Using modern desert animals to elucidate past desert evolution is confusing enough in scientific articles. In the desert, it can be bewildering. Coming suddenly face-to-face with a creosote bush–eating grasshopper one windy day, I had trouble seeing how “normal” evolution had produced it, whatever the continent. It was improbably stout for its meager food plant, colored a snazzy black and chartreuse better suited to a NASCAR rally than a dead-looking shrub in the middle of nowhere. It seemed more like something hatched out in a nuclear test site than a product of natural selection, old or new.
California desert frogs and toads can also bewilder. They may be less diverse than South American ones, but they can be pervasive at the right time of year. I’ve camped in gaunt canyons where red-spotted toads filled the night with ear-ringing song and tree frogs basked with surprising nonchalance in full sunlight. The frogs lounged within hopping distance of creek pools and shady crevices, but they stayed in the sun longer than I could. While my skin turned hot pink, theirs faded to cool grayish white. Their permeable skin, which absorbs water as well as evaporating it, is thought to insulate them. Still, I never saw any desert tree frogs soaking up water in a creek. They have a reputation for avoiding water except to breed or dodge predators.
Despite such complications, there is an evolutionary logic to the way anurans inhabit deserts. As amphibians, the “lowest” land vertebrates, still largely breeding in water, they seem marginal desert dwellers, and they are. Most California desert frogs and toads live near permanent or reliably intermittent water. Only spadefoot toads depend on puddles that occur briefly at unpredictable intervals, because they can live dormant in the ground for long periods, awaiting the next big rain. And some places are too dry even for spadefoots.
The other North American amphibian group, the salamanders, is even more marginal. Until 1970, they were unknown in California desert. Then a collector discovered two rare species congeneric to the common coastal slender salamander (Batrachoseps). They belong to a group that can breed out of water, but that hasn’t helped them much in desert. They live under rocks in a few isolated canyons where dense shrubs overgrow spring-fed stream courses. Since they also resemble Mexican and Central American genera, herpetologists see them as ancient relicts “occupying habitats associated with exposures of these ancient rocks for a long period, perhaps since early Tertiary times.” They seem likely relicts of Daniel Axelrod’s Madro-Tertiary woodlands.
California desert frogs and toads also seem likely relicts of a moister Madro-Tertiary world. As Axelrod observed, even the most desert-adapted, the spadefoot toads, live in moister places as well. As Frank Blair and his colleagues suggested, however, the reptiles—the “next step up” in vertebrate evolution—are harder to place as Madro-Tertiary relicts, not least in that so many kinds are thriving and abundant in today’s California desert.
Reptiles are popularly regarded as desert “naturals” because they are common in deserts and because, anyway, they belong in such godforsaken places along with scorpions, tarantulas, and other creepy-crawlies. From a progress-minded evolutionary viewpoint, however, there is no reason to associate reptiles especially with deserts. Darwin’s surprise at the abundance of tortoises and lizards on the dry Galapagos reflected this.
Reptiles are most abundant and diverse not in deserts but in tropical forests (little Costa Rica has 68 lizard species, from six-foot iguanas to cricket-size geckoes, and 127 snake species, from twenty-foot boa constrictors to worm-size blind snakes), and with good reason. Their respiration, circulation, and digestion are less efficient than birds’ or mammals’, so they compete better where food is abundant and digestible, the climate warm and moist. Although they do have nonpermeable skins and amniotic eggs, with embryos enclosed in a membrane that protects them from water loss, they remain more vulnerable to extremes than animals with livelier metabolisms. Deserts are among the planet’s most extreme environments, with sparse, coarse food and huge climatic fluctuations.
Then why do more kinds of reptiles live in California’s deserts than in its warm temperate coastal woodlands, where food and climate are more equable? Many more amphibians live in coastal woodland than in desert. I’ve found dozens of salamanders of several genera in a few miles’ walk there. If desert reptiles are relicts of the Madro-Tertiary flora and fauna as desert amphibians seem to be, shouldn’t reptiles be at least as diverse and abundant in California’s coastal woodlands?
Some reptiles are abundant in coastal woodland. Pacific pond turtles can throng waterways. Garter and gopher snakes are common sights, as are rattlesnakes. Ring-necked snakes, sharp-tailed snakes, king snakes, and rubber boas make regular appearances. Coastal woodland fence lizards, alligator lizards, and skinks are even commoner than snakes. Still, as I’ve said, reptiles seem peripheral in coastal woodland, central in desert.
The centrality of some reptiles in the scorching, barren desert makes a kind of sense. Desert tortoises carry insulated houses on their backs, store water in their bladders, and are strong burrowers. If rains fail they simply stay underground, and they live a long time. Their aplomb when encountered is impressive—usually just a perfunctory hiss and casual withdrawal of legs and head—and they were abundant before pet collectors and ORVs. The western Mojave around Red Rock Canyon had as many as two thousand per square mile, and they remain common on the nearby Desert Tortoise Preserve.
Snakes are elusive in desert. A typical encounter was when, after days at Joshua Tree National Park during which I saw not the smallest, drabbest serpent, I almost ran over a big coachwhip near the entrance to the I-10 freeway. Named for their braidlike scalation, coachwhips hunt in the open and so are more likely than most species to at least be glimpsed. I wanted to take that one back into the park, but when I’d swerved and screeched to a stop it was already long out of sight. Harry Greene, a snake expert, marveled that coachwhips “almost defy the laws of physics sometimes” while he tried to catch “an enormous magenta” one that “seemed to fly, not always touching earth.”
Apparent scarcity is an aspect of desert snake success, however. Field guides show that California’s desert snakes are more diverse than coastal ones, and they are probably more abundant. Alta California’s desert has five rattlesnake species for example—its coast has three—two of which are confined to the extreme south and also inhabit desert. Snakes are the most recently evolved reptiles, and their ability to live sinuously and virtually invisibly in the interstices of things is an aspect of their novelty. It gives them a big advantage in a land of heat, drought, and vigilant predators.
Desert lizard prominence is harder to understand. Although most extant lizard groups appeared no earlier than most living bird and mammal ones, they still seem the most primitive reptiles, the closest in body plan to the first amniotic egg-laying sprawlers that evolved from Paleozoic tetrapods. Edward Cope, an early analyst of the “amphibian to reptile” transition, thought the first reptiles resembled “the farm fence lizards of today.”
Lizard survival gear seems rudimentary for the exacting desert environment. Most lizard teeth are simple pegs or spikes in contrast to mammals’ elaborate tool kit for slicing, piercing, shearing, and grinding. Some like the Gila monster supplement their teeth with poison, but they lack snakes’ highly specialized fangs. And Gila monsters’ poison doesn’t avail them in the superbarren California desert: like giant cactuses, they largely stop at the Arizona border. Herbivorous lizards may have cusped or serrated teeth, but even they process food relatively poorly. Reptile jaws are weak compared to mammals’ because they are made of a jigsaw puzzle of small bones instead of a few large ones, so they can’t chew very well.
Lizard physiology is surprisingly vulnerable to desert climate, as Raymond B. Cowles demonstrated—callously—in the 1930s:
I spent a number of days in testing the upper temperature limits of snakes and lizards by exposing them one at a time to the impact of the sun in their natural habitat. The test required my squatting in daytime heat and allowing the tethered animals to run toward but not reach nearby shade. Watching these supposedly heat-demanding animals quickly die from high temperatures in their chosen home climate was a thought-provoking experience. Here was I, a heat-generating animal with a naked, unprotected skin, surviving even longer exposure while dozens of reptiles were killed in minutes by overheating. Some of the smallest reptiles died within about sixty seconds after being scooped out of their underground shelters into the blazing sun of the surface.
Lizards are thus even more dependent than birds and mammals on behavioral expedients to prevent overheating. Smaller ones bury themselves or move into crevices in midday, while larger ones stay near shade. When zebra-tailed and leopard lizards startled me with their midday athletics at Mitchell Caverns, it was because I was startling them from under sheltering bushes. Their goal in fleeing so briskly was to get under another bush.
Nevertheless, like mad dogs and Englishmen, lizards do go out in the midday sun and, like Englishmen, thrive. To quote another nonsense verse from the mad Englishman who coined the word “boojum”:
“You are old,” said the youth, “and your jaws are too weak, For anything tougher than suet;
Yet you finished the goose, with the bones and the beak—Pray, how did you manage to do it?”
Like Lewis Carroll’s goose-eating Father William, lizards have surprising talents. Although their reptilian background prevents them from politely masticating their food, their jaws are well adapted to gaping, grasping, and gulping. And they have methods of dealing with the desert climate besides hiding under bushes. When they bask in the sun during cool mornings, the blood supply to skin and extremities increases, which quickly warms the rest of the body. When they start to overheat, surface blood flow decreases. They can also change overall skin color to stay cool or get warm—becoming lighter in hot places and darker in cool ones. They have a sense organ for such feats on their heads—a small round “pineal eye” that includes a lens, pupil, and iris. It can’t really see, but it evidently senses ambient light, because its removal impairs activity and reproduction cycles.
Because they don’t need to maintain a constant body temperature, moreover, lizards can tolerate a greater range of it than birds and mammals, which helps them to spend cool weather dormant in the ground and hot weather going out in the midday sun. Desert iguanas can amble around eating creosote bush flowers with body temperatures between 104 and 107 degrees Fahrenheit. Raymond Cowles observed that some desert lizard species could survive “an occasional and very brief 113 degrees F,” a body temperature he would have found the reverse of thought provoking if he had tried it himself.
Indeed, herbivorous lizards compensate for poor chewing ability by using high body temperature to aid digestion. Chuckwallas normally start feeding at body temperatures of 100 degrees Fahrenheit or higher. A little sunbathing sprawled on a boulder soon raises their ungainly-looking bodies above the desert’s much lower nocturnal ambient temperature. Their genus name, Sauromalus, translates from Greek as “flat lizard.” An eighteen-inch-long individual may have a four-inch-wide abdomen.
Perhaps most important from an evolutionary viewpoint, lizards need much less food than birds and mammals, since their metabolisms use less energy. Cowles certainly would have found food for thought if he’d tried living on creosote bush and burroweed. Lizards can fast longer, and large ones can stay dormant in the ground during droughts, living off their fat. When conditions are right, a given area can support many more lizards than mammals or birds, allowing greater scope for survival and adaptation.
If Darwin had paid more attention to lizards, he might have changed his mind about deserts as evolutionary backwaters. It may be generally true, as he wrote of desert dwellers in The Descent of Man, that “nearly all the smaller quadrupeds, reptiles, and birds depend for safety on their colours.” But he had described showy orange, yellow, and red coloration in his land iguana specimens from the “very sterile” Galapagos, although he passed it over in his later evolutionary thinking. Desert lizards can wear surprisingly brilliant colors, as John Van Dyke noted, albeit vaguely: “The lizards are many in variety, and their colors are often very beautiful in grays, yellows, reds, blues, and indigoes.”
There has been nearly as much argument about desert lizard colors as about cactus thorns. Van Dyke thought they could change to match their surroundings, “having traced with surprise the faintly changing skin of the horned toad produced by the reflection of different colors held near him.” This raised the question as to whether they simply react automatically to environmental stimuli—as with the lightening or darkening of skin in response to heat or cold—or can change color in response to visual perception of their surroundings. And then there was the social question. Some naturalists maintained that lizards use colors simply to impress rivals in territorial disputes. Others thought they also use colors to impress prospective mates, precisely the competitive sexual selection that Darwin thought stressful desert life would minimize.
Whatever the reasons, California desert lizards can produce a definite if subtle rainbow effect, especially in mating season. Tiger whiptails are checkered blue-black and yellow, with a strange violet bloom. Spiny lizards have blue bellies and throats along with blue flanks. Zebra-tailed lizards sport pink or orange throats and yellow flanks as well as the blue and black flank bars I saw at Cinder Cone Lava Beds. Desert iguanas show pink and orange along their flanks. Male leopard lizards have pink bellies; females may have orange-spotted necks and flanks in spring, a blush indicating that they already have mated and further suitors need not apply.
The peacocks of desert lizards are collared lizards (Crotaphytus), leopard lizard relatives of rocky areas. Named for their black and white necks, collared lizards have big heads that—with their habit of running on their hind legs—make them resemble tiny tyrannosaurs. If dinosaurs were as colorful, they would have been impressive indeed. Local populations within their large range may have green, blue, red, or yellow throats along with blue flanks. As with leopard lizards, gravid females may have orange markings. I’ve seen peacock blue collared lizards, although California individuals have been less spectacular, with green-yellow speckled bodies and bluish heads. One of these, a juvenile, was earnestly trying to catch a side-blotched lizard not much smaller than itself.
The more scientists observe lizard coloration, the more complex it seems. When Barry Sinervo, a biologist at University of California, Santa Cruz, studied the abundant little side-blotched lizard for two decades, he found a bewildering variety of color “morphs.” Males can have throats and flanks colored dark blue, yellow, or bright orange. They can also have striped blue and yellow throats and flanks, blue and orange striped throats with yellow flanks, or blue and yellow striped throats with orange flanks. Females can have throats and flanks colored yellow or bright orange, or they can have blue and orange striped throats with orange flanks, or blue- and yellow striped throats and orange flanks.
Male coloration coincides with mating behavior. Blue males establish long-term bonds with females and stay near them. Yellow males sneak around trying to mate on the sly, and they mimic females to further their philandering. Orange males rush about trying to expel other males and to force their attentions on females. Chivalrous blue males cooperate to guard females, which helps them to exclude the sneaky yellows. Boorish orange males can break up blue associations, but they compete with each other so much that the yellows sneak in and mate with the females.
Female coloration also coincides with reproduction. Yellow females produce small clutches of large hatchlings, which makes their offspring more competitive when local population density is high. Like pickpockets, sneaky yellow males do better in crowds. Orange females produce large clutches of small hatchlings, which makes their offspring more competitive when local population density is low. Like Old West outlaws, boorish orange males do better in scattered populations.
One of Sinervo’s associates, Lesley Lancaster, found that the gray and tan markings on side-blotched lizards’ backs play a part in all this. Some lizards have sideways bars on their backs, which helps to camouflage them in vegetation. Barred yellow males can sneak around better. Some have longitudinal stripes on their backs, which help them to dodge predators. Striped orange males can swagger about in the open more safely.
Apparently the quantity of a hormone, oestradiol, in embryos affects marking patterns. Experiments suggest that male lizards’ colors can influence the amount of oestradiol that females secrete into their eggs, and thus the back markings of the hatchlings. If yellow males predominate near a female, she may produce more hatchlings with barred backs, giving yellow male hatchlings a survival advantage. If orange ones do, she may produce more hatchlings with striped backs. This would have pleased Edward Cope since it implies that, as he thought, organisms can respond to their environment in major physiological ways and pass on the responses to their offspring
Side-blotched lizards’ color-coded behavior is like a Mozart libretto, full of sartorial finery and amorous intrigues, disguises and cross-dressing. Hormone shifts can turn yellow males into blue ones—tenors into baritones. And how do the multicolored lizards get along? Do males with blue and yellow striped throats and orange flanks vacillate between gallantry, sneakiness, and boorishness? Do similarly spangled females produce similarly confused offspring? If side-blotched lizards try to practice Darwin’s basic sexual selection by choosing the most attractive mates, they must have complicated love lives.
I can’t say I’ve watched side-blotched lizards enough to verify all this. I haven’t seen blue lizards behaving particularly chivalrously or yellow ones acting any more sneakily than small lizards usually do when grabby giants loom over them. I can say that I’ve encountered an orange one that was, if not boorish, at least unusually bold—or foolish. Perched on a rock at the mouth of Porcupine Wash, he or she seemed unintimidated at my approach and almost let me touch his or her striped back before discretion got the better of hormones. But the little lizards’ variety of hues must have reasons.
Other, less-studied lizard species probably have unplumbed depths of mating behavior. A theater critic as well as a professor, Joseph Wood Krutch reviewed a zebra-tailed lizard pair’s performance enthusiastically:
Besides the advances and retreats which are the essential features of all courtships, this one consisted principally of poetic speeches or amorous arias, though I could not be sure which since the sounds were completely inaudible to me, at least through the window. The male would mount some two-inch elevation, raise himself high on his front legs, inflate his throat until he looked like a small iguana, and then give voice to some sort of utterance which shook his whole body from head to tail. His lady would listen intently, move a little closer, and then edge away again when her suitor approached to ask what effect his eloquence had produced.
Krutch’s zebra-tailed serenade is scientifically dubious. Some lizards can squawk or bark, especially if disturbed. I’ve heard that the Cahuillas named the chuckwalla for a sound it makes. Only the small, mostly nocturnal geckos are known to make or respond to less rudimentary vocalizations as part of mating. But who knows? “Ears comprise the reception system and are well developed in most lizards,” write two lizard specialists. “Other acoustic signals might include seismic signals generated when large lizards interact aggressively: the possibility is virtually unstudied.”
Lester Rowntree, a prominent California botanist, was singing to herself while collecting plants one day when a lizard emerged from under a boulder, climbed on her knee, and “showed an enormous capacity for large doses of song, closing his eyes in absurd abandon and opening them whenever I shut up, his eyelids sliding back to reveal pleading orbs. This went on for some time till I finally . . . placed him, limp with emotion, on the boulder.”