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This edited article about prehistoric animals originally appeared in Look and Learn issue number 954 published on 3 May 1980.
Sloths are proverbially lazy – in the rainy season they camouflage themselves with mould rather than move. But have they, perhaps, outlasted their more active ancestors for that very reason?
The tree sloth is the slowest moving of mammals and amply deserves to be named after the last of the seven deadly sins – it spends only ten per cent of its time making any movement at all.
Sloths are nocturnal animals which live in the tropical forests of South and Central America, from Argentina to Nicaragua. Their long, curled toes hook round branches, allowing them to hang idly for most of their lives: even when they die they retain their grip on their branches. Their diet consists of leaves, shoots and fruits which take about four days to pass through their stomachs, and they only amble down from their trees about once a week to relieve themselves. Their arms are longer than their legs and, on the ground, the sloths have to pull themselves along because they are incapable of walking. Babies spend the first two years of their lives clinging to their mothers’ backs.
It is surprising, then, to discover that one of the ancestors of these creatures was seven metres long and lived a full and active life on the ground. This was the Megatherium, which grew to the size of a modern elephant and weighed several tons. It lived on plants and had teeth only on the side of its jaws. It was slow and rather cumbersomely built, but it could walk on its hind feet as well as on all fours.
There were several kinds of giant sloths in South America in the Pleistocene era around 500,000 years ago, and living alongside them were giant armadillos like the glyptodonts. They were as big as rhinoceroses, with armour-plating fitted so tightly together that they looked like huge tortoises.
All of these giants were edentata, an order of animals which has evolved into today’s sloths, armadillos and anteaters of South America. Edentata literally means without teeth. The anteaters, which catch their food with long, sticky tongues, are the only ones which do not have any. Armadillos can have up to 100, while sloths have a few simple pegs. These are not enamelled but constantly grow as they wear away.
We know nothing of edentata before two suborders which were alive in the Paleocene era 60 million years ago. No fossil remains have so far been discovered to link them with one common suborder, but they both have common characteristics that put them in the same nest. One of these suborders was the Palaeanodonta, which had two families which died out after some 30 million years.
The other suborder, Xenathra, was more abundant, producing a large number of genera and species throughout the continent. Its three infraorders determined the lines of armadillo, anteater and sloth that are living today. The sloth family tree comes from the Pilosa infraorder. This bore four families of ground sloths and one specifically of tree sloths. The most important families of ground sloths were the Megalonychidae and Megatheriidae. The Megatheriidae provided the first fossil deposits of sloths and they date from the Oligocene period (38-26 million years ago). Although the tree sloth is thought to have developed from this family, there are no fossil remains to connect them.
This family was small compared with the others and by the early Miocene it had evolved sloths like the Hapalops. These had claws on five toes and were 1∑3 metres long. They had few teeth but had a spout protruding from their lower jaws. After the Hapalops came the Nothrotherium, but again it was a dwarf compared to its contemporary Megatherium.
In the last stages of pre-history, when the land bridge linked the two American continents, several genera of sloths made their way north, while some found a passage across to the West Indies where they lived for a time. Curiously, the modern tree sloth is not a bad swimmer, but it is unlikely that these early sloths seriously took to the water.
There are no fossil remains of tree sloths and the ones living today all belong to the Bradypodidae family of the Pilosa infraorder. There are two genera – the Choleopus to which the two-toed sloths belong and the Bradypus, which contains the three-toed species.
Thirty-one members of this edentata order are alive and living in South America today. But some of them are in danger of dying out – the three-banded, Burmeister and giant armadillo, which grows to around 1∑5 metres long, as well as the tiny pink fairy armadillo of Argentina which is only 16 cm long. Their strange armour-plating can be turned into unusual mementoes and they are preyed on by poachers. The giant ant-eater, too, may be in danger.
Of the sloths, the three-toed Bradypus torquatus is now in danger of extinction. As with other sloths, its immobility and colouring have kept predators away. Now, however, the forests are being developed and the sloth is unlikely to run away.