Why do sloths hang upside down?

Scientists have discovered how the lethargic tree-dwellers have evolved to spend most of their slow-paced life upside down

Sloths, the lazy creatures famed (and envied) for their slow-paced, laidback lifestyle manage to spend most of their time hanging upside down because their internal organs are quite literally fastened in place, scientists have discovered.

The lethargic creatures can take up to a month to digest a single leaf, sleep for at least 10 hours a day and can only climb at a leisurely maximum speed of eight feet per minute.

The tree-dwellers head to the forest floor to go to the toilet just once a week, leaving scientists pondering how they are able to breathe with the full weight of all that waste matter pressing on their lungs.

At any one time, a third of a sloth's bodyweight is taken up by stored urine and faeces, making their stomach and bowel contents extremely heavy.

But scientists at the University of Swansea have now deduced that after spending 90 per cent of their lives hanging upside down, sloths have evolved 'attachments' in the abdomen.

These attachments work by anchoring organs in place such as the liver, stomach and bowel and prevent them weighing down on the diaphragm.

The team's findings are published in the Royal Society journal Biology Letters.

Rebecca Cliffe, one of the University of Swansea scientists who conducted the research at the Costa Rica Sloth Sanctuary, said: “For a mammal that spends a significant amount of time hanging upside down, this large abdominal weight pressing down on the lungs would make breathing very costly in terms of energy, if not impossible.

"Sloths have solved this problem by anchoring their organs against the rib cage. They have multiple internal adhesions that bear the weight of the stomach and bowels when the sloth hangs inverted.

“They generate just about enough energy from their diet to move when and where required, but there is not much left in the tank afterwards.

"It would be energetically very expensive, if not completely impossible, for a sloth to lift this extra weight with each breath were it not for the adhesions. The presence of these simple adhesions therefore really is vital."

Co-author Professor Rory Wilson, also from the University of Swansea, said: "Nothing that sloths do is normal. They are quite the most extraordinary and 'off-the-wall' mammals I have ever come across and yet we know so very little about them."