Jacopo Mortola on how he discovered the marsupial

But the creature that performs this clever trick is a newborn baby only 4mm (0.15 inches) long.

The Julia Creek dunnart is a marsupial mouse from Australia which gives birth to one of the tiniest of newborn mammals in the world. After only 12 days gestation, the pup is delivered to the pouch.

"It is a very small, transparent creature which cannot yet contract its lungs, but just wiggles with uncoordinated movements,"Professor Jacopo Mortola, from McGill University, Canada, told BBC News Online.

"Just looking at it, you realise that breathing would be an impossible task. It survives by exchanging oxygen and carbon dioxide through the skin.

Dunnarts are up to 25cm long and live in the Australian desert

"On the first day after birth, the skin contribution is total but gradually declines as lungs take over.

"By three weeks, the skin contribution is 50%, which is still an enormous amount if you consider that for other mammals, including humans, the skin contributes zero."

Breathing through the skin is the quirky way in which nature has solved the problem of keeping such an undeveloped pup alive.

But Professor Mortola says: "It is a zoological curiosity, but the interest is the implications for future studies of the regulation of breathing.

"How breathing follows the body's energy use so closely is a 200-year puzzle, but here we have a creature that doesn't have to breath. So we can explore, with a natural model, the link in a way that was unthinkable before."

Professor Mortola, and his co-workers at La Trobe University, Australia, proved the mouse pup was using its skin to breath by fixing a tiny mask to its face. This measured the lungs' contribution.