Nowhere do the California mountains rise more dramatically than in Death Valley. It is the lowest, hottest, driest place in the United States, and it looks it, a vast salt flat walled with escarpments so steep they seem to ascend with visible speed through the heat waves that distort the air. But to me the most dramatic thing about Death Valley isn’t the blazing salt but little animals that live, to a degree, in the salt.
My strongest memory of the valley is not of the famous scenery but of happening, after a morning of being roasted and sandblasted by the famous climate, on what appeared to be a mountain stream flowing through a coastal marsh. The patches of pickleweed and salt grass bordering it were white with salt and smelled like the seashore, but the little stream looked as clear and sweet as a Sierra meadow one. Meadow birds flew around—a killdeer, a common snipe, and a black phoebe that wagged its tail happily as it banged a silvery insect against the ground in preparation for eating it.
When I looked into the stream, lively swarms of water beetles and caddis fly larvae on submerged plants enhanced the arcadian impression, as did the darting of tiny fish. Walking along the bank, I was surprised to see that the water literally swarmed with the fish, although the wind-ruffled surface made it hard to get a good look at them. Eventually I found a sheltered spot where I could watch the fish chase each other in what seemed like mating behavior. Most were a mottled honey color, but many were pale blue with yellow heads and black-tipped fins. Some of the blue fish were quite small, but still very active. The honey-colored fish chased each other; the pale blue fish chased each other. The fish’s mad pursuits made the stream appear not just lively but frenzied, more like a hormone-driven singles’ party at the beach than a Sierran Arcady.
William Manly and his party were among the first white explorers to see similar fish as they made their thirsty way into Death Valley in 1849. They found the discovery encouraging, if not particularly impressive: “One night we had a fair camp, as we were close to the base of the snow butte, and found a hole of clear, or what seemed to be living water. There were a few minnows in it, not much more than an inch long. This was among a big pile of rocks, and around these the oxen found some grass.” As the fish implied, the water proved potable, so they passed a rare comfortable night and then moved on.
If seeing the fish in the marshy stream I encountered on the valley floor had encouraged the Manly party to drink, however, they would have been unpleasantly surprised. Whereas the fish in the rocky hole they found inhabited freshwater, the ones I watched in that bucolic-looking stream spend at least parts of their lives in water twice as salty as seawater. Yet all the fish belong to the same genus—Cyprinodon—and they are all called desert pupfish.
The fish the Manly party found may have been the Devil’s Hole pupfish (C. diabolis), a different species than the ones I saw in the stream, the Salt Creek pupfish (C. salinus). Both species are confined to a single place on the planet. The Salt Creek pupfish inhabits a few miles of that marshy creek. The Devil’s Hole pupfish lives on a small limestone shelf at the top of a water-filled fissure in the rocks. The two places are a few dozen miles apart as the crow flies, but the two species look and act quite differently.
Devil’s Hole pupfish are shorter and slimmer than Salt Creek ones, with larger heads in proportion to their bodies, and they lack their pelvic fins and banded markings—although I didn’t see them up close because their pool is gated to protect them. Over the years, their population can fluctuate between a few hundred and a few dozen. Their small habitat and food supply limit their numbers and they have had to adapt to unusually warm water, averaging about 93 degrees Fahrenheit. The Devil’s Hole environment is fairly stable, however (except when humans have interfered), so the pupfish undergo little competition for food or mates.
The Salt Creek pupfish population fluctuates annually from a few thousand for most of the year, when water is scarce, to many thousands when spring runoff fills the creek and adjacent marshes and food is abundant. Competition accelerates as population soars, and sexual activity peaks—as I saw. The pale blue fish were males chasing rival males away; the honey-colored fish were females chasing away females; the smaller blue fish were sexually precocious young males trying to get in on the action. It was a wonder any of them found time or space to actually mate.
These two species are among more than twenty isolated pupfish populations in the Death Valley watershed today. They include at least ten taxa: four species and six subspecies of a fifth species. Each species and subspecies has evolved distinctive forms and biotic adaptations. The Devil’s Hole and Salt Creek species are unusual because of the extreme conditions they undergo. Those under less stress behave more “normally” for the genus, with dominant males establishing and defending territories that females visit for breeding.
I watched bright blue males of a more “normal” species—the Amargosa pupfish—guarding their territories in large freshwater springs at Ash Meadows east of Death Valley. They seemed a lot calmer than the Salt Creek males, as though resting complacently between conjugal visits. Surprisingly deep and clear for their bleak surroundings, the springs were even more full of life than Salt Creek, including another native fish—a reddish silver minnow called speckled dace—and tadpoles and crayfish as well as water insects. They also contained illegally dumped guppies and mollies that have contributed to the extirpation of another endemic, the Pahrump killifish (Empetrichthys). I could see why Devil’s Hole is gated. A sign said that the springs are the local human water supply as well as the home of vulnerable species, but tourists were smugly skinny-dipping anyway.
Death Valley’s fish became famous as examples of evolution-in-progress at about the same time Stebbins published his “quantum” accelerated desert evolution ideas. A 1949 article on the region proclaimed:
When a biologist stands on the floor of a great Pleistocene lake, where strand lines are clearly visible on the mountainsides all around him, but whose waters have shrunk until a few short streams and isolated springs are the only remaining traces of fresh water in the midst of a desert, his imagination is likely to be captured by the occurrence of a dense population of fishes and other aquatic organisms in those waters. Knowing the importance of geographic isolation and small breeding populations in evolution (as deduced from genetic theory), he cannot overlook the fact that a natural experiment on the mechanics of speciation has been performed for him on a colossal scale.
Biologists began seeing these “islands” of aquatic life as a kind of desert Galapagos. In 1979, two fish specialists wrote: “Just as biological processes and resultant divergent forms of terrestrial life on the remote Galapagos Islands strongly influenced the thinking of Charles Darwin, recent studies of the aquatic islands of Death Valley have increased our knowledge of the evolution of life forms. . . . Evolution and differentiation of pupfishes and killifishes in the Death Valley System has been influenced by three primary factors: stresses of the environment (selection pressure), length of isolation, and habitat and population size. These three factors are the forces driving the process of evolution.”
Comparing Death Valley to the Galapagos has its limits, however. Confronted by tiny fish isolated in a salty stream or tiny pool in the desert, Darwin probably would have seen an evolutionary backwater where the fish interacted more with the harsh environment than with other fish, and where natural selection was correspondingly slow. The two biologists’ “three primary factors” influencing desert fish evolution sound more like the neo-Darwinian “three main reasons” that Stebbins cited in his 1952 article on accelerated desert plant evolution than Darwin’s ideas.
Stebbins’s first condition for accelerated desert evolution, scarcity of moisture, is certainly a limiting factor on fish. This scarcity has divided desert pupfish into many isolated populations, thus providing his second condition, a lot of genetic variability. The biologists didn’t mention his third condition—specialized structures preadapted for desert stresses. But desert pupfish and killifish clearly have great potential to live in salty, oxygen-poor, hot, or otherwise stressful water.
Devil’s Hole pupfish have provided a striking example of this. When biologists moved some of them from their rocky ledge to refuges as part of a conservation effort, new generations of the transplanted fish soon grew longer, with smaller heads in relation to their bodies than their parents. In effect, they began to be more like the “normal” pupfish in streams and larger springs because refuge conditions were more like those habitats, with cooler water and more food. This implied that the Devil’s Hole pupfish’s ability to survive in hot water with little food is not just an adaptation gradually acquired through natural selection, but a preadaptation, a genetic potential that can be expressed under extreme conditions like Devil’s Hole, and then switched back off again if water cools and food becomes more abundant.
Yet if Death Valley’s fish supported Stebbins’s ideas about desert as an evolutionary frontier, they didn’t really address the question of California desert origins. The limits of likening Death Valley to the Galapagos arise again here. Biologists can surmise that Galapagos iguanas evolved relatively recently on those islands (or earlier volcanic islands) because Galapagos iguanas live nowhere else. Death Valley fish are less unique.
The Pahrump killifish (Empetrichthys) is (or was) an endemic genus, but close relatives inhabit warm springs in Mexico. Speckled dace (Rhinichthys) has congenerics throughout North America, and the pupfish genus, Cyprinodon, has species scattered through the southern United States and Mexico and on the East Coast as far north as Maine. The pupfish family, the Cyprinodontidae, also occurs on other continents, and some South American and African genera are adapted to survive complete evaporation of waters they inhabit. They lay their eggs in the mud before dying, and the eggs hatch when the next rains fill the pools again. They’re called annuals, like wildflowers.
California desert fish aren’t so specialized, which might imply that they haven’t been adapting to dryness for as long as others. Or it might not. Salt Creek pupfish survive the winter, when most of their stream stops running, not by laying their eggs in bottom mud, but by sheltering in it themselves. Such things hint that Cyprinodon and other fish may have inhabited ancient California deserts as well as present ones. Recent genetic studies suggest that Death Valley’s pupfish have been isolated from other North American Cyprinodon populations for millions of years.
California desert pupfish generally are thought to descend from ancestors that inhabited a brackish estuary, the Bouse Embayment, in the lower Colorado Valley and Salton Sink regions in the mid-Pliocene epoch. Pre–ice age fish fossils found in Death Valley and other parts of the California desert include Cyprinodon and relatives of Empetrichthys, as well as other kinds of fish that still live in the Southwest—suckers, squawfish, and Gila chubs. As the Bouse Embayment withdrew southward and the Pliocene climate dried, the fish presumably lived in smaller bodies of water. Some may have been isolated in desert springs and salt creeks.
During the subsequent rainy ice age, however, the ancestors of today’s desert fish would have inhabited the then ample lakes and streams, not desert springs and salt flats. Judging from the transplanted Devil’s Hole pupfish’s quick reversion to pupfish “normality,” a transition from isolated Pliocene springs and creeks to huge Pleistocene lakes would not have been a big problem for those ancestors. As their genetic variability implies, pupfish may have been getting in and out of hot water for a very long time. Although they can change so quickly in response to environmental shifts, Death Valley’s fish may be as much about endurance as change, perhaps another instance of desert biology going in circles.