Sloths are not exactly known for their speed.
But giant sloths that lived millions of years ago were speedy when it came to evolution.
Thirty million years ago modern day sloths diverged from ancient giant sloths, which were, as their name suggests, much bigger – some weighing up to 3800kg, around the size of an elephant. Today's sloths tend to weigh between 4 and 6kg.
A new study in BMC Evolutionary Biology reveals that some species of ground sloth added more than 100kg to their body weight every million years.
That might not sound amazingly fast, but that's just because evolution usually happens so s-l-o-w-l-y on the time scales we're used to dealing with (i.e. how long is it until lunch?) even that quick evolution doesn't sound particularly impressive. But it is.
The two sloth families, Megatheriidae and Nothrotheriidae, included in this analysis emerged around 23 and 12 million years ago. They both became extinct just 11,000 years ago.
Large body size must have been favoured at the time, perhaps because of competition between species or the climate the sloths lived in, in order to drive this growth.