Giant Sloths and their demise

A recent article published in Science (The emergence and demise of giant sloths), on May 22, 2025 looked into these uniquely American creatures that over a period of 30 million years occupied different ecological niches from grasslands, the Andes, and Alaska to Tierra del Fuego, and even one that was aquatic. They grew gigantic but now they are tiny and arboreal. Humans wiped them out when they reached the New World between 30 and 15 thousand years ago.

The article has some interesting data, and concludes that "size disparity increased during the late Cenozoic climatic cooling, but paleoclimatic changes do not explain the rapid extinction of ground sloths that started approximately 15,000 years ago. Their abrupt demise suggests human-driven factors in the decline and extinction of ground sloths."

The paper reported that giant-sized sloths (over 1 tonne in size) appeared independently three times: once among the Megatheroidea, and twice in the Mylodonoidea. The authors found that:

"The primary sloth extinction event began around 15,000 years ago, during the widespread expansion of Homo sapiens in the Americas between about 30,000 and 10,000 years ago. A second extinction event had occurred by the mid-Holocene, after humans arrived in the Caribbean islands, between 9000 and 5000 years ago). These two extinction events were substrate- and size-selective, preferentially targeting partially/fully terrestrial and medium/large-sized sloths
...
he selective extinction of large sloths led to a substantial reduction in the average body size and ecological disparity of the clade, reducing ecosystem complexity and functional richness. Small arboreal sloths likely survived on continents because they avoided extensive human impacts, such as predation, as they dwelt in denser, less-accessible habitats high in the canopy and have considerably reduced muscle mass for mammals of their size".

Size comparison sloths and human. Credits

The image above by Gabriel Ugueto depicts an extinct giant ground sloth (Eremotherium laurillardi) and an arboreal and extant one Bradypus variegatus (Three-toed Sloth), and a human being for size comparison. Look at those claws! Myths were surely creatied based on these giant animals.

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Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2025 by Austin Whittall © 

On the Giant Sloth (an article from 1903)

An article published in "The Saint Paul Globe" (St. Paul, Minn.), on 8 February 1903 that can be seen online here, reported about the progress of Hesketh Prichard's expedition in search of the Mylodon (1900/1901) criticizing him for not exploring the deeper parts of the Andean forests.

It also mentions an encounter with a Patagonian Sloth, which took place around 1893, as follows:

Unless our memory serves us a trick, some half a score of years ago a party of explorers in Patagonia, among whom was a distinguished attache of a German university, encountered a monster answering perfectly to the description given by scientists of the Mylodon.
According as I remember the account to have been published in both American and European papers there were three scientists in the party and but one of these survived to tell the story of their adventures in Patagonia.
The German, according to this report, fell a victim to the mysterious reptile or mammal which was taken for the sloth and which was encountered in some vast virgin solitude on that inhospitable plateau of the Andes described by some who have been there as being beyond the possibility of actual words to convey its desolateness.
Whether it was a mirage of the pre-tertiary fossil which appeared to the travelers, for mirages are as plentiful in Patagonia as leaves in Vallambrosa, [Italian monastery, surrounded by forests. Ed.] or whether it was an actual great sloth, for these monsters might have retired to the fastness of such a remote region and even have their habitations yet in some of the vast caverns of the mountain ranges, cannot be told.

I haven't been able to find any refrence to an expedition with a Geman University member, but I did find the following note in an article published in the Buenos Aires Caras y Caretas magazine (No. 32, May 5, 1899 - online) written by Florencio Basaldúa, who wrote the following:

Breaking News from Lake Musters, reports the attack on the Mylodon by three expeditionary from the party of the former librarian of the La Plata Museum, and its escape due to the invulnerability of the monster's armour, and its aggressive rages; but it is certain that in the end it will fall prisioner of men.

The text quoted further up. Caras y Caretas, 1899

The librarian was Nicolás Illin, who in 1898 set out to hunt the Mylodon, it was one of several expeditions that were sent to search for the Mylodon. Illin also explored Lake General Paz (now Lake Vintter), but didn't find the creature.

Lake Musters is located next to Lake Colhue Huapi, which has its lake creature, and is fed by the Senguer River, where Muster's water tiger (the Iemisch?) was sighted.

Lake Musters is located in the steppe, it is an arid area, except for the water provided by the Senguer River and the lakes, little else is found here, only arid basaltic mesas, some salt flats and the Patagonian shrubs and tough grasses.

In June 1899, issue 35 of the magazine carried a political cartoon cover with a painting (see below) depicting hunters stalking a weird looking mylodon, that carried a British flag and had the word empréstito (loan) written on its fur.

Hunting the "Myllodon" [sic], Caras y Caretas, 1899 (online)

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Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2025by Austin Whittall © 

Mapinguari (continued)

Continuing with yesterday's post, I checked out the website of the TV series (Nightmare of the Amazon, National Geographic Channel, 2011) [7] in 2011 which has a photograph of David Oren, who did some scientific research on the Mapinguary, with a cast of a track left by the beast in the jungle. Below is the photograph: (from [7])

The image is not all that sharp, but it looks like two parallel depressions with a roughly ovoidal shape. And that brought to my memory a description of a similar paw print, which I quoted in my book.[1]

An American, Ned Chace, lived in southern Patagonia for a long period of time (1898 till 1929), and travelled extensively from the Pacific to the Atlantic and visited many mountain lakes in the region. He met plenty of people and must have heard many yarns during those long winter Patagonian nights. Below is one of them, related in third person by the authors the book that narrates his adventures: (bold text is mine)

"A friend of his had followed a track like that of a wooden shoe with two cleats across the sole, until he caught sight of what he took for a hairy pig as big as a bull. Just a glimpse he had. Once or twice, long afterwards, on a still night in a forest, beside a glacier, Chace himself heard a trumpeting, something like a steamboat whistle. That was long before there was a whistle on any Cordilleran lake." [3][1]

This text has two intersting bits of information:

1. The sloth yell. The one issued by the Mapinguary is described as "The horrible soul-wrenching humanlike cry" and when it is reproduced it sounds "loud high cry travels down the scale, ending as a low rumble" [9], could this be similar to the yells of the Patagonian animal, a "trumpeting ... steamboat whistle" heard in Patagonia? A shrill scream?

2. The Cleats. "a wooden shoe with two cleats across the sole", which in my opinion are similar to the tracks of Mapinguari shown in the photograph above.

I decided to check out what could "cleats" mean to an early twentieth century person. My mental image of cleats are the studs on a pair of sports shoes, but what exactly does "Cleat" mean?

cleat. noun ⁄ 'klet⁄
: a piece of rubber, wood, or metal that is fastened to the bottom of a shoe or boot to prevent slipping [4]

And the image of two strips of wood fastened across the sole of a wooden shoe reminded me of those traditional Japanese sandals, a hybrid between sandals and clogs, the Geta. They consist of a wooden base which is separated from the ground by two transversal wooden strips, a cloth thong fastens it to the foot (like those of flip-flops). Below is an image of a Geta:

It is not difficult to see that the print of a Geta in soft soil would reproduce the print described by Chace's friend and that it would look like the plaster cast shown by Oren above.

But this does not mean that the animal is a sloth (either contemporary or extant megafaunal one). In fact, what does a giant sloth paw print look like?

Below is a track left by a giant sloth at Guaminí in the Buenos Aires Pampas, Argentina some 30 kya. The area was a soft muddy region and is full of animal tracks, among which are some decribed as "Tracks of a Giant Sloth" [2]

It looks like a human footprint, no cleats or dual depressions, just a big long print. This human-like appearance led to some interesting debate back in the 1880s, when similar human-looking prints were found in Nevada, at a sandstone quarry (the image below is very similar to the Guaminí prints, and it should be since it also shows a giant sloth paw print) [6]:

The sandstone is about one and a half million years old and the size of the tracks led some to belive that they were the footprints of giants! But science soon took a look at the tracks and concluded that they were Mylodon tracks (its bones had been found in the same quarry).

The reason for the shape of the tracks is that these gigantic sloths walked with a pedolateral foot posture: with the weight of their bodies place on the external part of their feet so the sole of their feet faced inwards and the long claws pointed inwards and upwards. That explains why no claws were imprinted in the soft sand.

Further reading on the Nevada tracks:
LeConte, J. 1883. Carson Footprints. Nature, Vol. 28, Pp. 101-102.
Davidson, Geo., 1883, The Carson Fossil Footprints, Mining and Scientific Press. Davis, Sam P. (Ed.), 1912, The History of Nevada (Vol. II), Ormsby County.
An Interesting website.

The point is that these paw prints belonging to extinct giant sloths do not look like cleat marks at all (however in yesterday's post some witnesses claim that its tracks are like those of human footprints).

But perhaps they represent the rear paws (feet) and not the "hand" prints or front paw marks, because the sloths walked on two feet, like us, bipedally. This was confirmed in 1986 with the tracks discovered at Pehuén Có,another site with fossil tracks, in Argentina, which showed that the Megatherium sloth could and did walk on its two legs at will, and only walked on all fours when the terrain required it.

Maybe front paw marks are rare.

The image below shows a giant sloth walking on its four limbs, the rear ones are turned inwards as we described above, the body rests on the outer part of the foot.

The key are the front paws

Now comes the interesting part, the front paws are resting on the long nails, so the body's weight would press these nails into the soil and leave their imprint on the ground. Would this track mark look like two parallel cleats? If so, this would mean two (2) nails, the image has three (3)... Some explanation is needed if we are to accept the "claws equal cleats" theory.

The hands of sloths

The extant South and Central American tree sloth, the Bradypus is a three-toed sloth and possesses only three digits II to IV on the hand. Extinct sloths have a wide range of hand shapes: [5] "among megatheriids, primitive species of Eremotherium were pentadactyl [five digits] (albeit it with a short thumb and a fifth digit with only one phalanx) while the advanced species E. laurillardi was tridactyl [three digits], possessing only digits III-V, and of these only digits III and IV had unguals" [claws]. [5]

Below are some sloth hands (from D. Naish Tetrapod-zoology [5]),

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. From [5].

So it three fingered sloths with two claws did exist (i.e. Eremotheium laurillardi) as well as the megalonychid Choloepus which only has digits II and III. Sloth phylogenetic studies have shown that on a genetic basis, extant Bradypus (3 toes) are similar to megatherids while the two-toed Cholepus are closer to mylodontids. [8]

So two toed sloths were related to the Mylodons... which lived in South America and may be related to the Mapinguari, this could somehow explain the two claw marks of Mapinguari tracks.

But, would these sloths leave a two-claw-imprint on the ground? one that resembles the plaster cast shown at the beginning of this post? I have not been able to come across frontal paw prints of extinct giant sloths, but will keep on looking for them. If I do and they look like those of the Mapinguari cast, it would explain the odd shape of the cryptid's tracks.

Sources

[1] Austin Whittall (2012), Monsters of Patagonia, Buenos Aires: Zagier & Urruty
[2] Photograph by Sergio Bogan. (2013) Huella de la "Era del Hielo" Exploración y Ciencia. Universidad Maimónides. N°1 ISSN 2314-2855 pp. 24
[3] Le Moyne Barrett, R. and Barrett K., (1931). A Yankee in Patagonia, Edward Chace.. Boston: Houghton Mifflin. pp. 29-30.
[4] Merriam Webster, dictionary
[5] Darren Naish, (2012) The anatomy of sloths, August 30, 2012.
[6] Photograph from: W. M. Keck Museum University of Nevada Reno
[7] Nightmare of the Amazon, Nat. Geo. Channel. 2011.
[8] Greenwood et al Mol Phy Evol 2001 / See Poinar et al Current Biology 2003
[9] Marguerite Holloway, Discover Magazine, (1999). Beasts in the Mist. Sept. 01, 1999

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Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia Copyright 2009-2014 by Austin Whittall ©