and at times tears. In return they gain a new appreciation for an animal we all thought we knew but is really still a mystery.

Getting to the tropical deciduous forests of southern Sonora does not necessarily require a trip to the airport. In fact, it is easiest done by car (at least if you are already starting in the southwestern U.S. (California, Arizona, and Nevada) where most Desert Tortoise biologists are located. From Tucson, it is only an 8 hour and 40 minute drive, according to Google Maps. However, I’ve never made it quite that quickly. Less than 280 miles of travel is considered the “Hassle Free Zone,” a poorly worded campaign by the Sonora Office of Tourism to encourage U.S. travelers to feel comfortable driving to tourist destinations like Puerto Peñasco or San Carlos. For us, it implies that the trip is a hassle. 

Travel along the major thoroughfares in Mexico is actually quite easy as long as the highway is open. But such is not always true. On multiple occasions the Yaqui Indian tribe has staged demonstrations that blocked traffic along Federal Highway 15. This is generally due to civil protest between the tribe and the Government of Mexico over water rights. Sometimes it is possible to skirt around the protesters through agricultural lands, but other times it requires a long detour to the Federal Highway 16 instead. This has become such a regular detour that during our last visit in 2013 the local radio station regularly announced if cars and trucks were being let through at any given time, as if it were the regular morning traffic report.

The range of Agassiz's Desert Tortoise and Morafka's Desert Tortoise.

Other than being a bit unpredictable, the drive from Tucson to Alamos is an amazing journey that follows the story of the Sonoran Desert and the evolutionary history of the tortoise. The upland habitat surrounding Tucson is characterized by iconic saguaro cactus reaching for the sky and green-barked palo verde trees with little green specks that are barely recognizable as leaves. However, one of the most fascinating aspects about the Sonoran Desert is its relatively recent transformation into the assemblage of species we observe today as the Arizona Upland Sonoran Desertscrub. Just 12,000 years ago it was a very different ecological community. Prior to and during the Pleistocene (12,000 to 2.5 million years ago), the Sonoran Desert showed evidence of having a more “tropical” flora and fauna that extended throughout the region. It was not a desert at all; fossil evidence of a boa constrictor found at the mouth of the Colorado River dated to the early Pleistocene, and crocodiles and capybaras that once occurred in east-central Sonoran would certainly not find a home in today’s arid landscape. History explains the remnant populations of jaguars and ocelots here. Around 12,000 years ago, during the early Holocene, it was likely wetter and cooler than it is today, and the landscape was dominated by woodlands that were replaced by desertscrub as the region became warmer and drier. The Sonoran Desert is the youngest of the North American deserts but also has the greatest biodiversity.

Staring out the car window, the changing desert landscape might not be immediately evident, but relatively soon after crossing the border into Mexico the silhouettes of saguaro cactus on the horizon become mixed with the grander cardón cactus as well as organ pipe cactus. The weedy Jatropha (or limberbush) of the Tucson Mountains rise into trees and fill the air with their fragrant aroma. It is still the Sonoran Desert, but it is recognizably different just a few hours to the south. One of the goals of our project was to assess the connectivity of Arizona and Mexican populations of tortoises by measuring how much genetic exchange occurs between them. This is called gene flow. Tortoises may move slowly but radio-telemetry studies suggest they can be motivated to move long distances. A tortoise from Saguaro National Park on the outskirts of Tucson was tracked making an extraordinary 20-mile journey over the course of two seasons from one mountain range to another. During her adventure she encountered railroad tracks and roads and eventually researchers physically moved her across the interstate when she became stuck between the frontage road and the constant flow of traffic. She was named Thelma after the movie character because she was an adult female who just picked up one day and left. Later, from the same park another adult female made a similar trek away from her home but in the opposite direction. She was named Louise.

It was not until 2011, six years after the Mexican Tortoise Project began, that we got to sample areas in Mexico just south of the border. We already had a collection of samples approaching the border on the U.S. side, but concerns about safety delayed our sampling in Mexico. We ended up sampling several sites in the mountains surrounding Ciudad Caborca. If you Google Image “Caborca, Mexico” pictures of burned human carcasses with their heads, arms, and legs removed quickly fill the screen. The drug violence in Mexico is real; however, the risk is easily reduced if you are a smart traveler. The problem is not that tourists (or biologists) are particularly targets in the cartel violence near the border cities of northern Mexico, rather a team of biologists working in Mexico tends to travel in a large caravan of 4x4 trucks and SUVs, which is also how cartels travel; thus we might call unwanted attention to ourselves. To reduce the risk of problems we tried to travel only during the day, checked in with local authorities, and worked on private “ranchos” where our presence was known, access to the areas was limited, and we were often accompanied by ranch hands (vaqueros) who know the land and most importantly know where to find tortoises. All this preparation was only possible because our Mexican collaborators not only helped facilitate our permits and land access, but also coordinated our efforts to make the trips safe and productive.

When friends ask me if it is dangerous to work in Mexico I have to say yes, but not at all for the reasons they assume. When conducting fieldwork, I am much more fearful of the heat and the bees than the drug cartels. In the dry heat of the desert, Africanized bees are drawn to even the smallest drop of water. They surround the mouthpiece of your CamelBak and flock to the sweat covering your back. Open the cooler in the back of your vehicle and it becomes a beacon for bees to gather. It is inevitable that some of the crew will get stung on any outing, but the greater danger of disturbing a hive is always looming as you peer into the rock crevices looking for tortoises. The constant presence of killer bees is coupled with the greater threat of the overbearing heat. It was not uncommon to measure ground temperatures greater than 150 degrees Fahrenheit during our afternoon siesta (we look for tortoises early in the morning and late in the afternoon). Although our crew consisted of extremely field-worthy biologists — many who spent the months prior to the Mexico field trips working full time in the heat of the Mojave Desert — the heat can be overwhelming for even the most experienced biologist in Mexico, especially in August and September with the added humidity of the monsoon season.

Signs of heat exhaustion include headaches, dizziness, cramps, and even vomiting. We had several incidents where people could not make it past the morning survey before hitting these incapacitating symptoms. The worst was when one of our crew members experienced heat stroke; I could see she was disoriented upon returning to the vehicles, and as I tried to engage her in conversation she fainted. I caught her in my arms as she went into seizures. We were hours from a hospital so I immediately put ice on the back of her neck to cool the blood flowing to her already overheated brain and we got her into an air-conditioned vehicle. Once she gained consciousness, we had her eat salt along with drinking small amounts of water. This time we were lucky and our overheated crew member fully recovered, but, needless to say, their trip ended early. This exemplifies how dedicated our volunteers are to work in these conditions. Sound like a fun way to spend your summer vacation? 

The Mexicans we work with carry table salt and limes with them in the field, along with their water. When they stop for a rest break, they lick some salt, squeeze key lime into their mouths, and take a sip of water. It is like a tequila shot but without the booze. Although we tend to think of the biggest factor in experiencing heat exhaustion as being a lack of water, it doesn’t matter how much water you have if your water is warm and your body (and brain) reaches more than 104 degrees Fahrenheit and can’t cool down. In addition, you lose a tremendous amount of water and salt through your sweat. Even if you are “hydrated,” your salt balance is critical for normal muscular and neurological functions. This is why athletes drink Gatorade — to maintain these vital electrolytes. The limón y sal taken by our Mexican colleagues achieves this same objective — and whatever is not used in the field can be passed around with the tequila later that night. 

Comcáac people
Although the tortoise may be a threatened species, it is nowhere near the level of endangerment of the culture and language of the Comcáac people.

In the lab I have these hard-earned tortoise genetic samples collected near the border, and I have assessed their relationship to samples in Arizona. Over the evolutionary history of a Desert Tortoise, there was no international border. I have found a continuum of genetic relatedness between desert tortoises as far north as Kingman, Arizona, (Hualapai foothills) to Hermosillo, Mexico (a span of more than 500 miles). This is an unusually large distribution for any species with a pending Endangered Species Act conservation status. Of course tortoises do not occupy this entire space but are instead distributed across the landscape in small patches of habitat in the foothills and uplands described earlier. Each of these small populations is seemingly isolated from other populations except for the long-distance movements of a few individuals like Thelma and Louise. Successful attempts to move from one population to another may be few and far between, but in perspective of a tortoise lifespan, only one such movement that results in the transfer of genes from one population to another per generation (estimated as every 25 years for a Desert Tortoise) is enough to maintain genetic connectivity among populations. This may be particularly important for the maintenance of small populations that have declined as a result of drought or stochastic processes. As tortoises are long-lived and slow to mature, the recovery of a declining population may actually be dependent on immigration from neighboring populations, as the time to recover to viable population levels naturally may take longer than the reproductive rate for a small population can accommodate.

Until now tortoises have been moving across the Sonoran Desert landscape like this for thousands of years. The recent proliferation of man-made barriers like highways, canals, agricultural lands, urban areas, and now border walls have occurred within only a couple of Desert Tortoise generations. Tortoises are just one of the species across the Sonoran Desert that are greatly affected by habitat fragmentation, along with bighorn sheep, cactus ferruginous pygmy owls, and pronghorn. But tortoises are different in that the time it will take to see the effects are much longer because they are so long-lived. On one hand, their natural population structure does not yet exhibit obvious detrimental effects. On the other hand, once it does exhibit problems recovery will proceed very, very slowly — if at all. Such issues are relevant to the short-term politics fueling the physical construction of the border wall and the long-term effects it will have on our natural heritage.

Coming back to our driving excursion into Mexico, if we take a side trip to the west, to coastal Sonora, we find an interesting piece of the puzzle. Here we sampled tortoises on the Seri lands, the home of the Comcáac people, an indigenous group of historically seminomadic hunter-gatherers who are at home both in the desert and on the sea. In Gary Nabhan’s wonderful ethno-herpetological account of the Comcáac, Singing the Turtles to Sea, he describes a people intimately connected to the land and more specifically, the reptilian inhabitants they share it with. The Comcáac have been implicated in introducing chuckwalla and other animals to many of the islands in the Sea of Cortez, presumably to maintain sources of food if their panga (fishing boat) becomes stranded during inclement weather. The Comcáac culture also maintains an intimate relationship with Desert Tortoises, and they inevitably moved Desert Tortoises around their territories as well. A variety of taboos likely helped regulate their over-exploitation, however.

In 2010, we had the opportunity to assess the tortoise population in the Seri territory of western Sonora, Mexico, and we visited several sites on the mainland and two sites on Isla Tiburón with our local Seri host, Ernesto Molina. This area consists of a magnificent representation of Sonoran desertscrub that includes a beautiful mosaic of saguaro, organ pipe, senita, and cardón columnar cactus. Part of the impetus of surveying here was to revisit several areas where tortoise surveys were conducted in 2001 and 2002 in response to reports by the Seri Indians of high Desert Tortoise mortality. At that time, the survey team led by Mercy Vaughn, Felipe Rodriguez Garcia, Gary Nabhan, and Patricia West found very high rates of recent mortality at all sites that raised concerns about the status of the Mexican populations of the species. The effort in 2010 was an opportunity to revisit these sites, and, unlike the first survey effort, we had permits to collect blood samples for further disease testing. Although we did find tortoise carcasses, the good news was we did not observe nearly so many as were found in 2001 to 2002 or other signs of recent mortality. In addition, we found very limited incidence of upper respiratory tract disease that occurs, sometimes commonly, in populations in the United States.

I did find, however, some intriguing genetic differences that suggest an increased amount of genetic diversity in this coastal region. Unique lineages found on Isla Tiburón (that potentially predate the arrival of humans to the New World) suggest that tortoises naturally immigrated here during glacial periods when lowered sea levels allowed passage. The Seri may have moved animals as well, but these likely mirror natural patterns of movement where the tortoises are closely related to each other in the region anyway. The continued presence of tortoises in these coastal areas, despite most of Sonora experiencing oscillating vegetation (tropical, mesic, desert) during glacial minima and maxima, suggest it may have been an oasis or refugia for desert-adapted plants and animals. Further up the coast, in the Gran Desierto and isolated Pinacate volcanic range, it appears that desert conditions may have been maintained while the rest of Sonora transitioned from tropical forests to woodlands and back again before warming and drying finally sculpted the current range of the Sonoran Desert. 

The genetic connectivity of tortoise populations spans from Kingman, Arizona, through Hermosillo, Mexico, and is suggestive of a recent population expansion across the emerging Sonoran Desert from these coastal refugia over the last 100,000 years. This expansion and contraction likely occurred many times over the evolutionary history of this species, and we are only witnessing the most recent event. The genetic diversity captured in these southern coastal ranges suggests that this may be the source for the entire Sonoran Desert Tortoise population and likely other species as well.

Although the tortoise may be a threatened species, it is nowhere near the level of endangerment of the culture and language of the Comcáac people. For thousands of years, the Comcáac and their ancestors acted as stewards of the land and its inhabitants. Now less than 1,000 native Seri speakers remain, and their vast knowledge of Desert Tortoises and the desert ecosystem may soon be lost. Now consider that Western scientists like myself are only beginning to understand this unique region.

Back on Interstate 15 and heading south, a big change happens after leaving Hermosillo, about 250 miles south of Tucson. Here we start to see thornscrub. While thornscrub forests may appear like a landscape in transition, it truly is a unique vegetative community unto itself. I’d be happy to describe it to you if any of the experts on Sinaloan Thornscrub could describe it to me first. It looks like someone let only the most aggressive and most difficult-to-pull weeds overtake the garden, and then the weeds grew to massive proportions. Biologists can describe it as consisting of mainly short trees, shrubs, and cacti with many species being “thorny.” It is nearly frost-free and thus temperature limits its latitudinal and elevational distribution; and it has alternating wet and dry seasons. However, my favorite description is from Vaughn who, instead of the “wait-a-minute” bush acacias you encounter in a typical desert, describes walking through thornscrub as “wait-a-couple-of-minutes” bush. 

Here, something magical happens. We do not cross a major river or go over a mountain, we just look out the window and the vegetation changes. And so do the tortoises. The tortoises occupying these thornscrub forests and continuing into the tropical deciduous forests where we started our story are very different from the tortoises we left behind in the desertscrub — about 5 to 6 million years different. This is as divergent genetically as what is observed between Desert Tortoises in the Mojave Desert of California and those in Arizona that are recognized as separate species. However, the Mojave/Sonoran split is somewhat expected because these species are separated by the Colorado River, which prior to that was an inland sea called the Bouse embayment. This is a classic example of what biologists call allopatric speciation. What we are seeing in the transition from desertscrub habitat to thornscrub habitat to a casual passerby is nothing more than a change of scenery. So what factors are driving this tremendous amount of differentiation?

Answering this and related questions is enough to earn someone a Ph.D., and I decided that someone should be me. We have dubbed these southernmost Desert Tortoises “Sinaloan” Tortoises and our investigation so far has revealed behavioral and morphological differences as well as genetic distinction. Are we dealing with a distinct species, cryptic to science until now? Very likely. Most intriguing, however, are a few areas of overlap where we find both kinds of tortoises, Sonoran and Sinaloan, and we observe limited mixing (or hybridization) between the types. My role as an evolutionary biologist is to tease apart the evolutionary history of this animal, investigate its origins, and help to define its evolutionary potential. Has the Sinaloan type just been isolated in its own tropical refugium during long periods of climate change and is only now reuniting with its desert cousin, yet to determine their compatibility after their long absence? Or, have these two lineages been in contact time and time again and is natural selection against their intermixing slowing forcing them apart? Wolves and coyotes, which diverged from a common ancestor 3 to 4 million years ago, are capable of interbreeding and sometimes do; it may even be beneficial to introduce some new genetic stock into the population once in awhile. However, these canids obviously still maintain distinct lineages, fill different niches, and retain unique adaptations. Is this going on between Sonoran and Sinaloan Desert Tortoises? One thing we do know from Darwin is that speciation generally takes a long, long time, and in the present we are only seeing a snapshot of that process.

Even to a novice, the Sinaloan Tortoise immediately appears different. They can be strikingly yellow or orange, and their carapace is flatter and squarer than what is seen in typical Sonoran individuals. The scales on their forelimbs frequently protrude and can be very spiky. Is this just natural variation or are these adaptations selected for this more tropical environment? Phil Rosen, one of the project collaborators, hypothesizes this armor may be a defense against predators living in these tropical regions. They do share their home with jaguars, after all. We still have a lot of questions about what Sinaloan Tortoises eat, how social they are, what their home range is, their reproductive cycle, and even how far south their distribution extends. Our southernmost sample was collected south of the Rio Fuerte in Sinaloa. To hold one is to know you are holding something special.

Defining these taxonomic distinctions is necessary for effective conservation of tortoises inside and 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. There is growing interest in understanding the natural history of Desert Tortoises in Mexico, so that we can apply long-term approaches to management and conservation. Mexico is faced with a privileged situation in that we can make efforts to protect tortoises before they are at the edge of extinction. Unfortunately, many species do not receive this kind of valuable research attention until after their populations have declined.  

As a result of this ongoing research, I was asked to co-author an article in Especies magazine, highlighting conservation concerns of tortoises specifically in Mexico, for a Mexican audience. Especies is a popular newsstand publication with glossy photos and relatable text. I was thrilled to reach this audience and although I have published many peer-reviewed scientific articles, I believe my biggest impact as a conservation biologist can come from this type of media. The article was published in the summer of 2012, and a tortoise even graced the front cover. I happened to be traveling with my family in Mexico when the issue came out, so I tried to obtain a copy. In the small tourist town where we were staying I was approached by a street vendor trying to sell ponchos and blankets, and I told him in my broken Spanish that what I really wanted was “Esto, por favor,” and I showed him a picture of the cover. He looked at it and said, “Un minuto,” and he took the image to some other people in the back of his store. Although what I wanted was the magazine, when he returned he stated, “You want a Tortuga? I can get you a Tortuga out in the desert.” 

So, unfortunately it is easier to buy a protected species of tortoise in Mexico than a conservation magazine about tortoises. This is too frequently the punchline of conservation. What I do know from my research over the last eight years is that Mexico has something special, something worth saving. Perhaps we will name a new species from our work or just define the Mexican populations as unique. Regardless, tortoises are a treasure of the Southwest and in their history is the history of the Sonoran Desert itself. Winston Churchill said, “The farther backward you can look, the farther forward you are likely to see.” And although he was certainly not referring to tortoises, my investigation into these almost ageless creatures has given me much insight into what role we all play in preserving the future of the desert Southwest.


This 184-page issue, with over 150 color photos, covers a variety of stories ranging from editorials chronicling the death of a young conservationist to long-standing conservation programs in the field. The issue is complete with photo essays and interviews with prominent figures in our community, containing a total of 19 pieces that focus on a number of Threatened and Critically Endangered species from around the globe. From the deserts of Nevada where a rogue rancher battles the Federal Government for grazing rights on Desert Tortoise habitat, to the runways of John F. Kennedy Airport where Diamondback Terrapins search for suitable nesting sites — this issue covers the most pressing conservation topics facing tortoises and turtles today.

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