What Some Animals Will Do to Survive on an Island

An island can be an especially safe place for some animals to live. It's like a castle with a moat around it to keep strangers and enemies out. But unless they fly or swim, animals cant leave, either. If things go wrong, they can be stuck, not safe, on their island.

Maybe that's why some island animals can get, well, kind of weird.

Babies in Their Pockets

The bettong, the devil, and the eastern quoll. The bandicoot, the pademelon, and the potoroo. The names sound as if they belong in a video game or a fairy tale, but they are all the names of real animals that are found on Tasmania, an island that lies about 150 miles south of Australia.

Their names aren't the only thing unusual about these animals. They all belong to a group of mammals known as marsupials. Marsupial babies are very tiny and not fully formed when they're born. Their mother carries them in a pouch where they keep on growing. Even after the baby is fully developed, the pouch serves as a safe place for it to hide and an easy way for the mother to carry her offspring about.

Marsupials, which include kangaroos and koalas, are found primarily on the island continent of Australia and nearby islands. A few can be found in South America, and North America has its own marsupial, the opossum. But it's only on Australia and neighboring islands that marsupials are common. Elsewhere, placental mammals dominate. Placental babies grow inside their mother's body for a much longer time than marsupials, and when they are born they are much more developed. Dogs, horses, lions, tigers, mice, squirrels, whales, and humans are all placental mammals.

Why aren't marsupials more widespread? Why are there no kangaroos or koalas in America? Part of the answer has to do with how their island homes were formed and how animals adapt to an isolated island environment.

Drifting Continents

Long, long ago all the continents were joined together. Over time, the great landmass broke up and the continents slowly drifted apart. Before the big breakup, however, early mammal ancestors had already separated into placental and marsupial types, and both types began to spread across the landmass. Fossils show that marsupials were once common in what is now North America, for example, but they died off when placental mammals reached the area, perhaps because the bigger-brained placentals were better at competing for food.

When the land that became Australia broke free, however, only marsupial mammals were living there. As Australia drifted, more and more ocean separated it from the other continents, making it impossible for placental mammals to invade. Isolated, without competition from the placentals, Australia's marsupials thrived. Over millions of years, many different kinds of marsupials developed, each adapted to its own special environment and often taking on a way of life filled by a placental mammal elsewhere. So there are marsupial burrowers and tree-dwellers, meat-eaters and grazers, some that resemble cats and others that are similar to dogs.

Saved by Rising Seas

Some marsupials are so unusual they are found in the wild on only one small island. The Tasmanian devil, for example, gets the first half of its name from the island where it lives. It was given the second half by early European settlers. They called most other marsupials by less imaginative names, such as marsupial mouse, marsupial mole, and Tasmanian wolf. But the devil, it seems, was so strange it couldn't be compared to any other animal.

Scientists can tell from fossil evidence that the Tasmanian devil used to live in Australia as well. The devil died out there, however, when the first humans arrived and brought the dingo dog. Dingoes are able predators, and they wiped out the devil in Australia. But the dingoes didn't come to Tasmania. About 10,000 years ago rising sea levels cut Tasmania off from mainland Australia. Because Tasmania was isolated by seawater, the devil survived.

Where Birds Ruled

Marsupials weren't the only animals to benefit from living on an isolated island. When the land that formed New Zealand, an island country near Australia, broke away from the ancient landmass, there were no mammals on it. Bats, because they can fly, were able to colonize New Zealand's islands at a later date. But there were no ground mammals. No wolves or cats. No deer or sheep or pigs or rabbits. Over time, birds took over the habitat that ground mammals would have occupied.

Because they had no predators to challenge them, the birds gradually lost the ability to fly. Flying demands a lot of energy. Rather than wasting energy on growing strong wings, flightless birds developed stronger legs for running or longer beaks for digging-features more useful for life on the forest floor.

So on New Zealand, flightless bush wrens, rather than mice, scurried about the forest floor, and instead of squirrels, kokakos climbed through the tree branches. The most impressive of the flightless birds was the moa, which reached a height of 12 feet. New Zealand's answer to the giraffe, it dined on leaves and shoots high in the trees. Unfortunately, the island's first human settlers, the Maoris, who came to New Zealand between 1150 and 1350, liked moa meat. Unused to predators and unable to fly away, the giant bird was an easy target, and by the mid-1700s the moa was extinct.