But sloths move with such pervasive slowness that their entire world seems intrinsically and permanently different from ours. I would almost conjecture that a fixed slow-motion camera occupies their cranial space, and that they gauge all their movements by this markedly different clock upon the world. Do we, and most other creatures, appear to them like the Keystone Kops in movement, or the Munchkins in raised pitch? Or do our frenetic paces (compared with their stately step) constitute the only external world they know, recorded in their brains as the slothful equivalent of “objective reality”? If one of El Greco’s uniformly tall and thin people stepped out of a canvas featuring no other person but himself, and then entered our world, would we all appear ridiculously squat and fat, or would he know nothing else (by virtue of a few centuries’ experience with human gawkers in art galleries, but never even a glance at confrères on other canvases), and therefore view us as ordinary and archetypal? But sloths do know other sloths and must also perceive a differently paced external world as well. Perhaps they don’t notice the difference; perhaps they are merely amused; perhaps they don’t care. I would love to know.
In any case, philosophical speculation aside, I have never been so powerfully moved by a sense of pervasive difference for something so basic as a pace of life. Hanging upside down, and grasping a tree branch by all fours, sloths move along hand over hand, and so very slowly—not (apparently) for reasons of immediate caution, but in accord with their own concept of normality. They stretch out an arm to reach their leafy food with the same utter languor. The algae that grow on their hanging hairs, imparting a green tinge to the entire body, almost seem to take hold because the animal can’t move away fast enough. (Yes, of course, I intend the last sentence only as a metaphor—but then I once heard that a rolling stone gathers no moss!)
At least my impressions are not idiosyncratic. Sloths seem to impact all Western observers in the same basic way. Englishmen named them with a word meaning “slow” by etymology, thus identifying sloths with one of the Seven Deadly Sins as well. Every other language that I know uses the same designation. They are paresseux in French, perezoso in Spanish, and pigrizia in Italian—all meaning “lazy” or “indolent.” They are ignavus in Linnaeus’s Latin, meaning the same thing—and Linnaeus formally named a sloth genus Bradypus, meaning “flow-foot” in Greek. As I stood watching a sloth high in a tree at Manuel Antonio National Park, I heard a group of German tourists speaking about a “foul” animal. I thought that they just didn’t care for the poor creature, but then I remembered that faul is German for “lazy,” and that a sloth auf Deutsch is Faultier.
But how slow is slow? (As I wrote in introducing the topic of this essay, we can at least experiment and accumulate outward data in lieu of our real desire to get inside another animal’s head.) Early sources heaped the calumny of exaggeration upon a reality already genuine enough. Nehemiah Grew, the first scientist (according to the Oxford English Dictionary) to call them by their common English name, wrote in 1681, in his catalog of specimens owned by the Royal Society of London: “The sloath . . . An animal of so slow a motion, that he will be three or four days, at least, in climbing up and coming down a tree.” Linnaeus, when formally naming this creature in his mid-eighteenth-century Systema naturae, wrote: “tardissime et aegre incedit, vix uno die 50 passus” (“he moves most slowly and reluctantly, scarcely managing 50 paces in a day”).
Their step is, in fact, a bit brisker, though nothing to challenge Aesop’s tortoise. In the standard book on the subject, Function and Form in the Sloth, M. Goffert begins his chapter on “motor activity” by writing: “Sloths sleep or rest about twenty hours a day, performing perhaps no more than 10 percent of the work of a higher mammal of the same size.” Goffert then summarizes a number of careful studies devoted to measuring the speed of sloths. Their movement along a horizontal pole (a good experimental surrogate for their favored tree branches in nature) averages a stately 0.1 to 0.3 miles per hour, with maximal acceleration to a sprightly 1.0 miles per hour.
Since sloths are so evidently well adapted to motion upside down along tree branches, we should not be surprised that their infrequent right-side-up progression on the ground should be so painfully inefficient. With front legs longer than hind limbs, and with permanently curved digits that hook well to branches but permit only hobbling motion on the ground, sloths cannot manage more than 0.1 to 0.2 miles per hour on terra firma—scarcely enough to outrun a pursuing jaguar.
Several aspects of sloth anatomy and physiology correlate with their extreme slowness. Studies of contraction time show that, in Goffart’s words, “the muscles of the fastest genus of sloths were thus four to six times slower than their homologues in the cat.” Sloths also maintain a lower and more variable body temperature than almost any other mammal—a fact of undoubted relevance to their slow pace of life. Most mammals hold their steady body temperature just a bit below 100°F (as in our “standard” of 98.6°). Monotremes and marsupials, the egg-laying and pouched mammals of Australia and a few other places, operate at a considerably lower level; the duck-billed platypus, for example, maintains its minimally warm-blooded body at about 85°F.
Sloths belong to the exclusively New World mammalian order Edentata, including armadillos and three genera of South and Central American anteaters. Edentates maintain the lowest body temperatures among placental mammals. For example, two species of the sloth genus Bradypus varied between 82° and 90°F throughout the day, depending upon the outside temperature.
Yet, for all these attempts to approach the sloth’s inner reality with our best inferences from outward data, we have failed badly (and for the usual reason of inability to overcome our self-centered view), at least in popular presentations. From the name that serves as their definition and incubus, to our constant emphasis on their slowness, stupidity, and dull daily routines, we have conveyed an image of sloths as very low mammals doing very little of interest very high in the trees. This tradition began with a remarkable characterization by the great French naturalist Georges Buffon in his classic eighteenth-century compendium, the many-volumed Histoire naturelle. Buffon held sloths in maximal contempt among mammals, and expressed his derision (in his usual elegant prose) by explicit comparison with human abilities, rather than by any attempt to grasp the sloth’s own world of opportunities and dangers. Buffon wrote (my translation):
Whereas nature appears to us live, vibrant, and enthusiastic in producing monkeys; so is she slow, constrained, and restricted in sloths. And we must speak more of wretchedness than laziness—more of default, deprivation, and defect in their constitution: no incisor or canine teeth, small and covered eyes, a thick and heavy jaw, flattened hair that looks like dried grass . . . legs too short, badly turned, and badly terminated . . . no separately movable digits, but two or three excessively long nails . . . Slowness, stupidity, neglect of its own body, and even habitual sadness, result from this bizarre and neglected conformation. No weapons for attack or defense; no means of security; no resource of safety in escape; confined, not to a country, but to a tiny mote of earth—the tree under which it was born; a prisoner in the middle of great space . . . everything about them announces their misery; they are imperfect productions made by nature, which, scarcely having the ability to exist at all, can only persist for a while, and shall then be effaced from the list of beings . . . These sloths are the lowest term of existence in the order of animals with flesh and blood; one more defect would have made their existence impossible.
As if Buffon had not already heaped enough disdain upon sloths, he then argues that human misery arises from moral failures of conscious decisions, and not from inborn propensity. But only among sloths has nature decreed inherent degradation:
The disgraced sloths are perhaps the only creatures that nature has maltreated, the only creatures that offer us an image of innate misery.
Only at the very end does Buffon pull back a bit, wonder about the sloth’s own internal state (as this essay advises), and conjectu
re that things may not be so bad after all—for such an insensible creature might not grasp its own plight:
If the misery resulting from lack of feeling is not the greatest of all ills, then that of these animals, although very apparent, may not be real, because they appear to feel so little: their mournful appearance, their heavy look, their indolent insensitivity to any received blow, all announce their insensibility.
If I wished to praise sloths and launch a counterattack against Buffon, I could add quite a mouthful at this point. A conventional defense would emphasize neglected features that might inspire human respect. For example, general slowness notwithstanding, sloths can give a quick and nasty slash with those long and inflexible nails that Buffon denigrated (males do fight for usual mammalian reasons of sexual competition; and sloths will defend themselves since they truly can’t run away). Moreover, their torpor (and algal cover) do serve an adaptive function in forging inconspicuousness in the presence of enemies, and should not be interpreted as a burden of phyletic primitivity.
I could also point out, still framing a conventional defense by trying to arouse human attention, that sloths have evolved an array of interesting and unique features. For example, sloths are not a dying remnant, but a group in reasonable vigor with more than half a dozen species in two genera—Bradypus, the three-toed sloth; and Choloepus, the two-toed sloth. With just one or two other exceptions, all mammals have exactly seven cervical (neck) vertebrae (see chapter 16)—yes, even giraffes (where the usual seven are mighty long). But sloths, for some unknown reason, vary this nearly universal number. Choloepus has only six cervicals; while Bradypus has nine. As a result of these extra vertebrae, Bradypus can rotate its head through 270 degrees, or a full three-quarters of a turn!—not quite the full spinning of cartoon clichés (remember Pinocchio turning to display his school clothes to Gepetto), but the closest equivalent in the real world.
Too many sloth lovers, myself included, have tried to stick up for these maligned edentates by invoking such a strategy—that is, by making them either nice or interesting in human terms. Goffart, for example, continues to combat Buffon’s calumny two centuries later when he writes:
Though explorers often described sloths as expressionless, dreamy and stupid, those acquainted with them as pets find that they have a great variety of expressions. Tirler says that when its face is in repose a good-natured smile is forever on its lips. When relieving itself, Choloepus has an expression of quiet pleasure.
But the more 1 ponder the subject, the more I conclude that we should just try to know sloths as they perceive and record the world—and not just scan their repertoires for items that resonate with us, or bring us pleasure (including the sublime delight of a good outcome in the outhouse!). And yet, to really know, I need those sixty seconds within a Bradypus brain—and no power on earth can supply this gift and tool. So I ponder the riddles of ordinary human walking seen as Keystone Kop freneticism, or of reaching for a leaf at t’ai-chi speed as another creature’s perception of average pacing. So near into that skull of a distant mammalian relative; so far to know directly.
For my second Costa Rican favorite, I turned to the carrion-feeding raptors, particularly the turkey vulture. I united these birds with the maximally disparate sloths in my mind because both made me wonder so powerfully about “different worlds” in the heads of animals with lifestyles so starkly in contrast with our own choices and proclivities—the only world we can know directly. But I then discovered another connection quite unknown to me at the time. Both these creatures elicited maximal contempt from the greatest arbiter of historical taste—Georges Buffon. I have already quoted Buffon’s deprecations of sloths. Now consider his opinion of vultures:
The eagle attacks his enemies or his victims one on one . . . Vultures, on the other hand, join together in troops, like cowardly assassins, and would rather be robbers than warriors, birds of carnage rather than birds of prey. In this genus [vultures], there are those who gang up upon their prey, several upon one; and there are others intent only upon cadavers, which they rip apart down to the bones. Corruption and infection attract them, instead of repelling them . . . If we compare these birds to mammals, the vulture joins the force and cruelty of tigers with the cowardice and gluttony of jackals, which also unite in troops in order to devour carrion and tear apart cadavers. The eagle, on the other hand, has the courage, the nobility, the magnanimity and the munificence of the lion.
Buffon also tells us that we can easily distinguish vultures from eagles by the naked head and neck of the nasty carrion feeders versus the full feathering of the noble hunters. If this aristocratic French naturalist had known the supposed adaptive value of the vulture’s naked head, he would undoubtedly have demoted these birds even further in his estimation—for we now remember Buffon mostly for his celebrated motto “le style c’est l’homme même” (the style is the man himself). Vultures plunge their entire head deep into rotting corpses, and a conventional mat of feathers would soon become dangerously fouled, while gore does not adhere to the smooth and naked skin. To cite a standard source (Leslie Brown and Dean Amadon’s Eagles, Hawks, and Falcons of the World): “Without this denudation, the head feathers would become smeared and matted with gore and infection might occur.” (I have little sympathy for adaptationist scenarios in the “just-so story” mode, but this particular tale makes good sense to me, especially since the Old and New World vultures are not closely related by genealogy, but have independently evolved this highly localized loss of feathers—apparently for the same functional reason.)
Little about these birds could possibly be judged as pleasant in human terms. Of the turkey vulture, my Costa Rican source of observation, Buffon concluded in an adjectival frenzy: “They are voracious, cowardly, disgusting, odious and, as with wolves, just as noxious during their lives as they are useless after their death.” Consider the grandest of New World vultures, the great (and nearly extinct) California Condor, with maximal wingspan among all the world’s flying birds. I don’t wish to compromise the noble efforts now under way to save this magnificent species (for adherence to human ethical standards could scarcely be more irrelevant in our judgment of other animals), but descriptions of feeding condors can scarcely inspire any visceral affection.
In the standard source on condor behavior, written in the early 1950s before the population had declined so precipitously, Carl B. Koford describes how a group of condors rips and struggles so vigorously at a carcass that the whole complex (of feeding birds and dead food source) slides slowly downhill:
Carcasses up to the size of a deer are generally dragged downhill as the condors feed. Once I saw twenty condors feed on a young calf . . . Soon after vigorous feeding commenced the carcass moved down the slope steadily, attended by several struggling condors, until it was two hundred yards downhill from its original site.
In our current climate of emphasis upon “family values,” I won’t dwell upon details of their manner of feeding. Suffice to say that the hides of sheep, deer, and cattle (the major sources of larger carcasses) are hard to penetrate—and that condors therefore begin by ripping away at natural orifices, and sticking their smooth heads into the opening bounty.
But I would still give (almost) anything for sixty seconds inside a turkey vulture’s head. What does their world look like, as they circle silently above a carcass? What attracts them? What is their aesthetic? Does rot and corruption truly appeal—and, if so, the more the better, or only up to a certain point? Would I, the homunculus in the vulture’s brain, view (and smell) a dead cow on the plains as a human explorer might regard a pot of gold at the rainbow’s end, or an oasis in the desert?
As these questions emerged in Costa Rica, several thousand miles from my library, I did not realize that an old and substantial literature had developed on this very subject—or at least on the strictly limited and operational way that humans can approach such questions by probing from our restricted position outside the bird’s own conceptual world. In parti
cular, naturalists have long wondered and argued about how vultures find their prey.
This old issue immediately raises two questions that both set the puzzle and complexify the answer. First, birds, in general, are preeminently visual animals, particularly so for the raptors (eagles, hawks, and their relatives) that stand in close genealogical proximity to some vultures. But carrion might be found better by smell than by sight. Do vultures therefore use a most unbirdlike sense of smell to find their food? Second, as mentioned before, “vulture” is a functional term for large, carrion-feeding birds that have converged upon a set of common features from different genealogical roots. If we discover that one species can’t smell at all, we cannot conclude that another species (with a different evolutionary ancestry) might not use olfaction above all other senses.
The Old World vultures do, apparently, rely entirely upon sight. They take no notice of the most odoriferous parcel of deliciously rotting meat unless they can see the food. But some New World vultures do use smell as a primary sense. Debate has long centered upon the species I saw in Costa Rica, the turkey vulture Cathartes aura.
The argument goes back at least to Audubon, who, in 1826, read a technical paper before the Natural History Society of Edinburgh titled “Account of the habits of the turkey buzzard, particularly with the view of exploding the opinion generally entertained of its extraordinary power of smelling.” Audubon interpreted his ambiguous experiments as indicating that vultures could not smell, and located prey only with a keen sense of vision. He may have been correct for the species he studied—not the turkey vulture, as he thought and misidentified, but the black vulture, Coragyps. The issue therefore remained open for my Costa Rican species.
As criticism of Audubon mounted, his friend, the eminent American naturalist John Bachman, performed a second set of experiments, supposedly to confirm Audubon’s conclusion. He even gathered a group of learned and respectable citizens to observe his work and sign a document of assent (shades of Joseph Smith and official witnesses to the Mormon tablets).
Leonardo’s Mountain of Clams and the Diet of Worms Page 36
