Desert Tortoise (Gopherus agassizii)

Main sources: Stebbins, Robert, 1985, A Field Guide to Western Reptiles and Amphibians, Houghton Mifflin; Behler, John & Wayne King, 1979, The Audubon Society Field Guide to North American Reptiles and Amphibians, NY: Alfred Knopf. Additional reference: Van Devender, Thomas, ed., 2002, The Sonoran Desert Tortoise. Tucson: University of Arizona Press; Nagy, Kenneth, "Dry, Dry Again", Natural History 12/2002-1/2003, pp. 50-55.

[Numerous images of our tortoises follow the text below, which discusses Desert Tortoise adaptations at some length -- so if you want to skip the text, just scroll down past it.]

Note: The Mohave population of Desert Tortoises has been listed as an Endangered Species by the U.S. Fish and Wildlife Service (USFWS). Nagy, cited above, states that more desert tortoises are now living as captives in backyards of Los Angeles County than in the entire Mojave Desert -- p. 55 -- and the non-human predators which multiply near human concentrations -- feral dogs, coyotes, ravens, etc. -- are taking increasing tolls. The Sonoran Desert Tortoise is not listed as Endangered, but the USFWS informally consider it a species of concern. (J.M. Howland and JC Rorabaugh, p. 348 in Van Devender, ed., listed above.) See also these links to The Desert Tortoise Preserve Committee and The Desert Tortoise Council for valuable additional information, including bibliographic sources.

Land-dwelling "Chelonians" (the Order of double-shell-encased reptiles) possessing domed shells and elephantlike limbs are found on all continents except Australia and Antarctica. They range into some of the most arid parts of the world, and most are herbivorous. Our Desert Tortoise -- remains of which have been found in archaeological sites from southeastern California and southern Utah into Northern Mexico since the late Pleistocene -- has a strongly domed shell and shows prominent growth lines on all of its shields. Its forelegs are defensively scaled, long, flattened, and muscular -- adapted for burrowing; hind legs are round, stumpy, and elephantine.

The Desert Tortoise may be unique in its ability to allow its internal body chemistry to vary with changing environment. They begin each year in a state of dormancy, in slanting underground burrows three to six feet deep, which they have constructed (often in the vicinity of Creosotebush, at any rate where soil has been softened somewhat). These burrows provide relatively humid, cool but not freezing quarters during winter, when the tortoise burns very little energy and burrow humidity keeps down dehydration. (Recent studies show they lose hardly any weight during the winter hibernation.) A tortoise may use the same burrow for several years.

Tortoises emerge from hibernation in the spring, to find highly variable green food (herbs, grasses, etc.) in our Sonoran Desert climate. If the desert is green, they devour these plants, but if it is dry they eat whatever dry grasses are available. Rather than voiding excess water the plants have provided, the tortoises store their excess water, and it then can be reabsorbed through the bladder wall during dry periods. By the time of full summer drought, they retreat to their burrows, to endure "internal concentrations of chemicals well beyond those that would kill any mammal or bird and nearly all other reptiles" (Nagy, cited above, p.53). According to Nagy, where desert camels can tolerate an increase of 40% in their "osmotic blood concentrations" (containing toxic wastes), desert tortoises can tolerate as much as 200% (ibid.). Should summer and fall remain dry, they may stay in their burrows and go directly into hibernation in October, though they may emerge when they sense approaching summer thunderstorms and even scrape out earth depressions to catch whatever rainfall comes and drink it immediately. If they do get a drink before winter hibernation, they will then discharge their old, highly toxic urine (held perhaps since the preceding winter) and drink some more. Multi-year droughts however may overwhelm this adaptation, killing them. And a well-known adage tells people not to pick up and overturn desert tortoises, since by reflex they will void their stored water -- which may deter a predator but also endanger their later survival.

As you would expect, in our area Desert Tortoises are most active during the Monsoons and just after, though males court females during the spring as well as the summer. The females do not skip reproduction during dry years (as do some desert birds and reptiles), but lay their eggs in their burrows from May-July, which hatch out in August-October. During this time, drivers of vehicles on our uplands must be most alert for the presence along the wheel tracks of some very small moving "pebbles" as well as their more football-sized parents, all of whom tend to use these dirt tracks as pathways: see for example this image below:

The white strip at the base of this image, taken in August 1998 on the Pool Wash Ridge Road, is the front portion of the hood of the vehicle (which did stop in time). This tortoise was not as small as it looks -- it was probably 8-10 years old, and in width almost the span of a man's hand. Younger ones would be much more difficult to see especially in the context of many similar-sized pebbles.

Below is a youngster who was moving ambitiously across gravel-and-sand terrain in September of 2003. The carapace is no more than 2 1/2 inches long by our casual method of measurement, which is merely sheer distance front to back as viewed from above. Length along the curvature would be a good bit longer.

Click on the right-hand image for a closeup.

The closeup shows us a series of 5 or 6 rings -- indicating perhaps 4 years of growth? (Rings form only in years of good forage.) The animals below are all a great deal larger than this youngster.

Below Left: near the Windmill, September 2000 (this carapace was about 7 inches long -- again, by our overhead method of measurement); right: Pool Wash ridge road, August 2000; far below, right: July 2001

Sexual differences in tortoises include the length and angle of the gular horn (the forward portion of the lower shell), which is longer and upturned in males; the shape of the rear lower shell, which is concave in males, flat in females; and the length of the tail, which is longer in males. Since we try to keep our distance from our tortoises, we haven't regularly sexed them, but the Windmill tortoise shown above is probably female (small gular horn, below left), while both the Survivor Tortoise (long gular horn, below center -- this tortoise is further discussed below); and the logo tortoise shown at the page top are probably male (concave rear lower shell, long tail -- here, below right):

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Besides the vagaries of food supply, Desert Tortoises face many other hazards and difficulties while working to survive in the Saguaro-Juniper zone. Evidence of some of these appeared recently when we encountered one elder (below) on a primitive road above Hot Springs Canyon Wash on August 28, 2002, the day before the first substantial Monsoon rain of the season hit our lands. Interestingly, Desert Tortoises have been observed to anticipate coming rains in various parts of our North American Deserts, emerging out of their burrows a day or so before rain begins to fall on their terrain. Notice its dirty carapace (in contrast to others photographed here), which suggests the animal may have emerged from its den only a short time before. Anyway, the photos below at center and right show an interesting feature: an open crack on the right edge of the carapace just in front of the right rear leg.

Below left: a closeup of the head profile of this animal shows some distinctive features which should assist future identification. Below right: But who will need them? Viewing the same individual from the left-hand side, we see a much larger crack and a second crack also in front of the rear leg. This tortoise has suffered a partial squashing of the entire rear portion of its carapace, but has survived nevertheless.

While one might suggest a number of possible scenarios for this traumatic event (several people suggested a vehicle running over it, but this seems unlikely), our shared experience in Saguaro Juniper lands suggests that the injury very likely occurred from a fall -- down one of the numerous cliffsides, or deep, cleft-like ravines that are found in our area, and over the edges of which small creatures may well be borne in a flash-flood. Some of these precipices are close to a hundred feet of sheer drop. While exploring Saguaro Juniper lands around the edge of Hot Springs Canyon Wash, we have occasionally found skeletal remains of Desert Tortoises at or near the base of such ravines. One, which we call the Cleft, contained such a skeleton right at the base of its sheer waterfall. In the photos below, on the left a smaller cleft can be seen clearly, appearing as merely a narrow crack in the San Manuel Formation. But this crack runs well back into the Formation, and one can walk into it at the base for a considerable distance. In the center photograph, visitors to the Cleft sit protected from the bright springtime sun; the half-cylinder rock wall at the back is palely illuminated by much more indirect light bouncing down the water-eroded vertical chimney all the way from the wash top, perhaps 80 feet above. On the right, another visitor is dwarfed within a similar waterfall chimney carved into San Manuel Formation near Rabbit Ears Saguaro Hill. (We call this one a "Meditation Area" though no one should meditate there during one of our rainstorms!) (Click on the left and center images to enlarge them.)

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Some Desert Tortoise corpses have been found in washes just below clefts such as these, for example:

(Click on image to enlarge)

Another probable cause of such injuries is from rock-falls off cliffs or very steep slopes, especially after rains. Roadways along our washes often contain scatterings of large, freshly-eroded boulders following even fairly modest rain events. There is definitely some hazard for anyone walking at the base of such slopes during or shortly after rains. And of course any creatures walking our washes during flash floods are likely to be tumbled by rushing masses of rock. For example: On September 10, 2004, walking a short way above the Lower Hot Springs Canyon Windmill -- less than a week after the stream had run in flood all the way down to the San Pedro River -- we found this corpse of an elderly Desert Tortoise, below: (Click on each image to enlarge it.)

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This Tortoise must have been swept up by this last flood at a point well upstream, battered thoroughly by rocks and drowned, and eventually deposited at this spot against a Hymenoclea plant. You can tell it is quite old from the shell rings on the enlarged image at far right.

So Desert Tortoises may catch trouble from many directions -- from all sides, and above as well as below.

Two days after encounter with the survivor tortoise above, another was sighted walking along a different roadway above Hot Springs Canyon. This one was smaller than Survivor Tortoise, but appears to be fairly old:

Our banner-photo Desert Tortoise (displayed at the top of our page, above) was about on August 25, rapidly moving from grass clump to grass clump, enjoying the newly emerged greenery. Our camera was a vintage camcorder, so we didn't get very clear photos, but we think it was a male (also shown here below). They appear to waste little time once fresh grasses are out.

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