A Sloths Life Cycle
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.
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.
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.
Sloths, members of the order Pilosa and related to anteaters, are some of the world’s slowest mammals. They are categorized in two families: Megalonychidae (two-toed sloths) and Bradypodidae (three-toed sloths). Despite this taxonomic distinction, all sloths have similar life cycles.
Sloths spend most of their lifespan sleeping up to 18 hours per day. This is a slow-moving species that is mostly active at night. Its metabolism is also slow, which helps sloths survive bad injuries. Sloth gestation can stretch up to 10 months, after which a single baby is born. Newborn sloths cling to the mother’s abdomen and remain with the mother until at least five months of age. Three-toed sloths are so difficult to maintain in captivity that little is known about their breeding behaviour and other aspects of their life history.
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.
Sloths Lifespan
Arms, hands and hips
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 E. laurillardi was tridactyl, possessing only digits III-V, and of these only digits III and IV had unguals (Cartelle & De Iuliis 1995).
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.
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.
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...
- Ten things you didn’t know about sloths (done in 2007, now in major need of an update)
- What was that skull? (on glyptodonts)
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. Here’s the answer... (still working on it: a larger version will be uploaded to my deviantART gallery later today)...
Refs - -
- ., 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. NaturalHistoryMuseum of Los AngelesCounty, 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? ...
The name “sloth” was given to members of the Megalonychidae and Bradypodidae families to describe six species of animals that move very slowly. But aside from their lack of urgency, how much do you really know about these arboreal creatures?
In honor of Sloth Week, here are ten facts about sloths that you may not have known:
Sloths have an extremely slow metabolism
This one seems fairly obvious, but it is worth mentioning exactly how slow the metabolism actually is. As a frame of reference, it takes the average human around 12-48 hours to ingest, digest, and eliminate waste from food. Sloths can take 30 days to completely digest a single leaf. They rarely like to leave the safety of trees, though they will come down in order to relieve themselves. Fortunately for them, they can hold it all in and only evacuate their bowels once a week. Sloths can lose about a third of their body weight every time they defecate and urinate.
Sloth fur contains fungi and algae, for good reason
Sloths move incredibly slowly at less than three meters per minute, so their top line of defense against predators (eagles, jaguars, snakes, and poachers) is just hoping they aren’t seen. Sloth fur has two layers: an inner layer closest to the sloth’s skin that is short, fine, and provides warmth, and a coarse outer layer that has cracks, allowing for the growth of algae and fungi, which is fantastic news for the sloth. In this case, being filthy and covered in algae gives the fur a green tint, acting as an effective camouflage up in the trees.
As a bonus, researchers recently discovered that some fungi within sloth fur could have applications for fighting off certain parasites, cancers, and bacteria.
Their fur acts as a home for many insects
While there is a population of hematophagous insects that feed on the sloth itself, including ticks and mites, there is also a coprophagous guild of insects which have a commensal relationship with the sloth and feed on the fungi and algae in sloth fur. The coprophagous guild can have around nine different species of beetles, mites, and moths, with many members of each species. One sloth was found to have 980 individual Trichilium adisi beetles living within its fur.
The five species of sloth moths find a host and ride around with it, waiting until the sloth relieves itself (more on that later). After the sloth defecates, the moth lays her eggs in the dung. The larvae remain in the dung until they have reached the adult stage, where they will then find their own sloth to live on and continue the life cycle. As the moths die, they decompose within the fur, feeding the algae and fungi.
Wild sloths don’t sleep as much as we thought
After studying captive sloths, scientists determined that the animals slept up to 15-18 hours every day. However, a study published in 2008 utilized electroencephalogram (EEG) recordings to monitor wild sloths’ brain activity. They found that the average wild sloth sleeps about 9-10 hours a day; not much more than the average human. The researchers noted that “sleep in the wild may be markedly different from that in captivity,” and called for more research to explore all of those differences. Lack of threat of predation while in captivity is an obvious difference, though diet and other factors could be at play also.
Sloths have changed considerably over time
As we know them today, all six sloth species are arboreal creatures who grow to be 50-60 cm (20-24 in) in length. Ancient sloths were ground dwellers, and some species were up to 6 meters (20 ft) long and weighed over 5 tons; similar to an African elephant. These ancient sloths first appeared about 35 million years ago and went extinct around 10,000 years ago, after humans spread throughout the Americas.
Even two-toed sloths have three toes
Stay with me here; I know that sounds counterintuitive. While two-toed sloths indeed have two digits on their forelimbs (arms), they have three digits on their hind limbs (legs). Realistically, a better name for these sloths would be “two-fingered sloths.” It appears that the confusion came when the names were translated into English, as the Spanish word for digit (dedo) can be used to mean finger or toe.
They are surprisingly adept at swimming
Aside from defecating, sloths will typically only leave the trees to take a swim. During the rainy season, sloths will sometimes drop from a tree into the water and then swim to another tree. Using a stroke that sort of looks like a doggy paddle/breaststroke combination, sloths can swim up to three times faster than they move on land. Their incredibly slow metabolism affords them another advantage here, as they can slow their heart rate down to less than a third of its normal rate, allowing it to hold its breath underwater for over 40 minutes.
Specialized anatomy allows them to hang upside down
For the most part, sloths prefer to be hanging upside down. They are able to eat, mate, and give birth while hanging upside down, without the weight from their internal organs and waste products pressing on their diaphragm, affecting their breathing. Researchers discovered that sloths’ internal organs are actually anchored on the abdomen, keeping the weight away from the diaphragm. If this weren’t so, breathing would be much more energetically taxing, and could not be supported by their slow metabolism.
Their parental love isn’t unconditional
Many parents in the animal kingdom will go to great lengths to protect their young. Sloth mothers do care about their offspring, at least to an extent. For the first 3-8 months of a baby sloth’s life, it clings to its mother, even after it has been weaned. While holding onto her young, a female sloth will hiss and swipe their long claws at a potential predator, in order to keep her baby safe.
The limit to that devotion appears to end if the baby should slip and fall. Sloths are fairly sturdy animals, and the fall alone won’t kill most baby sloths. The real danger comes from the fact that the mother rarely comes to retrieve her fallen, crying baby. Realistically, she might not be able to get to her baby and get back up into the safety of the tree before a predator comes, so she doesn’t bother. It has also been suggested that perhaps the mother sees a potential defect in the baby and is just cutting her losses and letting it go.
Fortunately, sometimes the abandoned baby sloths are found by compassionate humans who find them before an eagle or jaguar. The orphaned sloths are temporarily taken into sanctuaries where they are rehabilitated and released back into the wild, as two-toed sloths have a very restricted diet and do not fare well in captivity. While the thought of a mother dropping her baby and leaving it to scream and die is an incredibly depressing thought, the noise that the sloths make is ridiculously adorable.
They break the rules on mammalian cervical vertebrae
Of the estimated 5,400 species of mammals on the planet, nearly all of them have seven cervical vertebrae. Blue whale, possibly the largest animal ever? Seven cervical vertebrae. Kitti’s hog-nosed bat, the smallest known mammal? Seven. Giraffes? Seven. Humans? You guessed it. Two-toes sloths can have 5-7, making those (and manatees, who have 6) the only species to have fewer than the highly-conserved number of seven. Three-toed sloths typically have 8 or 9 cervical vertebrae.
Sloth Behavior
How these anomalies evolved is not entirely clear, and is still debated among biologists. For most other animals, straying from seven cervical vertebrae is a severe birth defect that can either result in stillbirth or the animal doesn’t live very long, which likely played a role in the demise of woolly mammoth. However, the incredibly slow metabolism of the sloth may have made it less risky to have a variable number of vertebrae. For three-toed sloths, the extra vertebrae allow it to turn their heads up to an astonishing 270 degrees.