• Goto NETFUTURE main page
  •                                  NETFUTURE
                        Technology and Human Responsibility
    Issue #97      A Publication of The Nature Institute      November 3, 1999
                Editor:  Stephen L. Talbott (stevet@netfuture.org)
                         On the Web: http://netfuture.org
         You may redistribute this newsletter for noncommercial purposes.
    NETFUTURE is a reader-supported publication.
    Editor's Note
       Technology and the Three-toed Sloth
    What Does It Mean to Be a Sloth? (Craig Holdrege)
       A study in wholeness
    About this newsletter
                                  EDITOR'S NOTE
    Technology and the Three-toed Sloth
    This issue of NETFUTURE is given completely over to an attempt to build up
    a picture of the sloth -- primarily the three-toed sloth.  (I told you to
    expect the unexpected!)  A strange development, you may well think.  What
    does the sloth have to do with technology?
    Everything.  One way to describe technology -- and at the same time to
    summarize almost all the content of NETFUTURE -- is to say that technology
    expresses our tendencies to fragment and lose sight of wholes.  The
    article below shows the necessary counterbalance to this tendency, since a
    proper understanding of the sloth -- as of any other organism -- requires
    us to grasp a whole.
    I suggest that you read the entire article while keeping in mind this
    statement that occurs toward the end:  "Every detail can begin to speak
    sloth".  Don't underestimate what a revolutionary claim this is.  It has
    no place in conventional science, which is distinguished by the search for
    non-qualitative parts that do not speak the whole.
    Physics, of course, is the final perfection of this fragmenting drive,
    and, classically, it gives us the ultimate fragments -- featureless
    particles that can be aggregated and articulated side by side, but can
    never rise to higher unities (never, at least, without our surreptitiously
    introducing unifying principles that violate our original methodological
    commitments).  The oxygen atom in me does not speak "man" any more than
    the oxygen atom in the sloth speaks "sloth".  This is no accident.  After
    all, the resolve at the outset was to arrive at concepts without qualities
    -- "hard", quantitative concepts -- and yet only qualities can speak
    What science does give us with great success is apparatuses that work --
    technologies.  The entire method is defined so as to achieve this -- and
    little more.  So it is that we confront in technology marvelous
    capabilities that somehow leave little room for "man" -- or for "sloth" or
    "tree" or anything else in the natural world that might demand respect for
    its own meaningful integrity.
    Much depends on our learning to understand ourselves, our society, and the
    world around us with the kind of organic vision that the author of the
    following article is striving toward, however distant the final goal.  It
    is a vision that requires a great deal of the reader.  In particular, it
    requires imaginative work.  An author can point us toward a whole, but in
    the end only our own imaginative effort can create the actual "picture" of
    a whole.
    Once we have gained such a picture, all attempts to re-engineer organisms
    genetically become highly problematic.  In the first place, one becomes
    aware that the organism will draw the introduced genetic material into its
    own context and set its own stamp upon it, just as it assimilates all the
    other elements it draws from its environment.  If the organism is a sloth,
    then in one way or another it will make the material speak "sloth", (even
    if, because of the engineer's invasive role, the speaking must be grossly
    pathological).  This is why genetic transfers between organisms present us
    with so many surprises.  We are not simply transferring well-defined
    mechanisms; we are touching another being.
    Perhaps more importantly, we also become aware of our ethical
    responsibility toward the organism.  We find ourselves caught up in an I-
    thou relationship with it.  To begin knowing the other in its integral
    wholeness -- in its inner being -- is to begin knowing one's own
    responsibility to it.  There are actions we can take consistent with this
    wholeness, and other actions destructive of it.  We must ask at every
    step:  do we have the wisdom to discern the difference?
    And finally:  there are two prevailing temptations today.  One is to lose
    the organism behind a veil of abstract, if precise, concepts.  The other
    is to falsify the organism through sentimental anthropomorphism.  The
    answer for both temptations is to look and see what is actually there in
    the fullest qualitative sense -- a return to an observational science that
    places extreme demands on us for discipline and objectivity.
    Such, anyway, is the grand hope.  But for now relax, take a break, and
    spend a little time getting to know one of your fellow creatures on this
    delicately balanced planet.
    Goto table of contents
                         WHAT DOES IT MEAN TO BE A SLOTH?
                                  Craig Holdrege
       "One more defect and they could not have existed."  (George Louis
       Leclerc, Comte de Buffon)
       "Hence we conceive of the individual animal as a small world, existing
       for its own sake, by its own means. Every creature is its own reason to
       be. All its parts have a direct effect on one another, a relationship
       to one another, thereby constantly renewing the circle of life; thus we
       are justified in considering every animal physiologically perfect."
       (Johann Wolfgang von Goethe)
    We are losing animals.  I do not mean only numerically through the
    extinction of species.  I also mean we are losing them in our
    understanding.  The poet and scientist Goethe, however, set the stage for
    a sound holistic approach to studying animals, and others have developed
    his method further (see NOTE at end).  This essay is an attempt to show
    how we can begin to grasp something of the organic lawfulness inherent in
    an animal.  And the sloth, with all its unique and unusual features,
    almost seems to be prodding us to understand it precisely in this way.
    The Sloth in its World
    Even if you were to look hard and make lots of noise, you would most
    likely not see the most prevalent tree-dwelling mammal in Central and
    South America's rain forests.  The monkeys scurry off and perhaps scream.
    The sloth remains still and hidden.
    The rain forest is an ecosystem characterized by constancy of conditions.
    Being a tropical ecosystem, the length of day and night during the year
    varies little.  On the equator there are twelve hours of daylight and
    twelve hours of night 365 days a year.  The sun rises at 6 am and sets at
    6 pm.  Afternoon rains fall daily throughout most of the year.  The air is
    humid (over 90%) and warm.  The temperature varies little in the course of
    the year, averaging 25 degrees C (77 degrees F).
    Except in the uppermost part of the forest canopy, it is dark in the rain
    forest.  Little light penetrates to the forest floor.  The uniformity of
    light, warmth and moisture -- in intensity and rhythm -- mark the rain
    forest.  And it is hard to imagine a rain forest dweller that embodies
    this quality of constancy more than the sloth.  From meters below, the
    sloth is sometimes described as looking like a clump of decomposing leaves
    or a lichen-covered bough.  The sloth's hair is long and shaggy, yet
    strangely soft.  The fur is brown to tan and quite variable in its mottled
    pattern.  Especially during the wettest times of year, the sloth is tinted
    green from the algae that thrive on its pelage, which soaks up water like
    a sponge (Aiello,1985).
    Since the sloth moves very slowly and makes few noises, it blends into the
    crowns of the rain forest trees.  It took researchers many years to
    discover that up to 700 sloths may inhabit one square kilometer of rain
    forest (Sunquist, 1986).  Only 70 howler monkeys inhabit the same area.
    The sloth spends essentially its whole life in the trees.  It hangs from
    branches by means of its long, sturdy claws, or sits nestled in the forks
    of tree branches.  The contrast to terrestrial mammals in respect to
    orientation is emphasized by its fur.  Instead of having a part on the
    mid-back, with the hair running towards the belly, as is typical for
    terrestrial mammals, the sloth's fur has a part on the mid-belly and the
    hair runs toward the back.
    The sloth moves slowly through the forest canopy -- from a few to rarely a
    few hundred feet in twenty-four hours.  Sloths were found to move on an
    average of seven to ten hours of the twenty-four-hour day (Sunquist and
    Montgomery, 1973).  The remaining time sloths are asleep or inactive.
    (Resting is the term often used to describe the sloth's inactive periods,
    but this isn't a sloth-appropriate expression.  From what activity is the
    sloth resting?)
    Limbs and Muscles
    The sloth's ability to hang from and cling to branches for hours on end is
    related to its whole anatomy and physiology.  The sloth is about the size
    of a large domestic cat.  It has very long limbs, especially the forelimbs
    (Figure 1).  When hanging, the sloth's body appears to be almost an
    appendage to the limbs.  Feet and toes are hidden in the fur.  Only the
    long, curved and pointed claws emerge from the fur.  The toe bones are not
    separately movable, being bound together by ligaments, so that the claws
    form one functional whole, best described as a hook.
    The three-toed sloth
    Figure 1. The three-toed sloth. (Author's drawing after a photograph in Beebe, 1926).
    The two different genera of sloths are named according to the number of claws they possess: the three-toed sloth (Bradypus) has three claws on each limb; the two-toed sloth (Choloepus) has two claws on the forelimb and three on the hind limb. (There are many differences in detail between these two groups of sloths. Most of the specific information referred to in this essay pertains to the three-toed sloth, unless otherwise indicated.) With its long limbs the sloth can embrace a thick branch or trunk, while the claws dig into the bark. But the sloth can also hang just by its claws on smaller branches, its body suspended in the air. A sloth can cling so tenaciously to a branch that researchers resort to sawing off the branch to bring the creature down from the trees. All body movements, or the holding of a given posture, are made possible by muscles, which are rooted in the bones. Muscles work by means of contraction. While clinging, for example, some muscles in the limbs -- the retractor muscles -- are contracted (think of your biceps) while other muscles -- the extensor muscles -- are relaxed (think of your triceps). When a limb is extended (when the sloth reaches out to a branch) the extensor muscles contract, while the retractor muscles relax. All movement involves a rhythmical interplay between retractor and extensor muscles. It is revealing that most of a sloth's skeletal musculature is made up of retractor muscles (Goffart, 1971; Mendel, 1985a). These are the muscles of the extremities that allow an animal to hold and cling to things and also to pull things toward it. The extensor muscles are smaller and fewer in number. In fact, significant extensor muscles in other mammals are modified in the sloth and serve as retractor muscles. A sloth can thus hold its hanging body for long periods of time. It can even clasp a vertical trunk with only the hind limbs and lean over backward ninety degrees with freed forelimbs. As the sloth expert M. Goffart points out, "in humans this feat is exceptional enough to be shown in a circus" (Goffart, 1971, p.75). At home as it is in the trees, the sloth is virtually helpless on the ground. Lacking necessary extensor muscles and stability in its joints, a sloth on the ground can hardly support its weight with its limbs. Researchers know little about natural terrestrial movement of sloths. But on rough surfaces captive sloths have been observed slowly crawling (Mendel, 1985b). If they are placed on a smooth surface like concrete, their limbs splay to the side. In this position a sloth can only drag its body by finding a hold with the claws of its forelimbs and pulling itself forward, using its strong retractor muscles. Since the sloth's main limb movements involve pulling and the limbs do not carry the body weight, it is truly a four-armed and not a four-legged mammal. The dominance of the arms can be seen in the fact that the hand and feet do not develop as independent organs (like they do in monkeys, for example), but are essentially a continuation of the long limb bones, ending in the elongated claws. We can also understand the dominance of the retractor muscles from this point of view. The human being, in contrast to most mammals, has arms as well as weight-bearing legs. The arms are dominated by retractor muscles, while the legs have more extensor muscles. Moreover, the arm muscles that move the arm toward the body are stronger than the antagonistic arm muscles that move the arms away from the body. This comparison shows us that the tendency inherent in the arm -- the limb that does not carry body's weight -- permeates the anatomy of the sloth. A sloth becomes quite agile if the forces of gravity are reduced, as in water. In water a body loses as much weight as the weight of the volume of water it displaces (Principle of Archimedes). The body becomes buoyant, and in the case of the sloth, virtually weightless. Remarkably, sloths are facile swimmers. But they manage to move across water with little apparent effort. Where the forest canopy is interrupted by a river or lake, sloths often paddle to new feeding grounds. With no heavy mass to weigh them down, they float on their buoyant, oversized stomachs. (Sunquist, 1986, p. 9) With its long forelimbs the sloth pulls its way through the water, not speedily, but in a "beautifully easy going" manner (Bullock, quoted in Goffart, 1971, p. 94). On the whole, sloths have little muscle tissue. Muscles make up 40 to 45 percent of the total body weight of most mammals, but make up only 25 to 30 percent in sloths (Goffart, 1971, p. 25). One can understand how the reduction of weight in water allows them to be less encumbered in movement. Sloth muscles also react slowly, the fastest muscles contracting four to six times slower than comparable ones in a cat. In contrast, however, a sloth can keep its muscles contracted six times longer than a rabbit (Goffart, 1971, p. 69). Such anatomical and physiological details reflect the sloth's whole way of being -- steadfastly clinging in a given position, only gradually changing its state. The tendency to the reduction of muscle tissue can also be found in the head. There is a reduction in the number and complexity of facial muscles (Naples, 1985). Through the facial markings the sloth has an expressive face, but this is the expression of a fixed image, rather than expression through movement, since the facial area itself is relatively immobile. The outer ears are tiny and are essentially stationary. The sloth alters the direction of its gaze by moving its head, not its eyeballs. This rather fixed countenance is dissolved at the lips and nostrils, which are quite mobile. Paced Metabolism and Fluctuating Body Temperature ------------------------------------------------- Since sloths are externally inactive or asleep a good portion of the twenty-four-hour day and the remaining time is spent slowly moving and feeding, they perform about ten percent of the physiological work of a mammal of similar size (Goffart, 1971, p. 59). All metabolic processes are markedly measured in tempo. Sloths use little oxygen, breathe slowly, and the respiratory surface of their lungs is small. All metabolic activity produces warmth. Warmth is also needed for activity, for example, in the exertion of muscles, which in turn results in more warmth production. Birds and virtually all mammals not only produce warmth, but also maintain a constant body temperature. This is a striking physiological feat. A warm-blooded (endothermic) animal is like a radiating, self-regulating center of warmth. Warmth constantly permeates the whole organism. Most mammals maintain a constant core body temperature of about 36 degrees C (98 degrees F), which changes very little despite variations in environmental temperatures. For example, in a laboratory experiment a mouse's internal temperature changes only four tenths of one degree Celsius when the outer temperature rises or falls twelve degrees (Bourlie`re, 1964). Exceptionally, however, a sloth's body is not so permeated by warmth; in other words, it is not constantly prepared for activity. Its body temperature can vary markedly. Gene Montgomery and Mel Sunquist, who did extensive field research in Panama on the ecology and physiology of sloths, found that the sloth's body temperature fluctuated with the ambient temperature (Montgomery and Sunquist, 1978). During the morning as the ambient temperature rises, the body temperature rises. When found on sunny days, sloths were often on an outer branch, belly-side up and limbs extended, basking in the sun. Body temperature usually peaks at about 36-38 degrees C soon after midday. It then begins to fall, reaching a low point of about 30-32 degrees C in the early morning. The body temperature is usually about 7-10 degrees C higher than the ambient temperature. Although sloths are often active at night, their body temperature does not rise in connection with their increased activity. This shows, in contrast to other mammals, that the sloth's body temperature is less affected by its own activity than by the ambient temperature. As Brian McNab (1978) puts it, the sloth "almost appears to regulate its rate of metabolism by varying body temperature, whereas most endotherms [warm-blooded animals, i.e., mammals and birds] regulate body temperature by varying the rate of metabolism." Raising the outer temperature under experimental conditions is, as Goffart puts it, an efficient way of "'deslothing' the sloth," since it then moves around more actively. But if its temperature remains at 40 degrees C for too long, it can prove fatal. A three-toed sloth has an insulating coat of fur comparable to that of an arctic mammal, which seems at first rather absurd for a tropical animal. It has, like an arctic fox, an outer coat of longer, thick hair and an inner coat of short, fine, downy fur. These allow the sloth to retain the little warmth it creates through its metabolic processes. But, characteristically, the sloth cannot actively raise its body temperature by shivering as other mammals do. Shivering involves rapid muscle contractions that produce warmth. Clearly, the sloth is at home in the womb of the rain forest, which keeps constant conditions like no other terrestrial ecosystem. Not only by virtue of its coloring and inconspicuous movements does the sloth blend into its environment, but through its slowly changing body temperature as well. Feeding and Orientation ----------------------- Moving unhurried through the crown of a tree, the sloth feeds on foliage. We usually think of leaf eating as an activity done on the ground by mammals, for example, deer. There are, in fact, relatively few leaf- eating mammals in the crowns of trees, and the sloth epitomizes them. Leaves are an abundant and constant source of food, and plants need not be chased down. Sloths are literally embedded in and surrounded by their food at all times and in all directions. Tropical trees do lose their leaves, but not all at once. Sometimes one and the same tree may lose leaves on one branch, while it sprouts new ones on others. Sloths don't eat just any leaves. They seem to prefer younger leaves, and each individual animal has its own particular repertoire of about 40 tree species from which it feeds (Montgomery and Sunquist, 1978). A young sloth feeds together with its mother, often licking leaf fragments from the mother's lips. After its mother departs the juvenile at the age of about six months, the young sloth continues to feed from those species it learned from its mother. This specificity is probably a major factor in the inability to keep three-toed sloths alive in zoos. They usually die of starvation after a short period of time. In contrast, the two-toed sloth is more flexible and survives well in captivity, eating assorted fruits and leaves. A sloth does not search for leaves with its eyes. Its eyesight is very poor and it is short-sighted (Goffart, 1971, p. 106ff.; Mendel et al., 1985b). The eyes lack the tiny muscles that change the form of the lens to accommodate for changing distances of objects. As if to emphasize the unimportance of its eyes, the sloth can retract them into the eye sockets. The pupils are usually tiny, even at night. Clearly, a sloth does not actively permeate its broader environment with its vision, as do most arboreal mammals like monkeys. Sight and hearing (the latter also not very developed in sloths) are the two senses through which animals perceive and react to stimuli at a distance. The sloth makes little use of these senses, relying much more on a sense that entails drawing the environment into itself: the sense of smell. I placed a sloth, hungry and not too disturbed, on an open area under the bamboos, and planted four shoots twenty feet away in the four directions of the compass. One of these was Cecropia [a primary food of three-toed sloths] camouflaged with thin cheesecloth, so that the best of eyesight would never identify it, and placed to the south, so that any direct wind from the east would not bring the odor too easily. The sloth lifted itself and looked blinkingly around. The bamboos thirty feet above, silhouetted against the sky, caught its eye, and it pitifully stretched up an arm, as a child will reach for the moon. It then sniffed with outstretched head and neck, and painfully began its hooking progress toward the Cecropia .... Not only is each food leaf tested with the nostrils, but each branch.... (Beebe, 1926, p. 23) So we should not imagine a sloth looking at its food. Rather, a sloth immerses and orients itself in a sea of wafting scents. When the sloth is in the immediate proximity of leaves it feeds on, it will hook the branch with the claws of a fore- or hind limb and bring the leaves to its mouth. Having no front teeth (incisors), it tears off the leaves with its tough lips. It chews the leaves with its rear peg-like teeth. Unlike most leaf-eating mammals (for example, deer), the sloth lacks many deeply rooted, hard, enamel-covered grinding teeth. The sloth also has comparatively few teeth (18 compared to 32 in most deer). Moreover, the teeth lack enamel altogether and wear easily. In compensation, the teeth grow slowly throughout the animal's life. There is no change of teeth from milk to permanent dentition. Growth and wear are in balance. While feeding, the sloth is continuously chewing and simultaneously moving food backward with its large tongue in order to swallow. Sloths can feed in all positions, even hanging upside down. A young, captive two-toed sloth showed "decided preference for eating upside down in the manner of adult sloths at eight months" (Goffart, 1971, p. 114). The sloth can move its head in all directions, having an extremely flexible neck. Imagine a sloth hanging from all four legs on a horizontal branch. In this position the head looks upward (like when we lie in a hammock). Now the sloth can turn its head -- without moving the body -- 180 degrees to the side and have its face oriented downwards. As if this were not enough, the sloth can then move its head vertically and face forward -- an upright head on an upside down body (Figure 2)! When it sleeps, a sloth can rest its head on its chest. The three-toed sloth Figure 2. The three-toed sloth. Note the orientation of the head. (Author's drawing after a photograph in Grzimek, 1975.)
    The sloth's neck is not only unique in its flexibility, but also in its anatomy. Mammals have seven neck (cervical) vertebrae. The long-necked giraffe and the seemingly neckless dolphin -- to mention the extremes -- both have seven cervical vertebrae. This fixed mammalian pattern is abandoned by only the sloth and the manatee. The three-toed sloth usually has nine and the two-toed sloth has between six and nine cervical vertebrae. Centered in its Stomach ----------------------- Digestion in the sloth occurs at an incredibly slow rate. In captive animals "after three or six days of fasting the stomach is found to be only slightly less full" (Britton, 1941). Leaves are hard to digest and not very nutrient-rich, consisting primarily of cellulose and water. Only with the help of microorganisms in the stomach can the sloth digest cellulose, breaking it down into substances (fatty acids) that can be taken up by the blood stream. The sloth's stomach is four-chambered like those of ruminants (cows, deer, and so on) and is clearly the center of the digestive process. The stomach is enormous relative to the animal's overall size. It takes up most of the space of the abdominal cavity and, including contents, makes up 20 to 30 percent of total body weight. Nonetheless, digestion takes a long time. On the basis of field experiments, Montgomery and Sunquist (1978) estimate that it takes food about ten times longer to pass through a sloth than through a cow. Moreover, the sloth also digests less of the plant material than most other herbivores. Through its stomach a mammal senses hunger. Most grazing mammals spend a large part of their time eating, so that food is continuously passing through their digestive tract. The sloth is, once again, an atypical herbivore since it feeds for a comparatively small portion of its day. A small rain forest deer, the same size as a sloth, ate six times as much during the same period of time (Beebe, 1926). The howler monkey, which also lives in the canopies of Central and South American rain forests and whose diet comprises only about 50% leaves, eats about seven times as many leaves as do sloths. With its slow metabolism and digestion, the sloth's stomach remains full, although the animal eats so little. As a stark contrast, we can think of carnivores like wolves or lions that regularly, as a normal part of their lives, experience empty stomachs. Their hunting drives are directly connected to their hunger. Hunger brings about the maximal aggressive activity of these animals. When a lion has gorged itself on forty pounds of meat, it becomes lethargic and sleeps for an extended period of time. The sloth's constantly full stomach is a fitting image for its consistently slow-paced life as well as, it seems, a physiological condition for it: "starvation makes [sloths] hyperactive" (Goffart, 1971 p. 113). After about a week of feeding, sleeping and external inactivity, a change occurs in the sloth's life. It begins to descend from its tree. Having reached the forest floor, it makes a hole in the leaf litter with its stubby little tail. It then urinates and defecates, covers the hole, and ascends back into the canopy, leaving its natural fertilizer behind. (The two-toed sloth has no tail and leaves its feces lying on the leaf litter.) Characteristically, the feces, the product of sloth metabolism, decompose very slowly. The hard pellets can be found only slightly decomposed six months after defecation. Normally, organic material decomposes rapidly in the warm and moist conditions of the rain forest. For example, leaves decompose within one or two months (a process that can take a few years in a temperate-climate forest). Ecologically, sloth feces "stands out as a long-term, stable source [of nutrients] ... and may be related to stabilizing some components of the forest system .... Sloths slow the normally high recycling rates for certain trees...." (Montgomery and Sunquist, 1975, p. 94). Sloths contribute not only slow movement to the rain forest but slow decomposition as well! It is estimated that a sloth can lose up to two pounds while defecating and urinating, more than one fourth of its total body weight (Goffart, 1971, p. 124). If one imagines a sloth with a full stomach (which it always seems to have) just prior to excreting, then more than half of its body weight is made up of its food, waste and digestive organs! This quantitative consideration points to the qualitative center of gravity in the animal's life. But the sloth's stomach is more like a vessel that needs to remain full than a place of intensive muscular activity, secretion, mixing and breaking down, as it is in the cow, for example. Stretching Time --------------- The sloth researcher William Beebe wrote in 1926: "Sloths have no right to be living on this earth, but they would be fitting inhabitants of Mars, whose year is over six hundred days long." Beebe was deeply impressed by the way in which sloths "stretch" time, another way of characterizing their slowness. We have seen how this quality permeates every fiber of their day-to-day existence. It is therefore not so surprising to find that the development of sloths takes a long time. Sloths have a gestation period of four to six months, compared to a little over two months in the similar-sized cat. One two-toed sloth in a zoo gave birth after eight-and-one-half months. Initially more surprising was the rediscovery of a female sloth in the rain forest 15 years after she had been tagged as an adult. This means she was at least 17 years old, "an unusually long life span for such a small mammal" (Montgomery, quoted in Sunquist, 1986). Thus, regarding time, the qualities of the sloth certainly speak a unified language. Gravity and the Skeleton ------------------------ If we look for the embodiment of fixed form in the organ systems of a mammal, then we come to the skeleton. The bony skeleton gives the mammal its basic form and is the solid anchor for all movement. The limb bones develop their final form in relation to both gravity and their own usage. An injured quadruped mammal will lose bone substance in the leg it is not using, which does not carry any weight. Conversely, in the other three limbs bone matter is laid down to compensate for the increase in weight carried and muscular stress. The sloth has a special relation to gravity. As mentioned earlier, the limbs hold the hanging body; they do not carry it (Figure 3). The sloth gives itself over to gravity rather than resisting it and living actively within it via its skeletal system. A sloth kept on the ground in a box developed raw feet from the unaccustomed pressure (Beebe, 1926).
    Skeleton of a three-toed sloth
    Figure 3. Skeleton of a three-toed sloth (from Young, 1973).
    The other pole in relation to gravity is represented by hoofed mammals like deer, horses or giraffes. By virtue of their skeletal architecture they can relax their muscles and even sleep while standing. Their legs are solid, stable columns that carry the body's weight (Figure 4). In contrast, the sloth has very loose limb joints. In his detailed study of the limbs of the two-toed sloth, Frank Mendel (1985a, p.159) points out how unusual the "poorly reinforced and extremely lax joint capsules" are. This anatomical peculiarity allows a wide range of limb movement and is connected with the fact that the joints are not subject to compression as they are in weight-bearing limbs. Through clinging and hanging, the joints of a sloth are being continually stretched. Similarly, the sloth has a flexible, curved spine. The hoofed mammal, in contrast, has a stiff, straight spine, from which the rib cage and internal organs of the torso are suspended. A deer would be as ungainly in a tree as a sloth is on the ground.
    Skeleton of a horse
    Figure 4. Skeleton of a horse (from Tank, 1984).
    This contrast is mirrored in the teeth. Hoofed mammals have deeply rooted, very hard teeth with ridges of enamel that withstand the toughness of grass. Enamel is the hardest substance a mammal can produce, and, as already mentioned, sloth teeth have no enamel coating. In addition, more than in other mammals, the form and chewing surfaces of the teeth are sculptured during usage. "Since sloth teeth acquire their individual characteristics through wear, it is very difficult to distinguish the young of one genus from those of another based upon shape or location of dentition" (Naples, 1982 p. 18). In other mammals -- especially the grazers -- the teeth are preformed with all their crown cusps and ridges before they erupt. The sloth's teeth emerge as simple cones and take on a characteristic form in the course of life. The sloth is, in this sense, formed from the outside. In a related way we see this tendency in its coloring, which arises not only from hair pigmentation but also through algae from the surroundings. Similarly, its temperature varies with the ambient temperature. From a different vantage point we can say: incorporating solidity and stability into the skeleton allows a quadruped mammal to live actively within gravitational forces. In giving itself over to gravity, the sloth incorporates inertia. We see inertia in its movements and digestion. The sloth is a bit like the clump of leaves or the alga-covered tree trunk it outwardly resembles. Drawing In ---------- Active arboreal mammals, like monkeys, have, of course, nothing of the skeletal rigidity of ground-dwelling quadrupeds. They have flexible joints and muscular agility that allow for actively swinging, jumping, and grasping. A sloth lacks the quick and nimble dexterity of monkeys, although it possesses a flexible spinal column (especially in the neck region) and limber fore- and hind limbs. A sloth can twist its forelimb in all directions and roll itself into a ball by flexing its vertebral column. Characteristically, the sloth makes use of this flexibility for its slow movements while feeding and also for protecting itself from a predator by curling up into a ball. The monkey, in contrast, engages in light and springy movements. This leads us to a slightly different way of characterizing the sloth. A primary gesture is that of pulling in or retracting. It doesn't project actively out into its surroundings. We can see this tendency in the head. The head is the center of the primary sense organs through which an animal relates to its environment. As we have seen, the eyes and ears are not the sloth's main senses. The outer ears (pinnae) are tiny and hardly visible on the head and the eyes can retract in their sockets. Both of these characteristics reveal externally the functional status of these organs within the whole animal. They also let the head appear as a broadened neck. But this appearance also has a deeper anatomical basis, since the first cervical vertebra (the so-called atlas) is nearly as wide as the widest part of the skull. The skull itself is rounded and self-contained -- superficially resembling a monkey's skull more than a grazing herbivore's (Figure 5). Most herbivores have an elongated snout that they use as a limb -- standing as they do on all four legs -- to reach their food. The sloth's forelimbs have this function and thus its snout is short. The premaxillary bones -- important in forming the elongate mammalian snout -- are tiny in the sloth. Moreover, the upper jawbones (maxillae) and the nasal bones are also short in the sloth. The sloth's skull does not project forward. Skulls of monkey, sloth, and horse Figure 5. Skulls of a monkey (top, left), three-toed sloth (top, right), and horse (bottom). (Author's drawings.)
    We have seen that the sense of smell is the sloth's primary sense and that its gesture is to draw in, in contrast to the more outwardly projecting senses of sight and hearing. When we see these facts together with the others, such as the dominance of retractor muscles, then the sloth's special orientation to its surroundings comes more clearly into view. The Sloth as a Habitat ---------------------- As if to emphasize its passive, somewhat withdrawn character, the sloth functions as a habitat for myriad organisms. I have mentioned the algae that live in its fur, giving the pelage a greenish tinge. In addition to the usual ticks and flies that infest the skin and fur of other mammals, a number of sloth-specific moth, beetle, and mite species live on the sloth and are dependent upon it for their development. The sloth moths and beetles live as adults in the sloth's fur. Some species live on the surface and others inhabit the deeper regions of the fur. They are evidently not parasitic; their source of food is unknown. When the sloth descends from a tree to defecate and urinate, female moths and beetles fly off the animal and lay their eggs in the sloth's dung. The wings of one moth species break off soon after they inhabit the sloth, so that they are incapable of flying. Consequently they must crawl off the sloth to reach the dung. The sloth's relatively long period of defecation, which lasts a few minutes, gives the insects the time they need. In this way the slowness of the sloth serves these most "slothful" of sloth moths! The larvae develop in and feed on the dung (which, you remember, decomposes slowly). The larvae pupate in the dung and the winged adult moths (or beetles) fly off to inhabit another sloth. Various species of insects and mites inhabit any given sloth, and the numbers of specimens of each species varies greatly, ranging from a few to over a hundred. The sloth has been observed grooming its fur. This is typical mammalian behavior and does rid an animal of some of its "pests." From this utilitarian point of view, the sloth's grooming is not very effective. Typically, sloths groom slowly, and sloth moths "may be seen to advance in a wave in front of the moving claws of the forefoot, disturbed, but by no means dislodged from the host" (Waage and Best, 1985 p. 308). Clearly, the measured pace of life, the unique excretory habits, and the consistency of dung allow the sloth to be a unique habitat for such a variety of organisms. Sensing a Boundary ------------------ The expression of pain is a barometer for the way an animal experiences its own body in relation to the environment. Pain is one way an animal experiences the external world penetrating and harming its biological integrity. The researcher Hermann Tirler, who kept sloths at his home in Brazil's rain forest, reports: One evening it smells like a burned sloth, the burning odor coming from the adjoining room. Lo and behold, a dozing sloth is sitting on the light bulb of a lamp. Its behind is burning and smoking. But it remains sitting. I take it down, but it wants to remain on the bulb, clinging to the lamp and crying "aheeeee" in protest. (Quoted in Grzimek, 1975, p. 172; translated by CH) Sloths are reported to "survive injuries that would be deadly within a short time to other mammals" (ibid.). "I have known a sloth to act normally for a long time after it had received a wound which practically destroyed the heart..." (Beebe, 1926 p. 32). These examples show that the sloth does not seem to notice an intrusion of its boundaries and continues to live despite them. Its body is not imbued with sensitive reactive presence. A Summarizing Sketch -------------------- Let us return to the image of a sloth, high in the crown of a rain-forest tree, hanging from or nestled on a branch. In its outer aspect, the sloth blends in with its environment. There are no sudden or loud movements. The sloth's green-tinged, mottled brown coat lets it optically recede into the wood and foliage of its surroundings. And like the tree bark, the sloth's fur is teeming with insect life. The sloth's body temperature rises and sinks with the ambient temperature. The round form of its head is the anatomical image of the way in which the sloth does not actively project into its environment. There are no large, movable, reactive outer ears and the eyes are rarely, if ever, moved. The sloth has no protruding snout. It draws the scents of the environment, especially of the leaves it feeds upon, into its nose. But much of the day the sloth is curled up, unaware of the world around it. Even when awake, the sloth seems not to live as intensely in its body as other mammals, being quite insensitive to pain. The sloth does not carry its own weight; rather, it clings to an outer support. Its skeletal system is not characterized by stability, but by looseness. This laxity allows the sloth to adopt positions that would be contortions in other animals. The sloth makes mostly steady pulling movements with its long limbs, a capacity based on the dominance of retractor muscles. The sloth moves slowly through the crowns, feeding on the leaves that surround it from all sides, bathing, as it were, in its food source. The leaves pass through the animal at an almost imperceptibly slow rate. The sloth's stomach is always filled with partially digested leaves. Its dung disappears slowly, despite the warm and humid rain forest climate that normally accelerates decomposition processes. The sloth brings slowness into the world. This is not only true of its reactions, movements and digestion. It also develops slowly in the womb and has a long life span for a mammal of its size. Encircling the Unspeakable: The Animal as a Whole ------------------------------------------------- "To express the being of a thing is a fruitless undertaking. We perceive effects and a complete natural history of these effects at best encircles the being of a thing. We labor in vein to describe a person's character, but when we draw together actions and deeds, a picture of character will emerge." (Goethe, 1988, p. 121; translation modified by CH) "The being is not behind its manifestations; it reveals itself through the manifestations." (Steiner, 1994, p. 96; translation by CH) "The way to the whole is into and through the parts. The whole is nowhere to be encountered except in the midst of the parts." (Bortoft, 1996, p. 12) I have been painting a picture of the sloth. This picture does not and cannot encompass the totality of its characteristics. One can always discover new details. I am not striving for totality, but rather for wholeness. Our understanding hinges on our ability to overcome the isolation of separate facts and to begin to fathom the animal as a whole, integrated organism. When we begin to see how all the facets of the animal are related to each other, then it comes alive for us. Or, putting it a bit differently, the animal begins to express something of its life in us. Every detail can begin to speak "sloth," not as a name, but as a qualitative concept that a definition can do little justice to. The whole is elusive, and yet, at every moment, potentially standing before the mind's eye. I have tried to describe the sloth in a way that allows us to catch glimpses of its wholeness. I can now refer to such characteristics as slowness, inertia, blending in with the environment, receding or pulling in and not actively projecting outward. Each expression is a different way of pointing to the same coherent whole. Taken alone, as abstract concepts or definitions, they are empty. They are real only inasmuch as they light up within the description or perception of the animal's characteristics. But they are not things like a bone or an eye. They are, in context, vibrant concepts that reveal the animal's unique way of being. We can now return to the statements quoted at the beginning of this essay: One more defect and they could not have existed. (George Louis Leclerc, Comte de Buffon; quoted in Beebe, 1926) Hence we conceive of the individual animal as a small world, existing for its own sake, by its own means. Every creature is its own reason to be. All its parts have a direct effect on one another, a relationship to one another, thereby constantly renewing the circle of life; thus we are justified in considering every animal physiologically perfect. (Goethe, 1988, p. 121) Buffon was a well-known 18th-century French scientist. He studied many animals, among them the sloth. His statement refers, of course, to the sloth. He considered their characteristics to be defects. And they are, if you take the point of view of a horse, eagle, jaguar, or human being. As Beebe put it, "a sloth in Paris would doubtless fulfill the prophecy of the French scientist, but on the other hand, Buffon clinging upside down to a branch of a tree in the jungle would expire even sooner" (Beebe, 1926 p. 13). Buffon takes a standpoint outside the animal. I have followed Goethe's suggestion and tried to view the sloth on its own turf. I have made use of comparison, but not to describe what the sloth "should" have in order to be a reasonable animal. The animals described by way of comparison shed light on the sloth, allowing its uniqueness, its perfection, to stand out all the more perceptibly. Viewed in its own terms, each animal is perfect, but the perfection of the sloth is not the perfection of the elephant or mole. And perfection means here not the quasi-eternal, non- changing perfection of a crystal, but the perfection of a living, sentient being in constant interaction with its environment. It in no way precludes evolution. Is There a Cause of Slothfulness? --------------------------------- In his compendium on sloths (1971), M. Goffart includes one section entitled "Slothfulness." He describes observations in the field, experimental results, and the hypotheses of scientists concerning the causes of slothfulness. Various possible explanations are brought forth: small heart, slow speed of muscle contraction, low body temperature, low rate of thyroid function, and so forth. He describes the shortcomings of each particular hypothesis and concludes that the "evidence as to the real causes of slothfulness is thus far from complete" (p. 95). Goffart points out, for example, that the sluggish koala has a constant body temperature of 36 degrees Celsius. Since this is a normal body temperature for mammals, it seems evident that it cannot be causing the koala's sluggishness. Since causes are assumed to be general, he concludes that temperature will also not be the cause of slothfulness in sloths. Goffart assumes that the causes of slothfulness will one day be found; we are just lacking the necessary information. I question this assumption and believe that such an example shows, in fact, primarily the limitations of the conceptual framework. In treating aspects of an organism as potential causes, we conceptually lift them out of the organism. Then we think of them affecting things in the organism as though they were not part of it. By so doing we can think in general terms of the factor "body temperature" as a cause, as if separate from the organism. But every time we carry through this process we realize that our conceptual scheme doesn't fit reality, because we are confronted with mutual relations, all of which express something of the animal as a whole. If we drop this scheme, then it becomes interesting that body temperature evidently means two very different things in the koala and the sloth. Instead of looking for physiological causes that we assume have general validity, we look at the unique expression of physiological facts in the given context. We take the unique integrity of the animal seriously. It is second nature for a scientist to inquire after the causes of what is under investigation. Some would even say this is the task of science. But in the context of organisms this method alone is not adequate. Putting it a bit radically, biologists would do well to eradicate the term "cause" from their vocabulary and use the more modest and open term "condition". What physiological and ecological studies can show is how aspects of an organism provide mutual and changing conditions for each other. This knowledge is extremely valuable as long as we don't separate it from the organism as a whole. In fact, it can be the gateway to understanding the organism as an integrated whole. NOTE: For general expositions of Goethe's method see: Bortoft, 1996; Goethe, 1988; Steiner, 1988. For the zoological application of a holistic methodology see: Portmann, 1967; Schad, 1977; Schad (ed.), 1983; Riegner, 1985 and 1993; Kranich, 1995; Holdrege, 1998. Acknowledgments --------------- This is a revised version of an article that appeared in the Newsletter of the Society for the Evolution of Science, Vol. 14, No. 1 (Winter, 1998). My work has been made possible by grants from Future Value Fund, the Mahle-Stiftung, the Rudolf Steiner Charitable Trust, and the Waldorf School Funds. I am deeply grateful for their support of my research activities. References ---------- Aiello, Annette. 1985. Sloth hair: unanswered questions. In The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas, ed. G. Gene Montgomery. Washington: Smithsonian Institution Press. Beebe, William. 1926 The three-toed sloth. Zoologica VII:1-67. Bortoft, Henri. 1996. The Wholeness of Nature. Hudson, NY: Lindisfarne Press. Bourli`ere, Fran,cois. 1964. The Natural History of Mammals. New York: Alfred A. Knopf. Pp. 252 ff. Britton, W.S. 1941. Form and function in the sloth. Quarterly Review of Biology 16:13-43, 190-207. Goethe, J.W. von. 1988. Scientific Studies, ed. & transl. Douglas Miller. New York: Suhrkamp. Goffart, M. 1971. Form and Function in the Sloth. Oxford and New York: Pergamon Press. Grzimek, Bernhard. 1975. Grzimek's Animal Life Encyclopedia, Vol. 11 (Mammals II). New York: Van Nostrand Reinhold Company. Holdrege, Craig. 1996. Genetics and the Manipulation of Life: the Forgotten Factor of Context. Hudson, New York: Lindisfarne Press. Holdrege, Craig. 1998. Seeing the animal whole: the example of horse and lion. In Goethe's Way of Science: Toward a Phenomenology of Nature, ed. D. Seamon and A. Zajonc. Albany, NY: SUNY Press. Kranich, Ernst-Michael. 1995. Wesensbilder der Tiere. Stuttgart: Verlag Freies Geistesleben. McNab, Brian K. 1978. Energetics of arboreal folivores: physiological problems and ecological consequences of feeding on an ubiquitous food supply. Pp. 153-162 in The Ecology of Arboreal Folivores, ed. G. Gene Montgomery. Washington D.C.: Smithsonian Institution Press. Mendel, Frank C. 1985a. Adaptations for suspensory behavior in the limbs of two-toed sloths. Pp. 151-162 in The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas, ed. G. Gene Montgomery. Washington: Smithsonian Institution Press. Mendel, Frank C. 1985b. Use of hands and feet of three-toed sloths (Bradypus variegatus) during climbing and terrestrial locomotion. Journal of Mammalogy 66:359-366. Mendel, Frank C. et al. 1985. Vision of two-toed sloths (Choloepus). Journal of Mammalogy 66: 197-200. Montgomery, G.G. and M.E. Sunquist. 1975. Impact of sloths on neotropical forest energy flow and nutrient cycling. Pp. 69-98 in Tropical Ecological Systems, ed. Frank. B. Golley and Ernesto Medina. New York: Springer Verlag. Montgomery G. Gene and M.E. Sunquist. 1978. Habitat selection and use by two-toed and three-toed sloths. Pp. 329-359 in The Ecology of Arboreal Folivores, ed. G. Gene Montgomery. Washington D.C.: Smithsonian Institution Press. Naples, Virginia L. 1982. Cranial osteology and function in the tree sloths, Bradypus and Choloepus. American Museum Novitates 2739:1-41. Naples, Virginia L. 1985. The superficial facial musculature in sloths and vermilinguas (anteaters). Pp. 173-189 in The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas, ed. G. Gene Montgomery. Washington: Smithsonian Institution Press. Portmann, Adolf. 1967. Animal Forms and Patterns. New York: Schocken Books. Riegner, Mark. 1985. Horns, hooves, spots, and stripes: form and pattern in mammals. Orion Nature Quarterly 4:22-35. Riegner, Mark. 1993. Toward a holistic understanding of place: reading a landscape through its flora and fauna. In Dwelling, Seeing and Designing: Toward a Phenomenological Ecology, ed. D. Seamon. Albany, NY: SUNY Press. Schad, Wolfgang. 1977. Man and Mammals: Toward a Biology of Form. Garden City, NY: Waldorf Press. Schad, Wolfgang (ed.). 1983. Goetheanistische Naturwissenschaft, Bd. 3: Zoologie. Stuttgart: Verlag Freies Geistesleben. Steiner, Rudolf. 1988. Goethean Science. Spring Valley, NY: Mercury Press. Steiner, Rudolf. 1994. Theosophy. Hudson, NY: Anthroposophic Press. Sunquist, Fiona. 1986. Secret energy of the sloth. International Wildlife 16: 6-10. Sunquist, M.E. and G.G. Montgomery. 1973. Activity patterns and rates of movement of two-toed and three-toed sloths (Choloepus hoffmanni and Bradypus infuscatus). Journal of Mammalogy 54:946-954. Tank. W. 1984. Tieranatomie fuer Kuenstler. Ravensburg, Germany: Otto Maier Verlag. Waage, J.K. and R.C. Best. 1985. Arthropod associates of sloths. Pp. 297-322 in The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas, ed. G. Gene Montgomery. Washington: Smithsonian Institution Press. Young, J.Z. 1973. The Life of Vertebrates (second edition). Oxford: Clarendon Press. Goto table of contents ========================================================================== ABOUT THIS NEWSLETTER Copyright 1999 by The Nature Institute. You may redistribute this newsletter for noncommercial purposes. You may also redistribute individual articles in their entirety, provided the NetFuture url and this paragraph are attached. NetFuture is supported by freely given reader contributions, and could not survive without them. For details and special offers, see http://netfuture.org/support.html . Current and past issues of NetFuture are available on the Web: http://netfuture.org/ To subscribe or unsubscribe to NetFuture: http://netfuture.org/subscribe.html. Steve Talbott :: NetFuture #97 :: November 3, 1999 Goto table of contents

  • Goto NETFUTURE main page