Sloths Move Slow, Evolve Fast

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Sloths might be notorious for their leisurely pace of life, but research published last year shows they are no slow coaches when it comes to evolution.

Sloths, as we know and love them, are small, slow-moving creatures found in the trees of tropical rainforests. But modern sloths are pretty odd compared to their extinct relatives. Sloths (Folivora) are represented today by just six species in two families; the Megalonychidae (two-toed sloths) and the Bradypodidae (three-toed sloths). But 20,000 years ago there were perhaps as many as 50 species of sloth spread across the globe, and most were relatively large, ground-dwelling animals quite unlike modern sloths. While most modern sloths weigh in at a modest 6kg, extinct species such as Megatherium americanum and Eremotherium eomigrans could weigh up to 5 tonnes!

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“Today’s sloths are really the black sheep of the sloth family. If we ignore the fossil record and limit our studies to living sloths, as previous studies have done, there’s a good chance that we’ll miss out on the real story and maybe underestimate the extraordinarily complex evolution that produced the species that inhabit our world.” – Dr Anjali Goswami, University College London

Most species of sloth, including giants such as Megatherium, went extinct around 11,000 years ago. Although the two remaining groups are very similar in terms of body size, habitat and behaviour, they are actually quite distantly related, sharing a common ancestor around 30 million years ago. They are two outer branches on the sloth family tree, with many large, terrestrial sloths separating them. This strongly suggests that the two groups independently evolved their small bodies and arboreal habits.


Rapid Evolution in the Sloth Lineage

Research published last year in BMC Evolutionary Biology indicates that sloths have changed dramatically during their evolution, showing extremely fast evolution in body size over the last … years. Most research on sloth evolution has previously focussed only on extant sloths, but the authors point out this gives a distorted view of sloth evolution. They tested different statistical models of body size evolution against data for 57 species of living and extinct sloth. They found that models considering only extant sloths were poor at explaining evolution across the entire sloth tree. The best models highlighed multiple subgroups of sloth lineages that underwent evolution at different rates, presumably under the influence of different evolutionary pressures. And some of those subgroups evolved very rapidly.

The fastest evolving sloth lineages were the Giant sloth clades Megatheriidae and , showed increases in body size of up to 126kg per million years. This makes sloths some of the fastest evolving mammals known. Larger bodies may have been favourable at the time because of their benefits for competing with other species, or in enabling sloths to exploit a wider range of habitats.

The authors suggest that their method could be used to better understand evolution in many other groups of mammals. Incorporating fossil evidence is crucial if we are to capture the full picture in studies of evolution.

“There are many other groups, such as hyaenas, elephants and rhinos, that, like sloths, have only a few living species. But if we look into the distant past, these groups were much more diverse, and in many cases very different to their current forms.” – Dr John Finarelli, University College Dublin


The Loss of the Large

Why did our larger-bodied sloths go extinct? Most ground sloths, particularly the larger-bodied species, were lost as part of the Quaternary mass extinction event around 10,000 years ago. Some species disappeared earlier than this, and others lasted a little longer, with the most recent sloth extinctions occurring around 4,000 years ago. The Pleistocene Megafauna Extinction, as it is also known, is so-called because the mass extinction event killed off far more large-bodied creatures than small. In total, 55% of all mammal genera over 40kg were lost globally, with the greatest losses occurring in South America and Australasia. Among the casualties were the Giant Sloths, Short-Faced Bears, Sabretooth Cats and Giant Tortoises of the Americas, the Moas and Giant Wombats of Australasia, and the Mammoths, Cave Bears and Woolly Rhinoceroses of Eurasia. Many categorise this event as part of a larger Holocene mass extinction, which is ongoing today.

The causes of the quaternary extinction event are hotly debated to this day, with two main hypotheses dominating the discussion: climate and habitat change, and human hunting. In many cases, the dates of megafauna extinctions line up eerily closely with the dates of human colonisation. Large species are often targeted by human hunters, and larger animals tend to reproduce more slowly, making their populations slower to recover from hunting. Together these factors tend to make larger animals particularly vulnerable to human hunting and other activities.

Research has shown that those species that managed to survive the extinction event tended to reproduce faster. A few slow-reproducing species also made the cut, but they tended to live in closed environments where they lived nocturnal, arboreal lifestyles. Characteristics that would have minimised their exposure to humans. However, other scientists argue that there is stronger evidence that climatic changes around 10,000 years ago were responsible for the mass extinction, with slow-reproducing species again finding it more difficult to respond to rapid changes in the environment. Most probably, the pleistocene extinctions were a result of expanding human populations and climatic changes which resulted in less available food and habitat, and more pressure from hunting.

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