A Swansea University team has found out how sloths are able to spend up to 90% of their lives hanging upside down yet continue breathing normally.
The research found the mammals, which live in the rainforests of south and central America, have a way of fixing their internal organs to the rib cage.
These adhesions prevent the stomach, liver, kidneys and even the bowels and bladder from pressing on the diaphragm.
The research carried out in Costa Rica is published by the Royal Society.
The scientists say much is still to be learned about these elusive and endangered creatures - the world's slowest mammals - as even basic information such as their natural diet and habitat preference remains a mystery.
PhD zoology researcher Rebecca Cliffe, 24, is one of the authors of the paper, based on work at the Sloth Sanctuary of Costa Rica.
She 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 sloths energy expenditure by 7% - 13% when hanging upside down.
To a sloth, an energy saving of 7% - 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."
Prof Rory Wilson, of the College of Science at Swansea University, a joint author of the paper, 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.
"How foolish we would be to watch these creatures become victims of deforestation and habitat fragmentation and the like without having the slightest idea how to help."