There's no denying that sloths are weird creatures. They risk their lives every time they poo, their organs are stuck to their bones, and finding a mate looks like a nightmare.
Now we can add an extra feature to the sloths' repertoire: their metabolism is so weird, scientists have never seen it before in the animal kingdom.
Most mammals have something called a thermoneutral zone. It's the temperature range which is similar enough to our body temperature to keep us comfortable, without using much energy.
On either side of this zone (too hot or cold) we use a lot of energy trying to keep our core temperature comfortable.
Not all animals operate this way, though. Reptiles, for one, don't have such a zone. Instead, because metabolism works faster in hotter temperatures, they try to conserve energy when cold, and use lots of energy when hot.
Which brings us back to sloths. Researchers already knew there was a strong link between sloth body temperature and the temperature of the environment.
In general, the hotter the environment, the more hyper the sloth; this has led sloth metabolism to be likened to that of reptiles.
"An increase in temperature should, theoretically, result in an increase in metabolic rate. However, nobody really knows," the researchers wrote on the Sloth Conservation Foundation (SCF) website.
"Considering the sloths limited energy supply and the potential knock-on effects of a warming climate, we designed an experiment to find out."
The researchers from the SCF, Swansea University and Queen's University Belfast in the UK, monitored eight adult three-fingered sloths (Bradypus variegatus) in a metabolic chamber.
They worked out the resting metabolic rate, and then increased the temperature slowly from 21 to 34 degrees Celsius (69 to 93 Fahrenheit) – all temperatures that sloths would experience if they were in their jungle habitat.
But what they found was fascinating.
"It appears that sloths do not behave like mammals, reptiles or birds when it comes to their metabolic response to temperature," the team explain.
"They use very little energy when it is cold (just like a reptile), lots of energy in the middle (between 26-30 degrees C° [78-86 F°]), but then as they get too hot, they begin to use less energy again."
"This reduction in metabolic rate at high temperatures is the exact opposite of what typically happens in all other animals," they added.
This is the first known example of a mammal reversibly reducing its metabolism without doing something like hibernating; the sloths were still wide awake and aware of their surroundings.
So what does this mean? Why are sloths being so weird?
Well, the researchers aren't sure, but think it could have to do with the animals not wanting to get too hot in the scorching sun.
Sloths don't have much option when it gets too hot except to move into the shade and stay put; but having an overactive metabolism would just make you hotter.
"You depress your metabolism and you just sit still and wait for the heat to pass," Rebecca Cliffe, founder of SCF and lead author of the study, told National Geographic.
"So it does make sense, but it was totally unexpected."
This research has important implications as the world heats up, as a hot sloth might not be able to eat and mate as effectively.
But we still don't know how this mechanism actually works in sloths, and so that's what the researchers are looking into next.
Meanwhile, we can all enjoy this video of baby sloth sounds, because how cute are they!
The research has been published in PeerJ.