The explanation of why the male genitals of grasshoppers are so intriguing to taxonomists is fairly simple. To begin, these structures exhibit remarkable consistency within species but spectacular variation among species, even in cases where two kinds of grasshoppers seem to be otherwise similar in size, form, and color. The basis for these differences is probably, at least in part, a function of reproductive isolation. For a species to evolve and sustain genetic integrity, its members shouldn’t be trying to mate with other species. Species often have distinct body forms and colors that allow prospective mates to readily identify their own kind. Some grasshoppers use elaborate courtship rituals to ensure that the prospective mate is the “right one.” Not so for the spurthroated grasshoppers—a subfamily named for the conical protuberance that arises from between their front legs, giving the impression of an enlarged Adam’s apple or “spur throat.” Species in this taxonomic group (of which spretus was a member) are similar in appearance and undiscriminating in their foreplay.
For the spurthroated grasshoppers, sex is a lover’s leap. Males often hop onto almost any moving object of approximately the right size and color of a prospective mate, including females of other species and sometimes even other males. In the latter case, consummating the sudden relationship is, of course, hopeless. But in the former case, mistaken matings would seem possible. However, with his weirdly contorted genitals, the male is not able to insert his aedeagus into just any female. When a mismatched male is not summarily kicked off by the female, he spends long minutes tediously probing with his genitalia. But the elaborately sculpted tip of his penis simply doesn’t align properly with her genital tract. The result is that the tube that is extruded from the aedeagus in order to transfer the sperm doesn’t thread properly into the duct of the female that opens into her pouch that receives and stores the sperm. The system is a bit like a key-and-lock and without the male’s “key” finding a matching “lock” in the female, the effort ends in frustration. And so, one might ask, why don’t taxonomists use the female genitalia, the “lock” rather than the “key”? The explanation might reflect male chauvinism in science, but there is a more simple answer. Consider your front door—it is much easier to examine and describe the structure of your key than it is the inner form of your lock.
The value of the male genitalia for discriminating among species of grasshoppers was pioneered by Theodore Huntington Hubbell. Having earned his bachelor’s degree at the University of Michigan, he pursued graduate studies at Harvard. However, Hubbell left before earning his degree at the behest of a friend to teach at the University of Florida. While teaching, Hubbell managed to earn his doctorate at the University of Michigan and returned to his alma mater as the Curator of Insects in the Museum of Zoology in 1946. Hubbell became the museum director in 1955 and built a collection of Orthoptera (grasshoppers, crickets, and katydids) that now requires hundreds of drawers stacked into towering cabinets filling more than 5,000 square feet of floor space. Within this bonanza of biodiversity, Hubbell discovered the taxonomic Rosetta Stone of these creatures—the size and shape of the various knobs, spines, and twists of male genitals.
To be precise, Hubbell’s innovation was based on a rather simple insight, that different species had remarkably unique internal genitalia. Of course, the existence of the aedeagus was well known prior to his work. Plenty of morphologists had illustrated these graceful—and sometimes wickedly armed—structures, but nobody had thought of systematically comparing them among different species. It was here that Hubbell made his contribution by providing spectacular evidence of the diagnostic value of the concealed genitalia of grasshoppers. Soon, the entomological world came to accept this bizarre but effective approach to deciphering the identity of grasshoppers. And when the “gold standard” of taxonomy was applied to settling the identity of the Rocky Mountain locust, the answer was unequivocal. The phase theory had very nearly made the Rocky Mountain locust disappear as a species, but it was to be brought back into existence through its reproductive organ, a fitting means for perpetuating the life of a species.
In 1959, armed with Hubbell’s insights regarding the informative power of the male genitalia, the federal government’s foremost expert on orthopteran taxonomy declared in utterly unambiguous terms that the Rocky Mountain locust was not simply the migratory phase of sanguinipes (then mexicanus) but a separate species altogether. Not many scientists have the standing to settle such long-standing debates by fiat, but Ashley Gurney had earned the authority through a lifetime of deep devotion to entomology. He had served as a malariologist in World War II, and friends recounted his annoyance at having the Japanese interrupt his mosquito collecting through their incessant shooting. Having earned a doctorate in entomology at the University of Massachusetts, he was hired by the USDA and became affiliated with the Smithsonian Institution. The Smithsonian is in the big leagues of taxonomy and by the 1950s, Ashley Gurney had ascended to a position that allowed him to carry on with the job that God had given Adam—naming the creatures.
In one short sentence—backed by pages of diagrams and analyses—Gurney and his colleague, Arthur Brooks from Canada’s Department of Agriculture, established the taxonomic standing of the Rocky Mountain locust that has held for nearly half a century: “Our study of the aedeagus indicates that spretus is a distinct species.” They invited continued tests of their conclusion, but the genitalic evidence was unambiguous. Although similar in size, the genitals of mexicanus resembled a soft cotton mitten whereas those of spretus were more like a tough leather sheath. They examined the specimens that Brett claimed to have transformed into a spretus-like creature a decade earlier, and they found that there was “no approach to the aedeagus of spretus.”
This work by Gurney and Brooks became the new standard. They clearly understood the implications of their findings: The disappearance of spretus was not simply a matter of a biological variant of an existing species having been somehow suppressed. Rather, the disappearance was the extinction of the most abundant form of life ever to sweep across the continent. It could not be “re-created” in the laboratory by the rearing of an extant species under particular conditions. The Rocky Mountain locust was gone forever.
But with the taxonomic status of the Rocky Mountain locust being as resolved as such matters get in classical taxonomy (molecular analyses were yet to come), entomologists were left to face an even more compelling problem. If extinction is like an ecological murder, then we had finally identified the victim. There was no multiple personality, no switched identities, none of the oh-so-clever feints used in detective stories—just a body. So, how did a species that once blackened the skies, sweeping across a continent in swarms larger than any known biological phenomenon on earth, disappear forever in less than twenty-five years?
10
Beautiful Theories and Ugly Facts
EVEN THOUGH CHARLES BRETT HAD FAILED TO resurrect spretus from sanguinipes through alterations in temperature, humidity, and food, his experiments left no doubt that a steady ature, humidity, and food, his experiments left no doubt that a steady diet of alfalfa produced wimpy grasshoppers. And in light of these findings he proposed the first clear hypothesis for the demise of the Rocky Mountain locust. According to Brett:The disappearance of the Rocky Mountain locust seems to be in part explained by the greatly increased acreage of alfalfa west of the Mississippi River since 1900. M. mexicanus [now sanguinipes] is strongly attracted to alfalfa, but the studies reported in this bulletin show that alfalfa is unsuited for the best development of this pest. Grasshoppers grown on alfalfa are comparatively small, and sometimes malformed. Historically, the disappearance of the Rocky Mountain locust appears to have coincided with the spread of alfalfa throughout its breeding grounds.
Brett’s logic was simple: If spretus were the robust phase of sanguinipes , then a widespread plant that caused a sickly form could account for the disappearance of spretus. If spretus had been the Superman of grasshoppers, maybe we’d planted a botanical kryp
tonite across its landscape. Brett’s proposal stimulated some of the first systematic thinking about the causes of the Rocky Mountain locust’s demise. Although it turned out that spretus was not the gregarious phase of sanguinipes, alfalfa remained a viable suspect. Could it be that both species fared poorly on alfalfa and thereby began to decline in the West?
The importance of alfalfa in the story of the Rocky Mountain locust became a matter of contention almost as soon as the theory was proposed. As Brett’s work was being published, Robert Pfadt was coming onto the grasshopper scene. Born in Erie, Pennsylvania, in 1915, the thirty-four-year-old assistant professor had recently arrived at the University of Wyoming. He had developed an interest in grasshoppers while a student in Wyoming, continued these studies during his doctoral program at the University of Minnesota, and carried this interest back to the faculty position at his alma mater. Pfadt continued his work on grasshoppers for half a century, establishing a record of practical research that convinced the university administration to replace him with another acridologist after he retired in 1984—which is how my position came to be. His work in grasshopper ecology was not focused on the Rocky Mountain locust, but he was drawn into the tale of this creature through his eminently practical work on its kin.
In 1949, Bob published a lengthy work on his experiments concerning the role of food plants as factors in the ecology of sanguinipes . The relation of alfalfa to the health of grasshoppers was not quite so straightforward as Brett has suggested. It was true that this plant was a very poor food on which to rear nymphs of sanguinipes. If provided only with alfalfa, barely one in five nymphs survived to adulthood, and they weighed about half as much as grasshoppers fed on dandelion, their most healthful food.5
But Bob was one of the most careful and meticulous researchers of his day. He invariably valued quality over quantity, such that a single comprehensive paper was preferred to a dozen pieces of fragmented science. So, Bob conducted a separate series of experiments to discover whether alfalfa was indeed detrimental to adult grasshoppers. Much to his surprise, when healthy adults were fed alfalfa, versus other food sources, they had superior longevity and egg production. Alfalfa, it seems, was the grasshopper equivalent of a glass of Merlot with dinner: a fine dietary component for grown-ups but not for children. And this finding provided the key to a paradox. Despite Brett’s assertion that alfalfa was deleterious, entomologists had often observed that sanguinipes flourished in alfalfa fields. Bob attributed the infestations of this species in alfalfa to their consumption of weeds that infest the crop. Even a modest amount of dandelion or bromegrass in the diet of developing nymphs could offset the deleterious effects of alfalfa.
Bob not only cast doubt on whether alfalfa was invariably detrimental to sanguinipes (and, by inference, to spretus) but also questioned, on two other grounds, the validity of Brett’s claim that the planting of this crop could account for the disappearance of spretus. First, he noted that we knew very little about the plants on which spretus originally depended, so nothing could really be said about whether alfalfa had actually replaced anything of importance to the locust. Even if alfalfa was harmful to the insect—and this was not unambiguously the case—the effect would be marginal unless the crop had replaced a vital food source.
And this argument led to his second concern, which both reflected the contemporary thinking and shaped much of the subsequent discussion regarding the locust’s disappearance. Bob pointed out that no matter what the effect of alfalfa, it simply did not exist on a sufficient scale to have impacted spretus throughout its range. Even a deadly poison—and alfalfa fell rather short of this—would need to have been spread over an area much larger than the nooks and crannies of the landscape filled with alfalfa. This crop requires a great deal of water, and although it was abundant in irrigated valleys, the range of the locust was immense and included vast areas of dry uplands where prairie grasses could not be replaced by thirsty alfalfa fields. A decade later, Gurney fully endorsed Bob’s contention, noting that he’d also found no evidence of “any pronounced range-plant changes in Montana during the 1860s, 1870s, or 1880s, so far as the general disappearance or replacement of plant species is concerned.” It appeared that poor Charles Brett had it wrong again. Not only was sanguinipes incapable of giving rise to spretus, but alfalfa fields seemed to have nothing whatsoever to do with the disappearance of the locust.
With the dismissal of the “alfalfa theory,” a consensus began to form on the fate of the Rocky Mountain locust. In the 1950s, the environmental movement was dawning, and ecologists were thinking in terms of large-scale anthropogenic effects—continental, even global, changes—on other species and ecosystems. And this way of perceiving the growing conflicts between humans and nature fostered the conceptual agreement among entomologists that the disappearance of the locust, a species that stretched across millions of square miles, must have been caused by an environmental change of commensurate scale. Although the precise mechanism causing the decline of the species was not apparent—just as nineteenth-century evolutionists did not understand genetics—the fundamental nature of the process was obvious. The challenge was to find a sweeping change that was concurrent with the locust’s disappearing act.
THE ROLE OF BISON
When one thinks of a life form sweeping across the West, locusts are not likely to be the first creatures that come to mind. Rather, the iconic image of the bison fills this legendary place in the lore of the prairies. Although we associate bison with western grasslands, the range of these animals included almost every state, as well as northern Mexico and western Canada. In 1839, Thomas Farnham traveled through a herd of bison along the Santa Fe Trail—for three days. Farnham could see bison stretching fifteen miles in either direction along his forty-five-mile passage, so he estimated that the herd covered more than a thousand square miles. Some thirty years later, Major Richard I. Dodge, traveling along the Arkansas River, encountered the largest reliably measured bison herd. This outpouring of hide and hoof was fifty miles long and twenty-five miles across, and later calculations estimated a population of 4 million animals. These animals were the furry counterparts of the waxy locusts on the continent. Attempts to estimate the total population size of either creature are fraught with speculative assumptions, but the efforts are revealing.
The peak number of bison has been calculated in various ways. Early estimates were based on extrapolations from hunting records. Modern approximations of carrying capacity—a measure of the number of creatures that can be sustained by a particular resource base—set the average sustained population density at 26 bison per square mile throughout their range. Ecologists have also tried to apply historical observations in refining crude guesses from nineteenth-century hunters. Given that the population estimates come from such a diversity of sources, it is somewhat remarkable that they all fall within the same range. Our best guess is that there were 30 to 60 million bison in North America, prior to European settlement.
Estimating the peak number of Rocky Mountain locusts is an equally dicey affair. But let’s take the outbreak of the 1870s and presume that half of the reportedly infested area actually had locusts present at any given time. If so, then these insects were present across an area of 500,000 square miles (about twice the area of Texas). The carrying capacity of rangeland for modern-day grasshoppers is around 10 individuals per square yard. Using this figure—which is probably quite conservative, as we’re considering the locust during an outbreak—we’d end up with 15 trillion insects, or a couple thousand locusts for every person currently on the earth.
Now then, to put the bison and locust on equal footing, we need some common units. Ecologists tend to prefer biomass—the number of kilograms, pounds, or tons of living tissue. Let’s assume that the average bison weighed in at five hundred pounds, using the figure for a juvenile as a reasonable compromise between a mature male weighing a ton and a newborn calf weighing thirty-five pounds. If there were 45 million bison, then there were somewhere aroun
d 11 million tons of critters scattered over the continent. As for the locusts, let’s assume a weight of half a gram per locust, which is the size of sanguinipes. Given 15 trillion insects, their collective biomass would have been about 8.5 million tons. So, bison and locusts had similar, and rather phenomenal, masses of herbivorous tissue. And both were very nearly gone by the turn of the nineteenth century.
Bison were quickly extirpated in regions where they were not particularly plentiful in the first place and humans were relatively numerous. By 1819, there were virtually no bison east of the Mississippi River, and by 1840 these creatures had been wiped out to the west of the Rocky Mountains. However, massive herds still roamed the plains, where people were thinly distributed. These millions of creatures were ultimately doomed by a tragic conspiracy of sociology, economics, and politics. The beginning of the end of the bison came with the end of beaver. That is, by the 1830s, unfettered trapping had decimated the beaver populations, and the American Fur Company and the Hudson Bay Company switched from purchasing beaver pelts to bison hides. With the emergence of this market came the professionalization of hunting. However, bison carcasses are absurdly cumbersome to handle, and the coastal markets were far from the Great Plains. So the demand for bison flesh was restricted to tongues, the prized cut of meat, and millions were shipped to market—with the rest of the carcasses left to rot on the prairie.
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