Anteaters, armadillos and sloths; the original South Americans

South America is a joy for mammal-watchers; for the lucky and well-guided visitor mammalian treats include Jaguars, tapirs (three species), otters (four species, including the magnificent Giant Otter), camel relatives (Guanacos and Vicunas), Spectacled Bears, peccaries, a wonderful array of monkeys and various strange and impressive rodents including Capybaras, viscachas and agoutis. There are of course also many marsupials, but they are small and nocturnal and most are rarely seen. Famously the marsupials are synonymous with the early days of South America, as part of Gondwana, a mighty southern landmass that included Australia. However, all the other mammals mentioned above except the monkeys and rodents are recent arrivals (or at least their ancestors were), crossing the Isthmus of Panama from North America within the last three million years. (Since then the North American camels and tapirs have disappeared.)

Jaguar (Panthera onca), Cuiabá River, Pantanal, south-western Brazil; one of the most-desired
sightings in South America for any naturalist.
However all modern South American carnivores and hoofed mammals are recent arrivals.

The ancestral monkeys and rodents seem to have rafted from Africa 35 and 45 million years ago respectively, which certainly makes them 'old South Americans' and very different from their distant African relations, but they are far from the oldest.

Capybaras Hydrochoerus hydrochaeris, Pantanal, Brazil.

Ecuadorian Squirrel Monkey Saimiri macrodon, Yasuní NP, Ecuador.
Both the 'caviomorph' rodents and monkeys can lay legitimate claim to being 'genuine'
South Americans, but they are far from the oldest South American eutherians
(ie all South American mammals except the marsupials).

Until relatively recently there was a wonderful array of ancient South American eutherians, as well as big carnivores whose relationships to marsupials is debated. But the greatest diversity was in ancient hoofed mammals totally unrelated to any living herbivores. There were grazers which resembled horses, camels, rhinos, tapirs and elephants - but which were something else entirely. Some had disappeared through natural processes long before the North Americans arrived, but others survived until the last few thousands or tens of thousands of years. Not one of them has living descendants though, sadly.

However one ancient South American group certainly does. The 31 living Xenarthrans represent a very old line of mammals, the anteaters, sloths and armadillos, which are totally different from any mammals anywhere else. Today the armadillos form one Order and the anteaters and sloths another. Their ancestors, apparently leaf-eating tree dwellers like modern sloths, arose in South America somewhere between 60 and 80 million years ago (estimates vary) and developed and diversified in isolation from the rest of the world until the recent push by some into now connected North America. It is surprisingly difficult to find published evidence of the times of divergence of the three modern groups, but it seems that armadillos had emerged as a separate group by 45-48 million years ago. Within the other Order, the oldest sloth fossils are 34-38 million years old, and the anteaters seemingly appeared some 15 million years later. (I found this article probably the most helpful.)

Big Hairy Armadillo Chaetophractus villosus, Chilean Patagonia.

Brown-throated Three-toed Sloth Bradypus variegatus, Costa Rica.

Giant Anteater Myrmecophaga tridactyla, Pantanal, Brazil.

It seems remarkable that animals so dramatically different could be related, but 45 million years of adaptations is a very long time! Nonetheless features that they share, which reflect their relatedness and their differences from other mammals, include very simple dentition (no teeth at all in the case of the anteaters), internal testes, shared spinal peculiarities and very low metabolic rates (around half that of other eutherian mammals of equivalent size) and body temperatures (no more than 35 degrees and down to 30 for some sloths).

Armadillos are characterised by leathery armour (comprised of bony scales covered by horny skin and separated by flexible strips of skin which allow some species to roll into a ball for protection). They are extremely proficient diggers for grubs, ants and termites, and construct substantial shelter burrows. There are 21 living species in two families, but many more existed in the past, including a diversity of the giant heavily-armoured glyptodons (up to five metres long) and the extinct family Pampatheriidae, which were also large (up to 200kg) and well-armoured. There are two living armadillo families, represented by the following two species.

Six-banded Armadillo Euphractus sexcinctus, Pantanal, Brazil, above and below.
The 'bands' of scales are clear here, along with the armoured tail and powerful digging feet.
But don't try and count the bands, I found that didn't help at all!

Their eyesight is very poor (and they only see in black or white, or degrees of light intensity)
but they have a good sense of smell.

Nine-banded Armadillo Dasypus novemcinctus, Pantanal. This one is found from Uruguay to the
central US, the widest range of any armadillo. (However it only expanded into the US in the late 1800s.)
A big armadillo, they can weigh up 10kg and be over a metre long.
Note the long ears, and the comment above regarding the number of bands!

It is hard to imagine a more contrasting lifestyle than that of ground-living and -burrowing armadillos and the exclusively arboreal sloths, which specialise in tropical rainforests. There are six living species in just two families, but in the past they were a far more diverse group, with five whole families of ground sloths (sloths that 'came down') many of which were giants. The biggest, Megatherium spp., were the size of modern elephants. There was even a genus of five species of long-tailed aquatic sloths which fed underwater on marine plants - algae and seagrasses - which they harvested from the seafloor. The most recent giant ground sloths, members of at least four families, lived in South Americans with humans until about 10,000 years ago. Their near-simultaneous extinctions suggest a human involvement, and this is supported by the fact that ground sloths in the Antilles survived until 5000 years ago - when humans arrived.

Lifesize statue of Mylodon, three metres long and weighing a tonne, at the Cuevo del Milodón
in Chilean Patagonia. Well-preserved remains, including bones and skin, were found
here in the 1890s. It's an impressive sight!

The two living families comprise the Three-toed and Two-toed Sloths respectively (actually it should be 'fingered' - they all have three toes), but though they look almost identical they have been separated for 28 million years. It seems that if you're going to live upside down in trees, you're likely to look very much like a sloth! Because they do live most of their lives upside down, their hair grows in the direction of feet-to-body, unlike all other mammals, so that rain runs off. Because their legs don't have to support the weight of their body, sloths have a significantly lower proportion of body muscle than other mammals. When they do come down - which bizarrely they do, about once a week, to urinate and defecate, and carefully bury the waste, at great risk - they have to drag themselves along the ground.

Hoffman's Three-toed Sloth Choloepus hoffmanni on the ground, Tambopata reserve,
southern Peruvian Amazon. Above and below.

 

Brown-throated Three-toed Sloth changing trees, Myuna Lodge, northern Peruvian Amazonia.
The extra articulations in the lower spine enable them to extend at right angles from a branch too.

The green tinge faintly visible in the fur of this Three-toed Sloth is due to the presence of two species of
blue-green algae in grooves in the hairs. This assists with camouflage from aerial predators.
Moreover various species of moths, beetles and ticks also live in the fur.

The contrast between sloths and anteaters, in the same Order, is immense. The four anteater species are similar in form, but range in size from the tiny Silky Anteater Cyclopes didactylus, only 30cm long, to the huge Giant Anteater which can be well over two metres long. They all have a long tubular snout and no teeth, but a hugely extendable tongue, longer than the head. This tongue is equipped with little back-pointing spines and coated with thick sticky saliva. The Giant Anteater is solely terrestrial, but the other three climb and have prehensile tails. They move from ant or termite nest to nest, to avoid being excessively bitten or stung each time. A Giant Anteater may visit 200 nests a day and eat up to 35,000 ants, but not many from each colony. Anteaters lack stomach hydrochloric acid, but rely on the formic acid of their prey to digest them!

When you encounter a Giant Anteater in the wild - and Brazil's southern Pantanal is perhaps the most reliable
place to do so - it is obvious that its eyesight is very limited, and hearing only moderate, but its sense of
smell is very acute. It has very powerful claws for ripping into termite and ant nests; to protect them
the Giant Anteater walks on its knuckles.

In the Pantanal, where only a tiny area is reserved, Giant Anteaters seem to thrive alongside the cattle.

In size between the Silky and Giant Anteaters are the two very similar tamanduas (Northern and Southern) which can be up to 1.5 metres long, but usually less. They forage both on the ground and in trees.

Southern Tamandua Tamandua tetradactyla, south Pantanal. It took me many years of searching
to finally see one of these beautiful animals - the Pantanal truly is a wonderful place.

Unlike the Giant Anteater, tamanduas protect their claws by walking on the outside of their front feet.

Everything about South America is fascinating, but these very ancient South Americans are up there with the most fascinating aspects of all. I hope you can see them some day, if you've not already done so - they are very special indeed.

NEXT POSTING THURSDAY 2 APRIL.
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the next one; in the meantime there's lots to read in past posts!)
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