What Does It Mean to Be a Sloth?
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. Perhaps it might be better to say we've rarely taken animals as whole, integrated beings seriously and therefore they have never really come into view for us. For that reason our scientific and technological culture can so casually manipulate what it does not know. The more we get to know something intimately, the less likely we are to treat it in a purely utilitarian fashion.
Imagine a biotechnologist wondering what causes the sloth to be slow and pondering whether the animal could be mined for "slothful" genes that might be put to therapeutic use in hyperactive children. Or another who wonders whether the sloth might not be a good research model for testing the efficacy of genes from other organisms that enhance metabolic activity. As far as I know, no such research projects are in progress or being planned — and I am glad. But how easily we can come up with ideas that hover in splendid isolation above any deeper concern for the animal itself! We are intrigued and motivated by the seemingly boundless limits of doing the doable. We do not feel limited by ignorance of what we're dealing with.
This essay is an attempt to show how we can take steps to overcome some of that ignorance — of which we should nonetheless always be mindful — by beginning to grasp something of the organic lawfulness inherent in one animal, the sloth. With all its unique and unusual features, the sloth almost seemed to be prodding me to understand it in an integrated, holistic way. The poet and scientist Goethe set the stage for a sound holistic approach to studying animals, and others have developed his method further. 1 I have been influenced and inspired by their work in this study.
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 a tropical ecosystem characterized by constancy of conditions. 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°C (77° 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. On average, sloths were found to move during 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.
Figure 1. The three-toed sloth. (Sketch by Craig Holdrege.)
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. 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 hands and feet are essentially a continuation of the long limb bones, ending in the elongated claws and do not develop as independent, agile organs as they do, say, in monkeys. 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 the 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 (Archimedes' Law). The body becomes buoyant, and in the case of the sloth, virtually weightless.
Remarkably, sloths are facile swimmers. 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 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 sluggishly, the fastest muscles contracting four to six times more slowly 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, as the primary gateways to perceiving and taking in food, 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°C (97°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 (Bourlière 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 rose, the body temperature also rose. When found on sunny days, sloths were often on an outer branch, belly-side up with limbs extended, basking in the sun. Body temperature usually peaked at
about 36-38°C soon after midday. It then began to fall, reaching a low point of about 30-32°C in the early morning. The body temperature was usually about 7-10°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. According to Brian McNab (1978), the sloth "almost appears to regulate its rate of metabolism by varying body temperature, whereas most endotherms [warm-blooded animals — 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°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 (browsing) 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, although tree leaves are an abundant and constant source of food. 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, pp. 106ff.; Mendel et al. 1985). 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 penetrate 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.
Figure 2. The three-toed sloth. Note the orientation of the head. (Sketch by Craig Holdrege.)
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. 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.)
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 excrement "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.
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).