ZoOLogy > What's This? Breathing

But the axolotl is not your typical salamander. It lives its whole life underwater.

Most salamanders are like frogs and other amphibians: they start their lives in water , then lose their gills and grow lungs as they mature. As adults, they breathe air and live on land. This is how swimming tadpoles becoming hopping frogs.

But the axolotl never goes through metamorphosis. So as it grows, it keeps its fish-like fins and the feathery gills to get oxygen from the water.

How long can you hold your breath? If you're like most people, a minute would be a long time. But the elephant seal can hold its breath for up to two hours! This marine mammal can dive down nearly a mile (1,430 meters) while hunting fish and squid in the icy waters around Antarctica .

What allows them to survive for so long on just one lungful of air? Like us, elephant seals are mammals. Pound for pound, the seal has three times the amount of blood that you do. It can also store three times the amount of oxygen in its muscles as you can.

As any mountain climber knows, the air gets thinner and thinner the higher you climb. This means that there's less oxygen to breathe. But some animals, like the Ruppell's vulture, do perfectly well at high altitudes with low oxygen levels.

In fact, one of these large birds once collided with a plane 37,000 feet (11 kilometers) over Africa. That's higher than the peak of Mt. Everest! High-flying birds like the Ruppell's vulture have a protein in their blood that helps their bodies function with less oxygen.

When animals hibernate in the winter, they take in very little oxygen. A hibernating bear may take a breath only once a minute. How do animals get by on so little oxygen? Their body systems slow way down. But brains still need a lot of oxygen.

The Arctic ground squirrel solves this problem by "powering down" its brain in the winter. During hibernation, the connections between brain cells wither. Like most hibernators, the squirrel wakes up now and then. The connections in the brain "rebuild" themselves very quickly and the little animal drifts back to sleep.

Mammals like bears and squirrels are the most familiar hibernators. But many other creatures – like birds, amphibians, and reptiles – enter this winter "sleep" to conserve energy. Painted turtles hibernate in an unusual place: at the bottom of a frozen lake, without any oxygen at all!

Buried in mud, these reptiles can survive four or five months without oxygen. They get energy  from their body tissues, which cause a harmful substance called lactic acid to build up. The calcium in their hard, heavy shells reacts with the lactic acid, making it harmless.

Many creatures respond to cold by shutting down and using less oxygen, but the bumblebee does the opposite.

When temperatures drop to a chilly 40°F (5° C), the bumblebee keeps flying even when other insects can't. Running its flight muscles in the cold takes more oxygen, but the payoff is worth it: since other nectar-feeders can't fly, bumblebees have first dibs on flowers!