Natural historians have dedicated themselves to the noble and fascinating task of trying to understand, in the deepest way accessible to us, the amazing variety of life on our planet. The best possible procedure immediately runs into Tennyson’s limit of proximity
with impossibility. I go eyeball to eyeball with some other creature—and I yearn to know the essential quality of its markedly different vitality. I cry to God the Gatekeeper of scientific knowability: Give me one minute—just one minute—inside the skin of this creature. Hook me for just sixty seconds to the perceptual and conceptual apparatus of this other being—and then I will know what natural
historians have sought through the ages.
But this god stays as silent as Baal, who would not answer the loud and fervent pleas of his 450 prophets, even when Elijah mocked them for the impotence of their deity. I can only look from the outside (or cut into the inside, but flesh and genes do not reveal organic totality). I am stuck with a panoply of ineluctably indirect methods—some very sophisticated
to be sure. I can anatomize, experiment, and infer. I can record reams of data about behaviors and responses. But if I could
be
a beetle or a bacillus for that one precious minute—and live to tell the tale in perfect memory—then I might truly fulfill Darwin’s dictum penned into an early notebook containing the first flowering of his evolutionary ideas during the late 1830s: “He who understands
baboon would do more towards metaphysics than Locke.”
Instead, we can only peer in from the outside, look our subject straight in the face, and wonder, ever wonder. Still, considering how far our methods must lie from the unreachable optimality of dwelling within, we have managed pretty well in a world without metempsychosis. Our indirect methods have taught us a mountain of things about horses,
but if you wished to learn even more, wouldn’t you rather be Whirlaway in the stretch, than interview Eddie Arcaro afterwards?
I came face to face (many times) with this old paradox during a recent trip to Costa Rica, a nation justly celebrated for its maximal attention among poor and tropical lands to the health and preservation of remaining natural environments—a position not only ethically
correct, but also potentially profitable both to a nation itself, and to all of us. Two Costa Rican animals cast a particularly enigmatic stare at me, and elicited the old frustrating thought that if I could only get inside their different world for a minute, I might understand. Small mammals and insects strike us as frenetic; some reptiles and amphibians seem overcome with torpor. But we are not
overwhelmed with the difference, if only because all these creatures vary their routines and paces: a squirrel can sit rigidly still, while an “immobile” frog catches insects on a lightning tongue.
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 conjecture 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.