The Eager Pursuit of Sloths

The following is an article from The Annals of Improbable Research.

(Image credit: Flickr user MicroMacroMicro)

by Alice Shirell Kaswell, Improbable Research staff

Sloth, sloth, sloth. Sloths. Yes, their name is synonymous and homonymous with a certain style of sin. But scientists pursue them for other reasons, too.

The animals move—something they do on occasion—in what can seem to be mysterious ways. They hang upside down from tree limbs, and sometimes amble that way, there. On the ground, ambling right side up is their preferred way to get from here to slightly over there. They often snooze.

A study called "Three-Dimensional Kinematic Analysis of the Pectoral Girdle During Upside-Down Locomotion of Two-Toed Sloths" appeared two years ago in the journal Frontiers in Zoology.
"Three-Dimensional Kinematic Analysis of the Pectoral Girdle During Upside-Down Locomotion of Two-Toed Sloths (Choloepus didactylus, Linné 1758)," John A. Nyakatura and Martin S. Fischer, Frontiers in Zoology, vol. 12, no. 7, July 12, 2010, p. 21–36. 

(Image credit: Flickr user Josh More)

John Nyakatura and Martin Fischer of Friedrich-Schiller- Universität in Germany analyzed the "suspensory quadrupedal locomotion" of two sloths. They concluded that an earlier biologist had exaggerated, but only slightly, in proclaiming that "of all mammals the sloths have probably the strangest mode of progression."

Others hesitated less to say more. A 2007 book called Victorian Animal Dreams: Representations of Animals in Nineteenth-Century Literature and Culture gets a bit slothful about facts. It says: "As the early Victorians well knew, the living descendants of the sloth family not only spend their lives suspended upside down in trees, but are also incapable of walking, and are, in fact, so slow that moss grows on their fur."

Victorian Animal Dreams: Representations of Animals in Nineteenth-Century Literature and Culture, Deborah Denenholz Morse and Martin A. Danahay, Ashgate Publishing, 2007, ISBN 978-0- 7546-5511-4.

Slothful Use of Hands and Feet

There has been some more careful research, in the sense that it is research undertaken more carefully. In 1981 Frank C. Mendel of the State University of New York, Buffalo, published a painstaking analysis called "Use of Hands and Feet of Two-toed Sloths During Climbing and Terrestrial Locomotion," in the Journal of Mammalogy. Four years later, Mendel published a not quite identical paper called "Use of Hands and Feet of Three-toed Sloths During Climbing and Terrestrial Locomotion."

"Use of Hands and Feet of Two-Toed Sloths (Choloepus hoffmanni) During Climbing and Terrestrial Locomotion," Frank C. Mendel, Journal of Mammalogy, vol 62, no. 2, 1981, pp. 413–21. State University of New York, Buffalo.

"Use of Hands and Feet of Three-Toed Sloths (Bradypus variegatus) During Climbing and Terrestrial Locomotion," Frank C. Mendel, Journal of Mammalogy, vol. 66, no. 2, 1985, pp. 359–66.

Sloths in Pennsylvania

Sloths, native to south and central America, do now and then get around. Robert Enders published a paper in 1940 called "Observations on Sloths in Captivity at Higher Altitudes in the Tropics and in Pennsylvania":

"Observations on Sloths in Captivity at Higher Altitudes in the Tropics and in Pennsylvania," Robert K. Enders, Journal of Mammalogy, vol. 21, no. 1, 1940, pp. 5–7.
Enders transported two sloths from their native Panama to his workplace at Swarthmore College in Pennsylvania, where they soon died.

Hair, and What Dwells Therein

In 2010 University of Helsinki researchers examined sloths' hair:

"Molecular Evidence for a Diverse Green Algal Community Growing in the Hair of Sloths and a Specific Association with Trichophilus welckeri (Chlorophyta, Ulvophyceae)," Milla Suutari, Markus Majaneva, David P. Fewer, Bryson Voirin, Annette Aiello, Thomas Friedl, Adriano G. Chiarello, and Jaanika Blomster, BMC Evolutionary Biology, vol. 10, no. 1, 2010, p. 86.

The authors report:

A wide variety of organisms have been reported to occur in the grooves and cracks of sloth hair, including cyanobacteria and diatoms; and among their fur, moths, beetles, cockroaches and roundworms. However, the greenish color of the hair, which is most evident in three-toed sloths, is due to green algae, which in most cases have been identified as Trichophilus welckeri....

Phylogenetic analysis reveals that sloth fur hosts a number of green algal species and suggests that acquisition of these organisms from the surrounding rainforest plays an important role in the discoloration of sloth fur. However, an alga corresponding to the morphological description of Trichophilus welckeri was found to be frequent and abundant on sloth fur. Phylogenetic analysis demonstrated the retention of this alga on the fur of sloths independent of geographic location.

Sloths Sleep

A German/Swiss/Panamanian/American team studied slothly sleep:

"Sleeping Outside the Box: Electroencephalographic Measures of Sleep in Sloths Inhabiting a Rainforest," Niels C. Rattenborg, Bryson Voirin, Alexei L. Vyssotski, Roland W. Kays, Kamiel Spoelstra, Franz Kuemmeth, Wolfgang Heidrich, and Martin Wikelski, Biology Letters, vol. 4, no. 4, 2008, pp. 402–5.

The authors explain:

We performed the first electroencephalogram (EEG) recordings of sleep on unrestricted animals in the wild using a recently developed miniaturized EEG recorder, and found that brown-throated three- toed sloths (Bradypus variegatus) inhabiting the canopy of a tropical rainforest only sleep 9.63 hours d-1, over 6 hours less than previously reported in captivity.... [Our] results suggest that sleep in the wild may be markedly different from that in captivity.

Sloths and Feeding

Sloths fascinate people in part, in large part, because of the ways sloths relate to food.

A German/Peruvian team of scientists published a study in 2011:

"Disgusting Appetite: Two-Toed Sloths Feeding in Human Latrines," Eckhard W. Heymann, Camilo Flores Amasifuén, Ney Shahuano Tello, Emérita R. Tirado Herrera, and Mojca Stojan-Dolar, Mammalian Biology-Zeitschrift für Säugetierkunde, vol. 76, no. 1, 2011, pp. 84–6.

The authors report:

The first observation of the unusual feeding habit took place on 3 November 2001. At around 2000h, a sloth was detected hanging underneath the wooden bars of our latrine. It was scooping with one hand from the semi-liquid manure composed of faeces, urine, and toilet paper and then eating from the hand.... Since this first observation, we obtained more than 25 additional records of sloths visiting and feeding in the latrine until 2007 when the latrine was fenced with wire mesh.

Not entirely unrelated is the study:

"Masticatory Apparatus of the Sloths," Harry Sicher, Fieldiana, Zoology, 29, no. 10, 1944, pp. 161-168.

Sicher says, "The present investigations disclose the curious fact that the mechanism of mastication in the two-toed sloth is, in a sense, of an opposite type from that in the three-toed sloth."

Some scientists study sloths as food:

"Sloths in the Diet of a Harpy Eagle Nestling in Eastern Amazon," Mauro Galetti and Oswaldo de Carvalho, Jr., Wilson Bulletin, vol. 112, no. 4, December 2000, pp. 535–6.

The authors report:

Prey remains of a nestling Harpy Eagle (Harpia harpyja) over 15 months in eastern Amazon, Brazil included 11 two-toed sloths (Choloepus didactylus), 9 three-toed sloths (Bradypus variegatus), and 1 gray four-eyed opossum (Philander opossum)....

Other investigators found frequent use of sloths by Harpy Eagles, but not to the extent we found (Rettig 1978, Izor 1985). Sloths comprised about 36% (Rettig 1978) of the prey items for Harpy Eagles in Guyana....

Our work shows that at least one pair of Harpy Eagles of the eastern Amazon frequently uses sloths to feed their nestling, even more than observed in Guyana (Rettig 1978, Izor 1985).

Sloths Doing the Unexpected

Sloths can shock experts:

"Agonistic Behavior by Three-Toed Sloths, Bradypus variegatus," Harry W. Greene, Biotropica, 1989, pp. 369–72.

The authors give this eyewitness account:

A three-toed sloth (Bradypus variegatus) rapidly ascended a cecropia tree, fought briefly and intensely with another adult male, and descended even more rapidly.... The social system, visual abilities, and activity budgets of sloths are probably more complex than previously thought.

On the Perceived Slothfulness of Sloths

No matter what else they say they're studying, scientists and amateur sloth sleuths always, down deep, wonder about the behavior that screams at us whenever someone says the word "sloth":

"Neuromuscular Responses of Sloths," James F. Toole and Theodore H. Bullock, Journal of Comparative Neurology, vol. 149, no. 2, 2004, pp. 259–70.

The authors report:

Because sloths are among the slowest moving of mammals, it is of interest to determine whether the rate limiting factor is in their motivation, central nervous system, or their muscles.... Our data suggest that although muscle mechanics undoubtedly contribute to the slow movements of the sloth, they are not the only rate limiting factor.

Always comes the big question: "Why are sloths so slothful?" Toole, he of the aforementioned neuromuscular study, earlier asked it directly and bluntly. He was, even then, investigating whether sloths' nerves and brain are sloppily slow:

"Why Are Sloths So Slothful?" James F. Toole, Transactions of the American Clinical and Climatological Association, vol. 82, 1971, pp. 131-5.
The author ran a series of tests, and concluded:

You will recall that myotonia is an abnormality of skeletal muscle in which movement is slow in performance and delayed in cessation.... The result of [our] experiments is that we cannot verify our initial suspicion that the sloth might be myotonic.

In a discussion at the end of the paper, he deepens the mystery:

The heart rate is very slow. It, however, is an unusual heart. Dr. Peter Poole at San Diego who studied the ability of the heart to contract was astonished, as was I, that animals thought to be dead continued to have heart beat when it was removed from the body much as the turtle heart might. Even when put in a calcium-free perfusion for twenty-four hours, the heart papillary muscle continued to contract and evidently was almost indestructible.

(Image credit:Flickr user Brian Gratwick


This article is republished with permission from the January-February 2013 issue of the Annals of Improbable Research. You can download or purchase back issues of the magazine, or subscribe to receive future issues. Or get a subscription for someone as a gift!

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