ADVERTISEMENT
  About the SA Blog Network













Tetrapod Zoology

Tetrapod Zoology


Amphibians, reptiles, birds and mammals - living and extinct
Tetrapod Zoology Home

The anatomy of sloths

The views expressed are those of the author and are not necessarily those of Scientific American.


Email   PrintPrint



Everybody loves sloths, and whenever we talk about sloths we have to remember that the two living kinds (Bradypus – the four species of three-toed sloth – and Choloepus – the two species of two-toed sloth) are but the tip of the iceberg when it comes to sloth diversity. This article – an excerpt from Naish (2005) (though with citations added that were absent in the published article) – briefly reviews the anatomy of fossil sloths, though there are references to the living forms where appropriate.

A typical fossil sloth can be imagined as a rather bear-shaped, shaggy-furred mammal with particularly powerful forelimbs, a barrel-shaped ribcage, a stout tail, prominent curved hand and foot claws and a markedly broad, robust pelvis.

Skulls from various sloths. a. Eremotherium, a Pleistocene megatheriid known from both South and North America. b. Nematherium, a mylodontid from Early Miocene South America. c. Scelidotherium, a scelidotheriine mylodontid from the Pliocene and Pleistocene of South America. d. Thalassocnus, a nothrotheriid from the Miocene and Pliocene of the Pacific coast of South America. e. Acratocnus odontrigonus, a choloepodine megalonychid from the Pleistocene of Puerto Rico. f. Acratocnus simorhynchus, an even shorter-snout choloepodine, from the Pleistocene of Hispaniola. Not to scale. Images by Darren Naish, redrawn from various sources. From Naish (2005).

Sloth skulls are diverse in form and range from the deep and broad, snub-faced morphology seen in Bradypus and some megalonychids to the elongate almost horse-like skulls of megatheriids and others (Gaudin 2004). Some megalonychids had a domed cranium resulting from marked enlargement of the sinuses within the frontal bones. The sloth palate is rugose and covered in pits and grooves and there are distinctive deep laminae that descend ventrally from the pterygoid bones (Gaudin 2004). The tip of the sloth mandible is usually spout-shaped and there is a foramen, representing an external opening of the mandibular canal, on the side of the lower jaw. In sloths with particularly long-rooted teeth there is a distinct bulge on the ventral margin of the lower jaw.

Skull of the Pleistocene mylodontid sloth Glossotherium, (c) Natural History Museum, London. Photo by Darren Naish.

Sloths have peculiar teeth. They do not possess deciduous teeth but have a single set of high-crowned, open-rooted teeth (Bargo et al. 2006) that grow continuously throughout life, and the lack of a replacement dentition has made it difficult to homologise sloth teeth with those of other mammals. Incisors are absent, and it is not really possible to distinguish between the similar premolars and molars. The living tree sloth Choloepus, as well as some mylodontids, megalonychids and nothrotheriids, possess caniniform teeth separated from the other teeth by a diastema. The upper caniniforms of these sloths are ahead of the lower caniniforms and, while some evidence suggests that the upper caniniform in Choloepus is a true canine, this probably isn’t the case for the lower caniniform. In the Pleistocene megalonychid Megalocnus from Cuba, and in certain other genera, the two most anterior upper jaw teeth have been described as ‘pseudorodentiform’ and are more incisiform than caniniform.

Sloth teeth lack enamel and are composed instead of two different kinds of dentine plus an outer layer of cementum, the softer dentine forming the innermost region of the tooth. When sloth teeth erupt they are devoid of the cusps and basins seen normally in mammalian teeth and are simple and cylindrical in form. As the teeth occlude against those in the opposite jaw, valleys and cusp-like structures are formed as the two kinds of dentine erode differentially (Naples 1989, 1995). Some fossil sloths had squarish or subrectangular teeth and, in these forms, transverse ridges between the valleys are particularly prominent.

Arms, hands and hips

Arm of the mylodontid Glossotherium from Pleistocene South America. Image Darren Naish, from Naish (2005).

The forelimbs of most sloths are about subequal in length to the hindlimbs, the most prominent exceptions being the long-armed tree sloths of the genus Bradypus. Mylodontids had a particularly prominent olecranon process on the ulna. Recent studies have shown that the length of the olecranon process relative to the rest of the ulna is a good indicator of digging ability in mammals as the olecranon provides the attachment area for the triceps, the main muscle used in digging. Forelimb bone strength in mylodontids was also high and shows that the forelimbs were resistant to impact with the ground (Bargo et al. 2000). Furthermore, the wide, straight and relatively flat claws of these sloths resemble those of living mammals that dig. Accordingly, mylodontids seem to have been proficient diggers that unearthed roots and tubers and they may even have constructed burrows.

Sloths are amazingly diverse and unusual in hand morphology. Among megatheriids, primitive species of Eremotherium were pentadactyl (albeit it with a short thumb and a fifth digit with only one phalanx) while the advanced species Elaurillardi was tridactyl, possessing only digits III-V, and of these only digits III and IV had unguals (Cartelle & De Iuliis 1995).

Some of the diversity present in sloth hands. L to r: Pleistocene mylodontid Glossotherium robustum, in which the hand is pentadactyl; Pleistocene megatheriid Eremotherium laurillardi, in which digits I and II have been lost; Pleistocene megatheriid Megatherium americanum, in which the thumb is absent. Not to scale. Drawings by Darren Naish, redrawn from various sources.

Bradypus, a taxon that’s notable for being outside the clade that includes the majority of other sloth lineages (Gaudin 2004, Pujos et al. 2007), possesses only digits II-IV on the hand, and the megalonychid Choloepus only has II and III. Several sloth groups exhibit fusion of various manual phalanges, including of both phalanges in the thumb (in Eremotherium) and of the two phalanges at the base of the third digit (in Thalassocnus), as well as fusion of metacarpals to carpals.

Enormous flaring pelvis of the megatheriid Megatherium. Note human silhouette in background providing rough idea of scale. Photo (c) Natural History Museum, London, taken by Darren Naish.

The sloth pelvis is massive and broad and unusual in that the ischia are connected to the vertebral column (in most tetrapods only the ilia are connected), a feature that sloths share with all other xenarthrans with the sole exception of Cyclopes, the Pygmy anteater. The femur in fossil sloths varies from robust to very robust with the femora of giant megatheriids being shaped like a wide rectangle. The tibia in most fossil sloths is proportionally short and is also massively constructed. As is true of the hand, some sloth groups reduced the number of toes with only three present in some megatheriids.

Mummified sloth skin preserved in the arid caves of Chile, Argentina, Arizona and Nevada provides excellent information on ground sloth skin and fur. Small bony ossicles were embedded in the skin of the mylodontids Mylodon, Glossotherium and Paramylodon, and probably also in Eremotherium, but are definitely not present in the mummified skin of Nothrotheriops. The fur itself was either yellowish or reddish brown.

Locomotion and posture

The configuration of the ground sloth foot and ankle indicates that most of these animals were plantigrade (that is, they placed the entire surface of the foot on the ground). However, it was argued as early as the 1840s that at least some ground sloths walked with a pedolateral foot posture: that is, with most of the weight supported by the outer margins of the feet. This bizarre configuration meant that the dorsal surface of the foot faced laterally.

Right foot of the giant megatheriid Eremotherium. Though this is how the foot looks in lateral view: the lateral surface of the foot is the anatomical dorsal surface. Only digits III-V were present in this sloth, and of these only digit III possessed a claw (this was curved toward the midline, and its tip is thus not visible in lateral view). Image by Darren Naish, from Naish (2005).

The centre of gravity in the ground sloth body and the strength of their hindlimb bones, pelvis and vertebrae indicate that at least some forms could walk bipedally. Fossil trackways confirm this. Most sloths have hands and hand claws that appear well suited for the manipulation of foliage and the robust tail seen in most fossil sloths suggests that they may have sat in a tripodal posture when foraging and eating. The tripodal abilities of ground sloths have proved inspirational to palaeontologists working on other fossil tetrapod groups.

Living tree sloths are good swimmers so it seems reasonable to assume that ground sloths were too. However, a few fossil sloths reveal morphological features which indicate that they were habitual, rather than occasional, swimmers and amphibious habits have been suggested for both scelidotheriine mylodontids and nothrotheriids. One group of nothrotheriid seems to have been truly semi-aquatic (Muizon & McDonald 1995, Muizon et al. 2003, 2004).

For previous Tet Zoo articles on sloths and other xenarthrans, see…

And – - seeing as this is another article on Cenozoic South American megafauna, I’m sure you’re wondering how it’s going with that montage I featured here back in July’s toxodont article. Heres the answer… (still working on it: a larger version will be uploaded to my deviantART gallery later today)…

South American Cenozoic megafauna - now with sloths!!

Refs – -

Bargo, M. S., De Iuliis, G. & Vizcaíno, S. F. 2006. Hypsodonty in Pleistocene ground sloths. Acta Palaeontologica Polonica 51, 53-61.

- ., Vizcaíno, S. F., Archuby, F. M. & Blanco, R. E. 2000. Limb bone proportions, strength and digging in some Lujanian (Late Pleistocene-Early Holocene) mylodontid ground sloths (Mammalia, Xenarthra). Journal of Vertebrate Paleontology 20, 601-610.

Cartelle, C. & De Iuliis, G. 1995. Eremotherium laurillardi: the Panamerican late Pleistocene megatheriid sloth. Journal of Vertebrate Paleontology 15, 830-841.

Gaudin, T. J. 1995. The ear region of edentates and the phylogeny of the Tardigrada (Mammalia, Xenarthra). Journal of Vertebrate Paleontology 15, 672-705.

- . 2004. Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zoological Journal of the Linnean Society 140, 255-305.

Muizon, C. de & McDonald, H. G. 1995. An aquatic sloth from the Pliocene of Peru. Nature 375, 224-227.

- ., McDonald, H. G., Salas, R. & Urbina, M. 2003. A new early species of the aquatic sloth Thalassocnus (Mammalia, Xenarthra) from the Late Miocene of Peru. Journal of Vertebrate Paleontology 23, 886-894.

- ., McDonald, H. G., Salas, R. & Urbina, M. 2004. The youngest species of the aquatic sloth Thalassocnus and a reassessment of the relationships of the nothrothere sloths (Mammalia: Xenarthra). Journal of Vertebrate Paleontology 24, 387-397

Naish, D. 2005. Fossils explained 51: sloths. Geology Today 21 (6), 232-238.

Naples, V. L. 1989. The feeding mechanism in the Pleistocene ground sloth, Glossotherium. Natural History Museum of Los Angeles County, Contributions in Science 415, 1-23.

- . 1995. The artificial generation of wear patterns on tooth models as a means to infer mandibular movement during feeding in mammals. In Thomason, J. (ed) Functional Morphology in Vertebrate Paleontology. Cambridge University Press, pp. 136-150.

Pujos, F., de Iuliis, G., Argot, C. & Lars, W. 2007. A peculiar climbing Megalonychidae from the Pleistocene of Peru and its implication for sloth history. Zoological Journal of the Linnean Society 149, 179-235.

POSTSCRIPT: how could I write about sloths and not include this? …

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





Rights & Permissions

Comments 45 Comments

Add Comment
  1. 1. Halbred 1:52 pm 08/30/2012

    Excellent article–sloths are among my favorite mammals. A few questions:

    1) To which group of ground sloths are modern arboreal sloths related? Are there fossil representations of arboreal sloths? The morphological disparity between living, arboreal sloths and ground sloths has always fascinated me.

    2) What’s up with Eremotherium’s weird Digit V?

    3) How the heck would pedolateral foot posture evolve? I imagine that enormous heel bone evolved in concert with said foot posture?

    4) Any chance for an article on knuckle-walking chalicotheres in the future? ;-)

    Link to this
  2. 2. naishd 2:05 pm 08/30/2012

    Great questions. Only time right now for one brief response: as always, an improving fossil record has allowed us to work out that there is a continuum among fossil sloths, with some of the smaller and ‘mid-sized’ forms being capable of at least some climbing. Note that living sloths are almost at ‘opposite ends’ of the phylogeny: the lineage that ends with Bradypus diverged from other sloths very early on in sloth history (in the Eocene or Paleocene) and apparently lacks stem forms, while Choloepus is deeply nested within Megalonychidae and is close to a set of taxa that had moderately good or reasonably good climbing abilities. See in particular…

    Pujos, F., de Iuliis, G., Argot, C. & Lars, W. 2007. A peculiar climbing Megalonychidae from the Pleistocene of Peru and its implication for sloth history. Zoological Journal of the Linnean Society 149, 179-235.

    White, J. L. 1993. Indicators of locomotor habits in xenarthrans: evidence for locomotor heterogeneity among fossil sloths. Journal of Vertebrate Paleontology 13, 230-242.

    - . 1997. Locomotor adaptations in Miocene xenarthrans. In Kay, R. F., Madden, R. H., Cifelli, R. L. & Flynn, J. J. (eds) Vertebrate Paleontology in the Neotropics: The Miocene fauna of La Venta, Colombia. Smithsonian Institution Press (Washington, D.C.), pp. 246-264

    Darren

    Link to this
  3. 3. neovenator250 3:18 pm 08/30/2012

    Always love seeing fossil sloths get some recognition! Anyone know any good references for differentiating between the North American taxa? I have a lot of great work from the South American side, but there doesn’t seem to be nearly as much from the northern hemisphere (to be expected, I suppose, considering the group’s distribution). Tooth morphology is a pretty standard fare for some, but most of the material tends to be postcranial. Size could potentially differentiate E. eomigrans, but still leaves a lot of room in the smaller taxa

    Link to this
  4. 4. Dallas Krentzel 6:47 pm 08/30/2012

    I’ve seen most of the information you’ve presented here before; however, I’ve never heard of the evidence that Choloepus’ upper caniniform is a true canine. Where does this data come from? I’ve always thought the convergence of sloth caniniforms (and incisiforms) to be really neat, given the idea that they evolved from cheek-teeth, and I always just took it on face value that because the upper caniniform occluded anterior to the lower that they must not be homologous to true canines. But guess it’s definitely possible for at least the upper tooth to still be a true canine. But how can we tell?

    Link to this
  5. 5. David Marjanović 7:06 pm 08/30/2012

    I’ve seen a paper about a climbing megalonychid – not hanging like the extant sloths, but climbing like a monkey or at least a koala. I’ll dig it up later…

    has made it difficult to homologise sloth teeth with those of other mammals.

    Isn’t the same true of the other xenarthrans that have teeth?

    Could it be that the ancestral xenarthran was a specialized myrmecophage?

    some evidence suggests that the upper caniniform in Choloepus is a true canine

    What is that evidence?

    Link to this
  6. 6. LeeB 1 7:07 pm 08/30/2012

    A great article.
    The remarkable thing about the ground sloths was their diversity, from ones as small as modern sloths up to multi ton Megatheres and Lestodontine Mylodonts.
    And in the late Pleistocene there were four species in North America, a dozen in the Antillean islands of Cuba, Hispaniola and Puerto Rice; and 20-25 species throughout South America from the lowlands to the Andes and from the tropics to Patagonia.
    The living tree sloths don’t seem to have much of a fossil record, which may not be too surprising given their arboreal habits in rain forests where fossil preservation is rare.
    Some of the Antillean ‘ground’ sloths are supposed to be semi arboreal, but how close they are to Choelopus seems uncertain, especially as they may have been isolated on the Greater Antilles for millions of years .

    The Andean Diabolotherium is now thought to be a rock climber; it keeps being found in caves in steep terrain.

    And several lineages of sloths seem to have become semi-aquatic; the Megalonychid Ahytherium aureum has been suggested to be semi-aquatic as well as the scelidotheres and nothrotheres mentioned above.

    LeeB.

    Link to this
  7. 7. Dallas Krentzel 8:59 pm 08/30/2012

    LeeB.,

    Couldn’t Diabolotherium just be a tree climber of mountainous cloud forests that was occasionally washed into the local caves upon death? Maybe climatic changes and potentially even further uplift have changed the environment since then. I skimmed through Pujos, et al. 2007′s description and I didn’t see anything but speculations of arboreality, although I need to give it a closer reading. They did say this though: “The forelimb characters mentioned above suggest arboreal capabilities for this taxon; it is noteworthy that Pujos (2002) interpreted the depositional environment of the Piedra Escrita site as representing an oasis.” I couldn’t get access to the 2002 paper in question and they don’t say anything more about it in that section, so take from that what you will. Looking at the photos of the cave sites, it does look like rather rough terrain, so maybe you’re right. Just think about something as clumsy looking as a sloth climbing rocks just has a funny feel to it, I guess.

    Link to this
  8. 8. Dallas Krentzel 9:01 pm 08/30/2012

    Damn it, no edit feature. I meant to say “Just thinking*…”

    Link to this
  9. 9. LeeB 1 2:26 am 08/31/2012

    Dallas,

    Diabolotherium is now known fron Peru, Chile and Argentina; a discussion of it’s possible rock climbing is in “Recent advances on variability, morpho-functional adaptations, dental terminology, and evolution of sloths”
    by Pujos et. al. 2012 in the J. Mammal. Evol;
    this is available on line.
    It references some other papers on Diabolotherium; the one discussing it’s occurrence in Argentina was also available on line the last time I looked.

    LeeB.

    Link to this
  10. 10. Dartian 2:43 am 08/31/2012

    Darren:
    Mummified sloth skin preserved in the arid caves of Chile, Argentina, Arizona and Nevada provides excellent information on ground sloth skin and fur.

    How do ground sloth skin and fur compare to those of extant tree sloths?

    I was a bit surprised that there was no mention in this article about the unusual number of neck vertebrae in extant sloths (Bradypus has more and Choloepus – or C. hoffmanni at least – has fewer cervical vertebrae than the typical seven of mammals). Are any fossil sloths with an unusual number of neck vertebrae known?

    Link to this
  11. 11. Mark Robinson 3:05 am 08/31/2012

    Another great article. I have a Q about the living arboreal sloths. I understand that they descend to the ground to defecate every few days. I assume that this would make them more vulnerable to predation, being easier to spot and being considerably less mobile once on the ground. I wonder why they haven’t altered their behaviour to just excrete whilst remaining up in the branches?

    …Antillean ‘ground’ sloths…
    I read that at first as “Ant-hillean ground sloths” and an image of a 3m Megalonyx tearing into a termite mound popped into my head.

    Link to this
  12. 12. Dartian 4:01 am 08/31/2012

    Mark:
    I wonder why they haven’t altered their behaviour to just excrete whilst remaining up in the branches?

    That’s a very good question, but nobody knows the answer to it (yet). Whatever is the reason, though, it must be something important because, as you suspect, descending to the ground does indeed make sloths very vulnerable to predators (Voirin et al., 2009; these authors end their paper by suggesting that “ground-based defecation behavior [...] will likely have a strong adaptive value that is yet to be discovered.”).

    Reference:
    Voirin, J.B., Kays, R., Lowman, M.D. & Wikelski, M. 2009. Evidence for three-toed sloth (Bradypus variegatus) predation by spectacled owl (Pulsatrix perspicillata). Edentata 8-10, 15-20.

    Link to this
  13. 13. naishd 6:03 am 08/31/2012

    Thanks for these many excellent comments. On the homology and identity of the caniniforms, the idea that the upper caniniform of Choloepus might be a true canine comes from Tim Gaudin, who told me about embryological work (published in French, if I remember correctly) that seemingly indicates this. I can’t find my notes on this, will pass on more info when I get it.

    Darren

    Link to this
  14. 14. naishd 7:28 am 08/31/2012

    Dartian asks (comment 10): “How do ground sloth skin and fur compare to those of extant tree sloths?”

    Here’s another excerpt from Naish (2005, p. 237)…

    “Both two-toed and three-toed sloths have specialized hairs that encourage the growth of bluegreen algae. In Choloepus, the algae grow in longitudinal grooves in the hairs, while in Bradypus there are transverse cracks that become progressively wider with age. During the wetter parts of the year, the algae blooms and the sloths turn green. Moths eat the algae and birds eat the moths. Again, it remains unclear whether this is a convergent feature unique to living tree sloths, or whether extinct sloths also grew algae on their fur. Extinct sloths for which hair is known lack either the longitudinal grooves or transverse cracks seen in living forms, but the remains of powdery green algae have been discovered on the hairs of some extinct sloths, so this relationship may have been widespread after all.”

    Darren

    Link to this
  15. 15. Jerzy v. 3.0. 7:54 am 08/31/2012

    Great article! Especially that even people aware of extinct ground sloths imagine just one, Megatherium-like creature, while there were 20 diverse species in South American Pleistocene alone!

    What about the alleged specialization of forelimbs of Megatherium for quick movements (and possible defensive function)?

    ***

    #12 Re: sloth defecation
    Nobody seems to investigate this. My theory is that faeces are an olfactory signal to other sloths considering climbing that tree, possibly warning that this resource patch is already occupied and advertising female presence to males.

    I guess also that defecating from the treetop would disclose sloth’s smell to predators far and wide. Important for a sloth which spends many days on just one tree, unlike eg. a monkey. Defecating on the ground can be therefore the least bad antipredator strategy.

    Both ideas are testable, especially giving a sloth the choice of two trees to climb, one with and another without another sloth’s faeces underneath.

    WIPCA (When Investigating Please Co-Author).

    Link to this
  16. 16. Dartian 9:01 am 08/31/2012

    Jerzy:
    Nobody seems to investigate [sloth defecation]

    Well, it is a pretty daunting subject to study – individual sloths only descend to the ground every three to eight days. You have got to be pretty darn patient to get enough useful data. (David Attenborough made this point in the companion volume to his classic TV series, Life on Earth; that book, incidentally, hasn’t aged nearly as badly as the telvision series has).

    My theory is that faeces are an olfactory signal to other sloths considering climbing that tree, possibly warning that this resource patch is already occupied and advertising female presence to males.

    Are there any observations suggesting that sloths actually inspect, or even detect, other sloths’ latrines? And don’t they typically move from tree to tree via tree branches rather than via the ground anyway?

    I guess also that defecating from the treetop would disclose sloth’s smell to predators far and wide.

    But the main threat to sloths in trees are raptors (especially the harpy eagle), which hunt mainly by vision, not smell. As long as the sloths stay up in the trees, they are pretty safe from mammalian predators (ocelots and other felids are known to prey heavily on sloths when these descend to the ground but they do not usually hunt sloths in the trees).

    Link to this
  17. 17. barndad 9:06 am 08/31/2012

    Re: Halbred 1) There has also been phylogenetic work done on extinct sloths using ancient DNA. Choloepus (2-toed) is close to Mylodon sp. Bradypus (3-toed) is close to extinct w.indian sloths Paracnus sp. Acratocnus sp. and Neocnus sp. The Shasta sloth (Nothrotheriops shastensis) may be close to Bradypus.
    Hoss et al PNAS 1996
    Greenwood et al Mol Phy Evol 2001
    See Poinar et al Current Biology 2003
    Clack et al Annals of Anatomy 2012
    There has even been the molecular identification of an unknown South American nothrotherid sloth from Argentine dung (Hofreiter et al. Quat Res 2003)

    Link to this
  18. 18. Heteromeles 11:24 am 08/31/2012

    Since I don’t have a dog in this particular fight, I’ll pitch out two different hypotheses for why sloths poop in middens.

    1. It’s about sanitation, and the simple way to find out is to see what happens to a sloth when it eats foliage that contains bits of its own feces. If it defecates from the treetops, it might be pooping on its own food, and for whatever reason (perhaps slow bowel movement rates) this may result in some horrendous build-up of pathogens or parasites. That can’t happen if it poops away from its food supply.

    2. Ancestral behavior. There was a huge, stratified deposit of Shasta ground sloth dung found at Rampart Cave in the Grand Canyon area. Perhaps sloths simply create middens? I’m not sure why pooping in a cave makes sense, although to be fair, other caves on the Colorado plateau have turned up things like mammoth dung. Still, tree sloths may be following an ancestral behavior. If one assumes it has no purpose, perhaps it hasn’t rendered them extinct yet, perhaps because no predator will stake out a midden for eight days in hopes of catching a sloth taking its weekly dump.

    3. Not a hypothesis per se, but a separate idea: sloth middens are learned behavior, where infants have simply copied their mothers for however long sloths have been around. After all, it’s fairly easy to litter train a cat. This is also testable, because there’s a sloth orphanage in Costa Rica, and according to this video (http://vimeo.com/16529550), young sloths have to be taught to defecate properly.

    Link to this
  19. 19. Heteromeles 11:40 am 08/31/2012

    Incidentally, I have the program Animal Planet did about slothville (slothville.com) saved on our DVR. The whole thing is so cute (yes, baby sloths are very cute) that it’s the best sleep aid and anti-depressant I’ve found in a long time. It also has a fair amount of sloth biology I didn’t know. Highly recommended, particularly if you’ve had a bad day.

    Link to this
  20. 20. Jerzy v. 3.0. 11:42 am 08/31/2012

    @Dartian
    I am not aware anybody investigated how sloths react to others faeces, that is why I am suggesting it.

    Anyway, sloths do travel on the ground. Tree canopies are not connected enough, remember that sloths cannot jump.

    About predation: I think every cat or large boid would easily climb the tree atfer any sloth and eat it. So predation by large birds (if it is indeed the case) is probably just a result that other predators cannot detect sloths.

    BTW: I myself was surprised when I saw a three-toed sloth on a row of Cercopia trees growing on the cattle pasture, where it must have walked several 100s of m on the grass for rather small food supply.

    Link to this
  21. 21. naishd 3:31 pm 08/31/2012

    A few additional, brief, slothy comments…

    Barndad (comment 17) says that “Choloepus (2-toed) is close to Mylodon sp. Bradypus (3-toed) is close to extinct w.indian sloths Paracnus sp. Acratocnus sp. and Neocnus sp. The Shasta sloth (Nothrotheriops shastensis) may be close to Bradypus”.

    It should be noted that molecular results are not wholly in agreement with morphological ones. Molecular studies often (e.g., Greenwood et al. 2001, Poinar et al. 2003) find Bradypus to group with megatheriids and/or nothrotheriids, and Choloepus to group with mylodontids. Conversely, morphological studies find Bradypus to be outside the clade that includes all other sloths, and Choloepus to be a megalonychid. I suppose we’d need a whole article devoted to sloth phylogeny and diversity to cover this issue properly.

    Some random things connected to the vexing question of why modern tree sloths defecate at ground level… Bradypus digs a hole [apparently using its tail as an auger] for defecating in; Choloepus doesn’t dig a hole and just poops on the ground. Is Bradypus therefore deliberately concealing its presence in a tree, while Choloepus is advertising it? Sloths are phenomenally abundant at some places, existing at really high densities in prime habitat such that several sloths can be found in the same tree. If a sloth approaches a tree from the ground (as they sometimes do), either concealing or advertising the prior use of the tree by other sloths could be to advantage. By concealing dung and therefore making the tree seem empty, maybe Bradypus is selfishly getting other sloths to join it (advantageous for breeding and/or safety from predators). By leaving dung tidily at the bottom of the tree, maybe Choloepus is leaving key information for other travellers.

    With reference to finding a lone sloth on a tree well separated from any other trees (comment 20), remember that people often rescue sloths from the roadside and place them in suitable trees, often in gardens or parks.

    On the evolution of the pedolateral foot posture – maybe the sloths that have it first evolved it in a scansorial or arboreal setting (where medially-facing feet were advantageous), and possessed it as an exaptation?

    Darren

    Refs – -

    Greenwood, A. D., Castresana, J., Feldmaier-Fuchs, T. &
    Pääbo, S. 2001. A molecular phylogeny of two extinct sloths. Molecular Phylogenetics and Evolution 18, 94–100.

    Poinar, H., Kuch, M., McDonald, G., Martin, P. & Pääbo, S. 2003. Nuclear gene sequences from a Late Pleistocene
    sloth coprolite. Current Biology 13, 1150–1152.

    Link to this
  22. 22. naishd 3:39 pm 08/31/2012

    Oh, and – on the diversity of North American sloths, I suppose the most current reference is…

    McDonald, H. G. & Naples, V. L. 2007. Xenarthra. In Janis, C. M., Gunnell, G. F. & Uhen, M. D. (eds) Evolution of Tertiary Mammals of North America, Vol. 2. Cambridge University Press, Cambridge, pp. 147-160.

    Darren

    Link to this
  23. 23. LeeB 1 5:35 pm 08/31/2012

    Diabolotherium is now known from Argentina, so I think it would be interesting to compare the DNA from the unknown sloth dung mentioned by Hofreiter et. al. with that of D. nordenskioldi ( some of whose remains have been found in high altitude caves where DNA is likely to have been preserved due to the low ambient temperature).

    LeeB.

    Link to this
  24. 24. Jerzy v. 3.0. 6:08 pm 08/31/2012

    Well, this was a row of trees and growing quite far away from any road, so the sloth (actually a female with small young) most likely got there on its own legs.

    In any case, because there are sanctuaries with larger number of sloths in SA, one could easily make short study on the reaction of sloths to the dung of their conspecifics.

    WIPCA (When Investigating Please Co-Author).

    Re: ancient sloth dung middens.
    I think famous Mylodon Cave in Patagonia had sloth dung deposits because ground sloths were chased there by Paleo-indians and kept some time as a living food store. Unless the interpretation changed three times since I read it? I wonder if the same could happen to Shasta ground sloths and mastodons in North America?

    Link to this
  25. 25. Dallas Krentzel 7:45 pm 08/31/2012

    LeeB,

    Thanks for the reference, that looks like a good read. I’m putting it on my ‘to read’ list.

    Link to this
  26. 26. Mark Robinson 3:21 am 09/1/2012

    Thanks for the responses re my Q on folivoran toilet etiquette. Dartian confirmed what I suspected – we don’t really know yet.

    That members of one extant genus bury their excrement while the other leaves it out for all to see/smell is new to me and certainly interesting (thanks, Darren).

    However, I would have thought that if there was some advantage to having other sloths join you in your tree (breeding, safety in numbers, someone to have a chat with), that that advantage would also extend to others of your species. Therefore, rather than the passive strategy of hiding your presence, which doesn’t differentiate between occupied and unoccupied trees, I would think that advertising your presence (perhaps with a nice mound of dung) would be the more effective strategy. So, this is clearly not a simple issue.

    Perhaps burying their waste prevents it from being disturbed by animals or washed away, and allows for it to remain useful as a signpost for longer? If it’s buried close to the tree that their using, it would probably be of nutritional benefit to the tree and therefore, in the longer term, the sloth itself.

    As for Choloepus leaving it out in the open, does it deposit a neat poo-pile at the base of the tree that it is using or does it move some distance away? If avoiding “sh!tting where it eats” was the only reason for descending, it would simply need to move below the height at which it feeds. (Not equating the two but interestingly C. didactylus is occasionally partial to human excrement – Heymann, E. W., Flores Amasifuén, C., Shahuano Tello, N., Tirado Herrera, E. T. & Stojan-Dolar, M (2010). “Disgusting appetite: Two-toed sloths feeding in human latrines”. Mammalian Biology 76: 84–86).

    Likewise, if the sloth signposting its presence was the only driver, I imagine that it could probably achieve this almost as effectively by descending only as far as a few metres above the ground before defecating. This would presumably make it more vulnerable than if it remained in the canopy but still safer than if it came all the way down to the ground. Perhaps the reduction in efficacy when dropping its “calling card” from a height is not worth the trade-off with the assumed increase in safety?

    Whatever the reason(s) for this behaviour, I agree that there is likely to be some moderate to strong advantage for it persisting. (The “this is the way we’ve always done it” argument doesn’t work for me). Since they are usually very quiet and discreet, perhaps they are just trying to avoid making a bit of a noise and giving away their presence?

    The fact that the sloths only come down once or twice a week, in of itself, suggests that they undertake this activity as infrequently as possible.

    Link to this
  27. 27. Dartian 4:39 am 09/1/2012

    Heteromeles:
    I don’t have a dog in this particular fight

    You do now! ;)

    If it defecates from the treetops, it might be pooping on its own food

    If that’s the (main) reason, why don’t all arboreal folivores defecate on the ground? Generally speaking, the dung of folivorous mammals (or even herbivorous mammals in general) is usually not all that pathogenic or parasite-infested anyway. And – as Darren has blogged and Mark in comment #26 points out – sloths may even deliberately eat (human) faeces!

    Perhaps we should try a comparative view: are there other arboreal mammals (especially folivorous ones) that typically descend to the ground to defecate? (To my shame, I have to admit that I do not know how/where koalas, for example, poop.) Most monkeys certainly don’t bother with climbing down from the trees to defecate* – which, incidentally, is one reason why monkeys don’t make good pets; they are difficult or impossible to house-train.

    * Some highly arboreal monkeys, such as spider monkeys Ateles, do descend to the ground for other reasons (notably, to eat mineral-rich clay), even though this increases their risk of falling victim to big cats and other predators.

    Jerzy:
    I think every cat or large boid would easily climb the tree atfer any sloth and eat it

    Opportunistically, sure. But hardly on a regular basis; among other things, the very fact that sloths are so utterly defenceless against even relatively small predators suggests to me that they do not really suffer from very high predation pressure – as long as they stay in the safety of the canopy, at least.

    Darren:
    “Bradypus digs a hole [apparently using its tail as an auger]

    It uses its tail. For digging. If this was any other mammal we were talking about, such a thing might be considered remarkable. With sloths, it’s just par for the course… ;)

    Link to this
  28. 28. David Marjanović 10:25 am 09/1/2012

    It uses its tail. For digging. If this was any other mammal we were talking about, such a thing might be considered remarkable. With sloths, it’s just par for the course… ;)

    I was going to say!

    Link to this
  29. 29. Zoovolunteer 10:56 am 09/1/2012

    Given that some sloths (horrifically) are known to eat human feces, is there any record of their eating their own in the way that rabbits and quite a few other herbivores are known to? It was my understanding that coprophagy is a way of reclaiming valuable nutrients produced in the hind gut, which seems something that an animal with a not very active metabolism might want to do in some circumstances.

    Link to this
  30. 30. Heteromeles 11:57 am 09/1/2012

    A couple of notes.

    –By “dog in this fight,” I should point out that I’m a botanist officially. I just happen to like animals too.

    –About mating. If you get a chance to watch the Animal Planet video, you’ll find out that female sloths in estrous scream, attracting nearby males. It’s a real problem at Slothville, because they don’t want their captive mature females mating with the local wild males. Similarly, males fight, especially when there’s a female in estrus nearby, and winners have been known to knock losers out of trees.

    In other words, sloths appear to be quite good at social communication, without using latrines at all.

    One of the fascinating problems Slothville has is that they get a fair number of baby sloths every year (often from road-killed mothers). While they can raise the babies to maturity, they’re pretty sure that captive-raised sloths are too incompetent to survive in the wild, so they keep them in captivity for the rest of their lives. They will release rehabilitated adults. This is one reason they don’t want the captive females mating: they’re chronically short of cage space. They’re also attracting people to study sloth behavior, because there isn’t such an accessible population of multiple sloth species in anywhere else.

    The thing that differentiates sloths from other arboreal herbivores is that they have very slow digestive systems for their size. On the surface, this appears to be a great system for building up intestinal parasites (remember, we’re talking about tropical forest), and eating the feces of other sloths in such an environment is potentially a bad idea. I should point out that koalas live in a rather different environment, as do rabbits.

    The bigger point is that, according to the experts (the people at slothville), they have to train all the babies to defecate properly. It’s entirely possible that learning how to crap in the woods is one of those indispensable parts of sloth culture, along with whatever else mother sloths teach their offspring that’s necessary for them to survive in the wild. The reason sloths defecate the way they do could be simply because that’s the way they were taught to crap, and there may even be cultural differences in different populations.

    Link to this
  31. 31. Dartian 1:50 am 09/2/2012

    Heteromeles:
    The thing that differentiates sloths from other arboreal herbivores is that they have very slow digestive systems for their size. On the surface, this appears to be a great system for building up intestinal parasites

    Endoparasites are a more serious problem for an animal with a slow digestive system? Why would that be the case? Please explain.

    A more general comment: Parasites are, obviously, bad for you and will lower your fitness, but usually they do not outright kill you (unless you’re an exceptionally weak or unlucky individual). Predators, on the other hand, do intend to kill you. Hence, any satisfactory hypothesis about why sloths take the trouble to descend to the ground to defecate should explain how the benefits with doing so are greater than the non-trivial risks (i.e., getting killed) associated with it.

    koalas live in a rather different environment

    Well, yes, but ecologically they are still pretty much the closest thing to a sloth among non-sloth extant mammals (slow-metabolism, relatively inactive arboreal folivores). Thus, their behaviour might perhaps provide some insight to that of sloths’.

    they have to train all the babies to defecate properly

    That’s interesting information, and potentially quite relevant to this discussion. Have these observations been published? If so, can I please have a reference? (No disrespect to Animal Planet, but I’d prefer a slightly more authoritative source.)

    Link to this
  32. 32. Heteromeles 8:51 am 09/2/2012

    @Dartian:

    –Yes, intestinal parasites can kill. Hookworms drain blood out of the intestinal lining, for instance. Bacteria can also kill. The thing about a long transit time is that it gives more time for any organism to grow in the gut. If you don’t want to read some simple, popular book like Parasite Rex, go dig out a basic textbook on parasitology. In humans, contamination of food and water by fecal contents is the normal way for intestinal parasites to spread.

    Now, I don’t know what’s in sloth guts(Wikipedia suggests transit times of up to a month, which I find amazing), but I am proposing this as a hypothesis for why deliberately crapping away from a food supply could be a good thing.

    Koalas are different because they are eucalyptus specialists, and their food requires specialized bacteria and massive digestion times simply to get sufficient nutrients, due (so far as I can tell) to the combination of sclerophylly and eucalyptus chemicals like menthol. Sloths break into two groups: two-toes, which are dietary generalists (including the occasional insect, possibly human feces, etc) and Three-toes, which reportedly focus on Cecropia. Cecropias typically use a combination of latex, tannins, and endemic Azteca ants to defend themselves, although there’s a debate about how effective the ants are (http://www.marietta.edu/~biol/costa_rica/animals/cecropia_ants.htm). Speaking as a PhD botanist, this is a different defense strategy. Eucalypts attempt to starve herbivores and have low nutrient densities in their leaves along with strong CHO-based chemical defenses (menthol, etc), Cecropias appear to have sufficient nutrients in their leaves, and attempt to these inaccessible chemically (latex gums up mouthparts, while tannins mess up proteins in mouths and guts). While Koalas and Sloths approach the problem in the same way (long digestion time), so far as I can tell, koalas are nutrient and possibly calorie limited (e.g. they can move fast at need, but sleep a lot because they have to), but it’s not at all clear to me that sloths are similarly limited, that if you could feed them better, they’d move faster. Instead, I’d argue it’s equally possible they are slow because stealth is their primary defense strategy. This may be another reason why they crap away from their feeding territories.

    As for training their babies to defecate, all I’ve seen (to repeat what I said above) is the video (you can see it at the link I posted above, (http://vimeo.com/16529550). I have no idea what’s in the sloth behavior literature. Since the videographers interviewed a grad student who was studying sloth behavior at slothville precisely because it was so little known, I suspect that papers are rather sparse. In the absence of a well-designed study, I’d trust the people who routinely raise orphaned baby sloths over academic speculators any day.

    Link to this
  33. 33. farandfew 3:35 pm 09/2/2012

    My money would be on Jerzy’s second hypothesis, predator avoidance, as the main reason. Given that falling faeces make a noise as well as a smell and that sloths periodically remain a very long time in the same small area and that most potential sloth predators can either fly or climb trees – given all these things I think it’s quite possible that sloths might be more vulnerable to predators if they didn’t have toilets.
    Sloths surely avoid predation primarily by not being detected. They presumably depend on the same strategy while descending to defecate also.
    But then, even if the primary purpose is predator avoidance, it would be unusual if some kind of secondary signalling function did not also develop. It would be hard to tease the two apart.
    Perhaps the best test would be to fake it: bury piles of sloth dung under some trees and scatter it under others and place camera traps to see if either attracts predators and which attracts more. You’d need a pretty big sample probably and it wouldn’t include the sound or work on harpy eagles, or prove that the other reasons weren’t good reasons too.

    Link to this
  34. 34. BilBy 4:44 pm 09/2/2012

    @farandfew “My money would be on Jerzy’s second hypothesis, predator avoidance, as the main reason. Given that falling faeces make a noise as well as a smell and that sloths periodically remain a very long time in the same small area and that most potential sloth predators can either fly or climb trees – given all these things I think it’s quite possible that sloths might be more vulnerable to predators if they didn’t have toilets.” But for an animal that doesn’t move much, it must make itself visible by moving, potentially along way, down a tree, to defecate, leaving a neat, compact package associated strongly with THAT tree, as opposed to scattered by falling a long way – that doesn’t seem to support the predator avoidance hypothesis. Crapping from a tree may scatter the dung, therefore putting it at the base of the tree seems like a signal associated with that tree, so I would prefer a deliberate signalling hypothesis. It’s difficult to know when there is nothing quite like a sloth to compare it with. But I’m really enjoying this thread :)

    Link to this
  35. 35. Dartian 5:29 am 09/3/2012

    Heteromeles:
    Yes, intestinal parasites can kill.

    You don’t need to tell me that. I’m perfectly aware of the fact that they sometimes do that. Which is why I qualified what I wrote with a “usually” in comment #31.

    If we’re going to get into some pointless Internet pissing contest about our respective expertises regarding vertebrate parasites, I might say as a personal aside that, back in my student days (when I had a brief flirtation with ornithology), I was involved with the study of the breeding biology of various seabirds. During that study, I saw plenty of (mainly juvenile) birds that were dead, dying, or sick from parasite infections; I also got to see dissected intestines full of parasites such as acanthocephalans (which, btw, are pretty nightmarish-looking things especially when magnified). So, yes, I’ve seen with my own eyes that parasites sometimes directly cause the deaths of their hosts.

    But, and here’s the rub, I’ve also seen that, in most cases and most of the time, they don’t cause the deaths of their hosts. Most individuals’ immune systems can deal with parasites (at least long enough until they’ve reproduced and are past their prime). This is why I pointed out that, of parasites and predators, the former are normally the lesser evil.

    (Or, just to be crystal bloody clear about this: A hookworm does not ‘intend’ to kill the sloth. An ocelot does. That’s the crucial difference between those two – at least for the purposes of this discussion.)

    The thing about a long transit time is that it gives more time for any organism to grow in the gut.

    But on the other hand, it would also give more time for the host’s immune system to mount up a defence against it, wouldn’t it?

    If you don’t want to read some simple, popular book like Parasite Rex”

    Does that book say anything about sloths?

    The primary sloth parasitology literature admittedly seems to be fairly limited, but what data there is suggest that endoparasites, by and large, do not seem to be a serious problem to sloths. For example, both Diniz & Oliveira (1999) and Sibaja-Morales et al. (2009) (the latter working with animals at the Aviarios Sloth Rescue Center – a.k.a. “Slothville” – in Costa Rica), only found moderate levels of parasite infestations in captive two- and three-toed sloths. Gilmore et al (2001) noted that, apart from a large number of arthropod ectoparasites, sloths harbour various kinds of pathogenic microorganisms, but these do not usually seem to cause serious morbidity. For example, Choloepus is known to be a vector of the flagellate protozoan Leishmania, which causes leishmaniasis in humans but does not seem to cause any pathology in the two-toed sloths. Incidentally, in the Neotropics Leishmania is transmitted by the sand fly Lutzomyia, which, at least AFAIK, is usually found near ground level – which would seem to suggest that sloths are usually stung by the sand flies when they descend to the ground to defecate.

    it’s not at all clear to me that sloths are similarly limited, that if you could feed them better, they’d move faster

    Don’t know about their locomotion but, in captivity, if you offer three-toed sloths sufficient supplemental heat throughout the day, their digestive ‘transit time’ can be significantly increased – so much so that they can be made to defecate every 24 hr (Raines, 2005).

    I have no idea what’s in the sloth behavior literature.

    Raines (2005) briefly comments on hand-rearing of juvenile three-toed sloths at the Aviarios Sloth Rescue Center; no mention of having to teach them how to defecate, alas.

    I’d trust the people who routinely raise orphaned baby sloths over academic speculators any day

    Trust issues aside, for someone who claimed not to “have a dog in this particular fight” you certainly do seem awfully passionate about your ideas.

    References:
    Diniz, L. de S.M. & Oliveira, P.M.A. 1999. Clinical problems of sloths (Bradypus sp. and Choloepus sp.) in captivity. Journal of Zoo and Wildlife Medicine 30, 76-80.

    Gilmore, D.P., Da Costa, C.P. & Duarte, D.P.F. 2001. Sloth biology: an update on their physiological ecology, behavior and role as vectors of arthropods and arboviruses. Brazilian Journal of Medical and Biological Research 34, 9-25.

    Raines, J. 2005. Captive health and husbandry of the Bradypodidae. Zoo Biology 24, 557-568.

    Sibaja-Morales, K.D. et al. 2009. Gastrointestinal parasites and ectoparasites of Bradypus variegatus and Choloepus hoffmanni sloths in captivity from Costa Rica. Journal of Zoo and Wildlife Medicine 40, 86-90.

    Link to this
  36. 36. farandfew 7:21 am 09/3/2012

    @BilBy
    I’m not sure that’s necessarily the case.
    If you are a predator and, for the sake of argument, a mammal and you encounter these two situations (piled/buried dung or scattered dung) what does it tell you in either case?
    Piled/buried dung says that there is, or was until recently, a sloth living in the vicinity. It probably doesn’t restrict itself to a single tree, rather this tree is part of the sloth’s current ‘patch’ of interconnected trees.
    If you’re a mammal you aren’t going to wait for the sloth to come back. You could climb the tree and search for it but that is likely to be more trouble than it’s worth as the sloth may well be hanging somewhere inaccessible and you can’t climb as well as a sloth.
    If on the other hand you’re in the parallel universe where sloths simply let drop. You might smell sloth dung or you might hear it. If you hear it fall it’s obviously fresh. Even if not, it’s probably fairly fresh because dung decays fast in tropical forests. If you’ve learned to hunt parallel-universe-sloths, you look up and you may well see the sloth. Perhaps you can see how to climb up and get it (here I’m feeling a bit shaky because I’m not sure that mammalian predators really are cognitively capable of this). In any case the falling dung will not ‘scatter’ far and will be more likely to be close to the place where the sloth does most of its feeding than buried dung at the base of a tree.
    Of course, in the real universe, if you see the sloth descending to defecate and it doesn’t see you until it’s within reach then that’s also bad news for the sloth.
    But I don’t think it’s obvious which strategy would be worse overall in terms of exposure to predators.
    At least it’s not obvious given my limited knowledge of sloths and their movements.

    Link to this
  37. 37. farandfew 9:37 am 09/3/2012

    I’ve just realised this is probably all untestable. Predator switching and all. You’d need very long timescales.
    With a dung detection dog and a whole bunch of video camera traps, you could determine predation rates on defecating sloths, though. Both those things are pretty expensive, however.

    Link to this
  38. 38. BrianL 12:54 pm 09/3/2012

    Do we know anything about the way ground sloths (especially the smaller species) avoided predation? Should we imagine them to have been rather faster than their living relatives so that they’d be more easily able to put those claws to use against potential predators? I know some sloths had light dermal armour, but is there anything else known about what defenses they had?

    Link to this
  39. 39. Heteromeles 1:21 pm 09/3/2012

    @Dartian: First off, thanks for the information–that does help quite a lot.

    As for the “dog in the fight,” I’m not an academic zoologist, so I don’t have a reputation to protect in that regard.

    The passion is because in my current work situation, I’m outside the academy, dealing with a lot of stupidity caused by people inside the academy who don’t take the time to deal with reality, at least when it comes to creating a conservation plan based on what’s happening on the ground (yes, I’m withholding details, because I’m trying to see if we can get to a workable plan without taking the academic side to court). The query “do you have a paper to support that?” comes across to me as, “well, if you don’t have someone else paying for your access to Web of Science, you must not know what you’re talking about,” whether or not it was intended that way.

    Link to this
  40. 40. Dartian 3:27 am 09/4/2012

    Farandfew:
    dung decays fast in tropical forests

    This is surely an important factor to consider if one postulates that sloths defecate on the ground in order to send intraspecific signals to other sloths. How long does sloth dung last in the heat and the humidity of the tropical rain forest floor, and for how long does the scent of the faeces linger on? If any detectable traces of dung disappear quickly, it would seem to limit the usefulness of scatting on the ground as a social signal.

    Heteromeles:
    The query “do you have a paper to support that?” comes across to me as, “well, if you don’t have someone else paying for your access to Web of Science, you must not know what you’re talking about,” whether or not it was intended that way.

    It wasn’t intended that way. Contrary to how it may perhaps seem, I do not blindly and systematically discredit popular sources, ‘grey literature’, or anecdotal information. It’s just that I far prefer to get important biological information from the primary literature. That’s why I routinely ask if such references are available. That’s also why I usually back up my own comments here on Tet Zoo with references; I do it so that others don’t need to take my word for anything – they can check it out for themselves from the original sources. (Any personal non-mainstream speculation or opinions that I express here I usually try to clearly identify as such.)

    Link to this
  41. 41. farandfew 7:42 am 09/6/2012

    @Dartian
    Warning, this comment will contain zero references to the primary literature :-)

    The last time I looked at this, which was perhaps a couple of years ago and maybe not that thoroughly, I couldn’t find any studies that explicitly reviewed the question of dung decay rates in tropical forest but many studies that consider the question with regard to a particular survey for a particular species in a particular location. A review of the decay rates found in these studies would be interesting provided it looked at the internal variance.
    I say that because my own experience, and what I’ve heard about an unpublished study, leads me to believe that the ‘decay rate’ varies hugely depending on some very fine-scale spatial factors. I put ‘decay rate’ in inverted commas because I think it’s misleading. In many (most?) cases dung does not ‘decay’ but is simply eaten by dung beetles, worms, etc. I suspect that the exact placement of the dung greatly affects the chance of it getting eaten. An obvious example; if placed on a rock, unlikely to be eaten by worms. Some animals do seem to prefer to defecate on top of rocks which suggests that animals which do use their dung as a scent signal may be able to make a big difference to its effectiveness through placement.
    Middens are likely to be an effective placement strategy, not only because the smell is more likely to build up and linger but also perhaps because of predator saturation (by ‘predator’ in this context, I mean dung-eater.
    What effect burying the dung would have, is hard to predict.
    My suspicion is that a sloth toilet would maintain the smell of sloth more consistently than haphazardly-scattered sloth dung so it probably is a better signal, but, on the other hand I suspect it would be a less useful clue to the sloth’s current exact whereabouts in a way that would be useful to a predator.
    All of this would require testing – it’s not a priori obvious which strategy is better as a signal, or which strategy is better for avoiding predators.
    But the fact that some hypotheses are much easier to test than others concerns me, seeing as there is no reason to assume the behaviour has only a single function.

    Link to this
  42. 42. David Marjanović 1:52 pm 09/6/2012

    Perhaps you can see how to climb up and get it (here I’m feeling a bit shaky because I’m not sure that mammalian predators really are cognitively capable of this).

    I’m sure cats are.

    The query “do you have a paper to support that?” comes across to me as, “well, if you don’t have someone else paying for your access to Web of Science, you must not know what you’re talking about,” whether or not it was intended that way.

    Write to the authors and beg for the pdf. More tedious, but free.

    Link to this
  43. 43. Heteromeles 4:37 pm 09/6/2012

    “Write to the authors and beg for the pdf. More tedious, but free.”

    David, with due respect, this is both patronizing and ignorant.

    As I’ve said repeatedly, I’m neither a sloth expert nor a professional zoologist. While I think this is a cool discussion that I’m willing to further, I’m not invested in it enough to bother to read the primary literature, especially since this is a blog, not a journal letters section.

    In the context of a blog discussion, which has a life of perhaps a week or two, it’s stupid to assume that I’m willing to a) hunt down all the papers, b) beg copies, which often takes another week to obtain, c) find the papers’ references, to get the papers that likely have the answers, d) beg copies of those, and e) reply to an offhand comment from one of you.

    Compare that with the process within the academy, where it takes you no more than an hour or two to do the same process online, using the journal and database access that your institution provides.

    Yes, you have a special privilege, paid for by your employer. Enjoy it, but don’t make the mistake of assuming that someone who lacks the same privilege is automatically ignorant, or that my disinterest in getting the papers stems from ignorance. If I’m doing something that personally matters to me, I’ll happily get the papers.

    Link to this
  44. 44. David Marjanović 6:21 pm 09/6/2012

    I didn’t mean for my comment to have a context. I replied only to the one sentence of yours that I quoted.

    Link to this
  45. 45. Laurence Clark Crossen 9:43 am 08/23/2014

    How is it possible for even giant sloths to have survived on the ground, even on continents when even elephants are taken down by lions and when the predators were also much larger then?

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Holiday Sale

Give a Gift &
Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now! >

X

Email this Article

X