Breathless.

Imagine a newborn mammal starting life without a single breath from its lungs. Impossible? Until recently, scientists thought so, too. They were astonished to find that a baby mouse from Australia called the Julia Creek dunnart breathes oxygen through its skin!

"That was thought to be unthinkable in mammals," says Jacopo Mortola, a respiratory (breathing) systems expert at McGill University in Montreal, Quebec. Amphibians like frogs and salamanders can breathe through their skin at birth.

How could dunnarts survive? At birth they weigh just 17 milligrams--about the size of a grain of rice! (Four-millimeter-long newborns reach an adult length of about 25 centimeters.) Dunnarts are so small at birth, Mortola reasoned, they don't need as much oxygen as larger newborn mammals do. He theorized that dunnarts absorb tiny amounts of oxygen right through their skin like newborn tadpoles.

To test his hypothesis, Mortola and Australian colleagues isolated newborn dunnarts in separate, sealed chambers. They extended tiny tubes from each mouse's mouth and nose to a second sealed chamber that collected air released from the lungs.

By comparing the amount of oxygen in the two chambers, scientists determined that, at birth, the mouse receives all oxygen through the skin without using its lungs. Dunnart lungs become more active as the animal grows. At three weeks the lungs provide two-thirds of the mouse's oxygen. (Researchers halted the experiment after three weeks because the facial tubes made the little dunnarts uncomfortable.) Mortola thinks that dunnarts breathe entirely with their lungs at about one month old

A newborn dunnart's sheer skin lets air pass through and delivers oxygen directly into blood vessels. "Its skin is so thin you can actually see internal organs without an x-ray," Motorola says. "What we have here is an animal that is genetically a mammal but behaves like an amphibian!"