Here is a fine, very well-fed individual of the species Spermophilus variegatus, also known as the rock squirrel. This one happened to be working the crowd outside the El Tovar on the south rim of the Grand Canyon, but the species ranges throughout the Southwestern states and Mexico, edging just barely into that part of the state of California that fronts the Colorado River, for instance my neighborhood here.
The rock squirrel is one of 18 or so Spermophilus species in the western part of North America. There are currently 42 species worldwide. Spermophilus itself — the name means “seed lover” — is one of three genera of true ground squirrels, along with Ammospermophilus, the antelope squirrels of the desert Southwest, and Cynomys, popularly known as prairie dogs. Close relatives of the three genera include marmots and chipmunks. Taxonomists group ground squirrels, chipmunks and marmots into a “tribe” within the squirrel family called Marmotini, which sounds like something you’d find on the menu in a spaghetti joint in Bozeman, or maybe a mixture of gin and wheatgrass juice.
Anyway, there are Spermophilus of various species living everywhere in Western North America from the beaches of the Arctic Ocean to southern Mexico. Their appearances vary widely, with some looking very much like your basic tree squirrels, some — the golden-mantled ground squirrel, S. lateralis, for instance — looking like glorified chipmunks, the flickertails (S. richardsonii) essentially trimmed-down prairie dogs, and the thirteen-lined and Mexican ground squirrels (S. tridecemlineatus and S. mexicanus, and I bet you can guess which is which) looking like nothing I’ve ever seen.
I found myself facing down a Spemophilus day before yesterday up at Cima Dome. I didn’t get a chance to ask it to display its tail. A tail with short hair would have made it a round-tailed ground squirrel, S. tereticaudus. Rock squirrels have bushy tails like tree squirrels. On reflection, though, I’ve decided it was probably a rock squirrel. Round-tails are low-elevation denizens, from what I can tell, preferring warmer, sandy wastes like the one I nearly got the Jeep stuck in about two hours before I saw the squirrel. The squirrel and I were up a bit, at around 5,500 feet. Also — another clue — it was hanging out in a big pile of rocks.
Still, I was a little hesitant to make the call because my squirrel field guide, Tamara Hartson’s Squirrels of the West, has a range map for the rock squirrel that implied it was unlikely I’d find any Spermophilus variegatus near Cima Dome. I decided, though, that the maps in Hartson were — how do I put this kindly? — drawn without reliance on primary sources. The range map provided for the one other Spermophilus living in the Mojave Desert, Spermophilus mohavensis, was, well, inaccurate. It shows the Mojave ground squirrel as ranging south and east of Joshua Tree National Park, almost to the Colorado River at Blythe. The actual southeastern limit of the Mojave ground squirrel is pretty much the Mojave River. That’s about a 130-mile error, about a two-fold increase in the actual range of the species.
The southeastern limit of the Mohave ground squirrel’s range is, and not by coincidence, the northwestern limit of its closest relative, the round-tailed ground squirrel. To a first approximation, each species lives on its own side of the Mojave River and nobody fishes in the middle. This becomes a little confusing when you remember that while there is water flowing in the Mojave River, it mainly does that flowing beneath several feet of gravel and sand. There’s nothing in that riverbed, 364.5 days out of your typical 365, that would provide the kind of barrier to migration — and thus gene exchange — that usually causes one species to diverge into two sibling species. A squirrel spotting a likely mate on the other side wouldn’t even have to get its feet wet to exchange some genes.
And in fact, there is a little bit of interbreeding going on along the river, which is a pretty romantic spot when you get right down to it, and squirrelologists have found a few hybrids between the two species here and there, mainly in disturbed areas where the typical behavior of each species may have been disrupted. The breakdown of the natural order of things has likely resulted in interspecies mating, and yet the Mormon church is strangely silent on the matter.
If there’s no barrier to the populations of squirrels meeting, and they can still interbreed, what was it that split the original species in two in the first place?
The Mojave River is still a possibility despite its current lack of, well, current. Before 6,000 years ago there was water flowing in the river, lots of it, running down off the slopes of the Pluvial-era San Bernardino and San Gabriel mountains, filling what are now dry washes and alkaline playas with water too broad for a squirrel to ford. There was a vast network of lakes and streams in the Mojave then, with water off the east side of the Sierra Nevada joining in from the northwest, filling Owens and Searles lakes and spilling over into the Panamint Valley and Lake Manly on the floor of Death Valley. For its part, the Mojave River filled immense lakes where Harper Lake is now, near Hinkley, and Coyote Lake, north of Yermo. Its main flow continued past them and cut a gorge now called Afton Canyon, filled up Soda and Silver Lakes near Baker, and ended up in Lake Manly as well.
It may be that there were other factors beside swollen rivers keeping the squirrels apart. The heart of the Mohave ground squirrel’s habitat in the northwestern Mojave, smack dab in the rainshadow of the Sierra, is thought to have remained somewhat arid during the Pluvial, and may have been a refugium of sorts where desert-adapted critters were able to escape competition from less-droughty species taking advantage of the wetter Mojave elsewhere. But when you map out the boundary between the ranges of the Mohave and round-tailed ground squirrels, it’s never farther than about 30 kilometers from the Pluvial Mojave River System waterline. Things dried up 6,000 years ago, so that’s a migration rate of about five meters a year, not unlikely given ground squirrel behavior as we see it today.
I love this kind of story so much. Who but a complete geek would take note of seemingly arbitrary boundaries between the ranges of two very similar squirrel species? The Mohave is a little more pink-gray than the round-tailed, and the underside of its tail is white where the round-tailed’s is solid cinnamon-colored; the first likes gravel and the second sand. These are differences all but invisible to the vast majority of people, especially in a genus as diversely-appearanced as Spermophilus.
But when you start paying attention to why the ranges of animals and plants and other living things are the way they are, when you start paying attention to how the arrangement of life on today’s Earth came to be the way it is, epics unfurl themselves before you. Caruthers Canyon, a valley near here full of incongruous oaks and manzanitas and other usually-coastal chaparral plants, reveals itself to be a redoubt, a place where hundreds of generations of plants have held on as mountain ranges rose, blocked the rain, made two hundred miles of desert between them and their nearest kin. Uninspiring, ratty rings of creosote bush become impossibly ancient matriarchs, already 5,000 years old or more when the First Dynasty struggled to power in Egypt.
And a vague line between the ranges of two drab species of burrowing rodent becomes mythic, Noachic, the heavens opening up, a millennia-long flood separating brother from brother, sister from sister, two histories diverging from a common root, one nation cleft into two and the two meeting again as strangers.
The desert is woven of these stories, and I could spent the rest of my life reading them and be content.