Spotlight on Vet Medicine
May 27, 2005
Sloth Standards
The unique physiology of two-toed sloths sometimes makes for puzzling pictures of their insides.
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| A two-toed sloth at the Zoo. |
A National Zoo two-toed sloth (Choloepus didactylus) was due for his physical exam. It was strictly routine—keepers reported that the animal’s behavior, eating, drinking, and activity levels were normal. The Zoo’s vets carefully watched the animal before its exam and they saw that the sloth was quiet, alert, responsive, had a good coat of fur, and appeared somewhat overweight. All in all, this appeared to be a healthy sloth, albeit perhaps a little fat.
A Strange Mass?While sluggish and seemingly innocuous, sloths do not take well to being handled and can inflict nasty scratches and bites on anyone foolish enough to try. For the exam, then, the sloth was under anesthesia while the vets took a blood sample, and completed a thorough physical examination. When the vets palpated the abdomen, it was too distended to identify any abdominal organs. So next on the list of procedures was a set of radiographs of the sloth’s thoracic (chest) and abdominal cavities.
While the thoracic radiographs were unremarkable, one view of the abdomen (below) showed a large round mass of fluid-filled soft tissue.
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The mass appeared to be squeezing the animal’s colon and kidneys out of their normal position. Further, the colon was bulging with digested food. (In the wild, two-toed sloths are largely leaf-eaters; in the Zoo they eat a variety of vegetables and fruit.) As a result, a follow-up exam was scheduled for four weeks later.
Not So Strange After AllThe follow-up exam included additional radiographs as well an ultrasound exam. The vets found that the sloth’s abdomen no longer felt distended to the touch and, as before, they saw no other external signs of poor health. Further, the radiographs (one view is shown below) and ultrasound no longer showed the mass seen in the previous diagnostic images and the positions of the colon, kidneys, and other internal organs were normal.
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It appeared that the mass seen four-weeks earlier was an enlarged bladder (shown below) filled with a large amount of urine that had since been emptied.
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Sloths Naturally “Hold It”
It turns out that two-toed sloths naturally retain both urine and feces, urinating and defecating at intervals that range from 3.4 to 4.6 days. (This was actually first documented in 1985 by Zoo scientists studying sloths in the Zoo’s collection.)
Their bladders are large, and sloths urinate up to 500 milliliters (almost 17 ounces) of urine at a time. Their rectal pouch is also large and the animals expel up to 235 milliliters (almost 8 ounces) of feces at a time. Depending on when in the excretion cycle a sloth is weighed, urine and feces may account for up to 30 percent of the animal’s body weight, which averages about 6 kilograms (about 13 pounds). This information supports the view that the enlarged bladder seen in the first radiographs was normal.
In the Wild
Two-toed sloths are native to warm, humid forests in northern South America, where they spend almost all of their time in the upper canopy of the forest at 24 to 30 meters (about 80 to 100 feet) above the ground. Sloths are notoriously sedentary. Studies in zoos report they are active only 25 percent of time, spending 45 percent of their time asleep and the remainder resting. They move very slowly as well, an average of 0.5 to 0.6 kilometers an hour (about one-third of mile per hour) when traveling in the trees.
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| Two-toed sloths rarely leave the treetops. |
On the ground, their movements are awkward and even slower, just 250 meters, or a little over 800 feet, per hour. But they descend to the ground only to change trees, which they do infrequently, and to simultaneously urinate and defecate at the base of the tree.
On the ground, sloths are at high risk of predation, easy targets of such predators as coatis, jaguars, margays, ocelots, and anacondas. Reducing the risk of predation by going to the ground very infrequently is probably why sloths evolved the ability to retain their urine and feces so long. Why they do not urinate and defecate in the canopy and let the droppings fall to the ground, as some other arboreal mammals do, is more puzzling. One possibility is that the smelly piles of feces left on the ground, called middens, are signposts indicating a sloth’s presence to others of its species. This may help individuals find mates.
Sloths live in Amazonia and in the Small Mammal House at the National Zoo. Look for them high in the trees.
More Information
Slow and Steady Sloths Note to Media: If you would like more information about this project, or any of the Zoo's animal health programs, please contact the Zoo's Office of Public Affairs.
