“We called him Tortoise because he taught us.” — Lewis Carroll

By DR. CAMERON BARROWS

When we think of iconic desert animals, species that epitomize not just survival, but the ability to thrive in a desert environment, chuckwallas, desert iguanas, fringe-toed lizards, and tortoises rise to the top of my list. One criterion would be that they are restricted to the boundaries of the deserts in which they live. Certainly chuckwallas, desert iguanas, and fringe-toed lizards, including all their close relatives are only found in deserts. There are five recognized species of chuckwallas, two species of desert iguanas, and at least seven, possibly eight, species of fringe toed lizards, and all are only found in deserts. What about desert spiny or granite spiny lizards? Both do well in deserts, but they do better in palm oases, or in the case of granite spiny lizards, they also thrive in montane chaparral and high elevation conifer forests, as long as there are big granite boulders. At first glance tortoises seem to fit that criterion as well, but with a deeper look, it could be that our Mojave Desert tortoise could be an outlier when it comes to desert living.

There are six extant species of North American tortoises: Mojave Desert tortoises, Sonoran Desert (Morafka’s) tortoises, Goode’s thornscrub tortoises, Bolson tortoises, Texas (Berlander’s) tortoises, and gopher tortoises. Together they comprise the genus Gopherus, or Gopher tortoises. Of these six species, the one that is referred to with the common name gopher tortoise (G. polyphemus) is restricted to Florida and adjoining states, so not a desert. Goode’s thornscrub tortoise (G. evgoodei) is the most recently described species (2016) and is restricted to subtropical deciduous forests of northwestern Mexico, so again, not a desert. Texas tortoises (G. berlanderi) live in southern Texas’ sparse shrublands and grasslands, and into northeastern Mexico, some of which is Chihuahuan Desert, but intergrades with subtropical habitats. So not a strictly desert species. The other three North American tortoises, Mojave Desert (G. agassizii), Sonoran Desert (G. morafkai) and Bolson Tortoise (G. flavomarginatus) are restricted to the Mojave-northern Colorado Desert, Sonoran Desert, and northern Chihuahuan Deserts respectively. As a group, these species prefer warm, seasonally dry, mostly frost-free climates (the Mojave Desert tortoise does occupy habitats where the winters are cold with frequent frosts).

More distantly related species of tortoises occur in warm climates around the world, including Africa, southern Europe adjoining the Mediterranean Sea, and southern Asia. There are those that occupy portions of South America, including Brazil’s rain forests, red-footed tortoises, (Chelonoidis carbonarius) and Brazilian giant tortoises (C. denticulatus). The Brazilian giants also occur in Columbia and Ecuador, making them, or their relatives, likely candidates for being the tortoises that were washed out to sea in a flood, and by chance survived days or weeks floating at sea, then making landfall on the Galapagos islands, populating those islands with what would become true giants. Gigantism is not exceptional among Galapagos tortoises. The Aldabra Island tortoise found on the Seychelles Islands in the Indian Ocean vies with the Galapagos Island tortoise as the current largest species. Even here in North America there were giant tortoises from the Oligocene through to the Pleistocene, nearly 34 million years ago up until just maybe 20,000 years ago; these tortoises were up to a meter or more in length. One of these giants was excavated from the badland formations of Anza Borrego Desert State Park. These giants were likely ancestors of the six North American tortoises alive today. Still, no desert habitat restrictions emerge when we look at tortoises worldwide or back in time.

Rather, we might consider our Mojave Desert tortoise a “desert avoider,” spending most of their lives underground in burrows they excavate, where both temperature and humidity are moderated. They only emerge when the temperature is just right and when there is a reason. Those reasons are typically limited to finding a mate or eating or drinking. Even then such movements outside their burrows are short-lived and rare, making any sighting of a wild tortoise a very special event. Tortoises can store water in their urinary bladder for up to a year.  So, unless it is a good wet spring with sufficient flowers filled with both nutrients and water, the best strategy for a tortoise is to sit tight in their relatively cool burrows with close to 100% humidity conserving both energy and water. When there is a good spring, or a good summer monsoon leaving pools of water, then tortoises emerge, void their increasingly toxic bladder contents, and then replenish their water and energy reserves, before going back into their burrow.

The adult female tortoises can store sperm from a single liaison with a male for two to four years. Then, when there is a wet spring and plentiful flowers, the female tortoises can produce one or more clutches of viable eggs, whether they encountered a suitable male that spring or not. When it comes to mating, female tortoises are picky. In an unpublished study, one researcher was observing male-female Mojave Desert tortoise interactions during the breeding season. In this population, all tortoises had been genetically analyzed so that the scientists knew the levels of relatedness between individuals. What she observed was that when adult females encountered males that were close relatives (fathers, uncles, sons, or brothers), the female refused to mate. Males were less inhibited, often persistent, but eventually would give up unless the female agreed. However, if the male was more distantly related, mating might occur. Moral correctness aside, what is interesting about these observations is that tortoises are hatched from eggs. The female lays her clutch of 4-6 eggs in the ground, covers them and then walks away. The baby tortoises emerge alone maybe 60 days later, never knowing who their mothers, fathers, uncles, or even siblings are.

Mating with close relatives increases the likelihood of “poor genes,” genes that do not support healthy development and reproduction, being expressed in subsequent generations. Yet, somehow, according to these unpublished observations, they know. Somehow, apparently by smell alone, the female tortoise can determine relatedness. I will add some corroboration. We were given two captive-bred, same clutch baby tortoises many years ago. As they grew it became apparent one was a male and the other female. When they matured, the male relentlessly tried to mate with the female, but she steadfastly refused. We gifted the male to someone else to give our female, “Shelly” some peace.

Mojave Desert tortoises are experts at “living in the slow lane”. They can live more than 60 years, perhaps up to a century. Breeding every year, or even eating and drinking in each season isn’t critical, if they can wait out the lean times in their burrow, avoiding the desiccating effects of living in a desert. The problem now is that due to modern climate change, desert droughts are longer, often lasting multiple consecutive years. Tortoises are very good at avoiding “desert conditions” by staying in their burrows, but there are limits. Eventually they must eat and drink. Eventually females must be able to consume sufficient nutrients to produce healthy eggs. Add habitat loss, ravens becoming very good at finding and eating baby tortoises, invasive inedible weeds crowding out the native wildflowers the tortoises need to survive, wildfires, diseases, and it is not surprising that the Mojave Desert tortoises are critically endangered. It’s death from a thousand cuts. The challenge is how to reverse this trajectory.

Nullius in verba – Go outside, tip your hat to a chuckwalla (and a cactus), and think like a mountain