They found a clear trend for the evolution of a larger and larger body size in sloths over time. This was not just true for ground sloths, where for example the family including Megatherium showed a huge average mass increase of kg per million years. These trends were also seen in the family Megalonychidae, which includes modern-day two-toed sloths, where there was an average body mass increase of 2.
Sloths first emerged 50 million years ago and began to spread throughout first South America and then North America, following the Great American Interchange. The results of this new study suggest environmental conditions during this period selected for ever increasing body size.
The researchers speculate this could have been due to the climate or competition with larger species. The arrival of humans in the Americas around this time is often believed to be the cause of these mass extinctions, although it is possible that the ice age around this period contributed as well. Whatever the cause, clearly only the small bodied tree-dwelling sloths, hidden in their tropical forest refuges, were able to survive these events, completely reversing the millions year old trend towards larger and larger sloths.
Reconstructing the trends revealed in this research would not have been possible without including data from the fossil record. As the researchers say, it is hard to infer from a group of small-bodied leaf eaters the existence of something like the giant Megatheriid ground sloths.
The researchers suggest sloths might not be the only group where modern day species are unrepresentative of overall evolutionary trends. They stress the importance of considering fossil species when looking at overall evolutionary trends, especially in other groups, such as hyenas or tuataras , where diversity today is a fraction of what it was in the past. Modern day sloths then, are revealed not just to be the oddest of mammals, but the black sheep of their own family.
Two-toed sloths are omnivorous , meaning they can consume plants and animals. Their diet includes fruits, leaves, insects and small lizards. Three-toed sloths, on the other hand, are almost entirely herbivorous plant eaters. Their diet consists primarily of leafs and buds from select species of tree, including the leafy cecropia tree.
But like many herbivorous mammals , sloths have a multichambered stomachs filled with symbiotic bacteria that can break down cellulose. Sloths digest food even more slowly than they eat it. In fact, it can take up to a month for a sloth to digest a single meal, according to the Jacksonville Zoo in Florida.
Their leafy diet isn't very nutritious, so they don't get much energy from it, which could be the reason for their sluggish lifestyle. Despite sloths being relatively defenseless creatures, their populations aren't struggling as a whole. Nonetheless, humans pose a threat to the continued survival of sloths through deforestation and poaching.
However, the maned three-toed sloth Bradypus torquatus , native to Brazil's rapidly shrinking Atlantic Forest, is classified as vulnerable to extinction and the pygmy three-toed sloth, found only on Panama's Escudo Island, is classified as critically endangered. Sloths are considered the world's slowest animal. They creep at such a slow pace that algae grows on their fur. This green algae, known as Trichophilus , grows only on the fur of sloths. More than half of all sloths harbor this algae, which gives their coarse, brownish-gray coats a greenish hue during the rainy season.
The algae works to the sloth's advantage. Having green fur provides these arboreal animals with camouflage to blend in among the trees.
Staying out of sight is the sloth's best defense against its predators, which include jaguars, harpy eagles and humans. All mammals, from humans to giraffes, have seven vertebrae in their necks — except for sloths and manatees. Two-toed sloth species have between five and seven neck vertebrae, while three-toed sloths have eight or nine.
Having a few extra neck vertebrae allows three-toed sloths to rotate their heads up to degrees. Sloths mate and give birth while hanging in the trees. Three-toed sloth babies are often seen clinging to their mothers—they travel by hanging on to them for the first nine months of their lives. On land, sloths' weak hind legs provide no power and their long claws are a hindrance. They must dig into the earth with their front claws and use their strong front legs to pull themselves along, dragging their bellies across the ground.
If caught on land, these animals have no chance to evade predators, such as big cats, and must try to defend themselves by clawing and biting. Though they couldn't be clumsier on land, sloths are surprisingly good swimmers. They sometimes fall directly from rain forest trees into rivers and stroke efficiently with their long arms. Three-toed sloths also have an advantage that few other mammals possess: They have extra neck vertebrae that allows them to turn their heads some degrees.
All rights reserved. Common Name: Three-Toed Sloths. Scientific Name: Bradypus. Type: Mammals. Diet: Herbivore. Size: 23 inches. Weight: 8. Size relative to a 6-ft man:. This photo was submitted to Your Shot, our photo community on Instagram.
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