Sloths are uniquely adapted to hanging around doing very little, scientists have discovered.
The ponderous creatures, which spend 90% of their lives upside down, have internal organs fastened in place to prevent them squashing their lungs.
Famous for their slow pace of life, sloths can take a month to digest a single leaf and only go for a "number two" once a week.
At any one time, a third of a sloth's bodyweight may be taken up by stored urine and faeces, making their stomach and bowel contents extremely heavy.
Until now, scientists could not understand how they were able to breathe with the full weight of all that waste matter pressing on their lungs.
The answer is that sloths have evolved attachments in the abdomen that anchor organs such as the liver, stomach and bowel and prevent them weighing down on the diaphragm.
Rebecca Cliffe, one of the University of Swansea scientists who conducted the research at the Costa Rica Sloth Sanctuary, said: "With an extremely slow metabolic rate and low energy diet, sloths are experts at saving energy.
"They have a very slow rate of digestion and can store up to a third of their body weight in urine and faeces. 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.
"We estimate that these adhesions could reduce a sloth's energy expenditure by seven to 13% when hanging upside down.
"To a sloth, an energy saving of seven to 13% is a big deal. 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."
The team's findings are published in the Royal Society journal Biology Letters.
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."
Some sloth facts:
:: Sloths live in the rainforests of south and central America. They spend 90% of their life hanging upside down from tree branches by their hind legs.
:: The sloth's chief survival strategy is camouflage. It is so sedentary that green algae grows on its coat, helping it blend into the surrounding vegetation.
:: Sloths sleep for 15 to 20 hours a day and even when awake spend much of their time immobile.
:: Sloths only defecate once a week and carry up to a third of their bodyweight in urine and faeces.
:: A sloth can take a month to digest a single leaf.