From living mammals as small as a baby's thumb to extinct mammals almost 5 meters tall, the class of animals to which we humans belong is one of the most wondrously diverse and successful of all life-form groups. The new "Extreme Mammals" show at the American Museum of Natural History in New York showcases that fact in an exhibition that's both scientifically rigorous and richly entertaining.
Only mammals like us - with our highly developed brains, our opposable thumbs and our urge to collect, understand and display - could have put together an exhibit as fascinating and varied as the "Extreme Mammals" show. It's an exhibit filled with surprises. (Click to view a slideshow of amazing mammals.)
Strange and stunning creatures
There is a model of an "Indricotherium," an ancient plant eater about 5 meters tall that weighed about 18 tons when it "galumphed" its way through primeval Central Asia about 35 million years ago. It looks a bit like a cross between a current-day rhinoceros, a horse and a cute, impossibly huge plush toy. (Click to watch a video of the model being built.)
The exhibit also features the skeletons of extinct horned rodents, as well as present-day tarsiers. Tarsiers are nocturnal Philippine primates with skulls so tight they can't move their eyes; they can, however, rotate their heads nearly 180 degrees. Indeed, it seems that when it comes to mammals, extreme is "normal."
The extraordinary range of weights within the Mammalian class is one example showcased at the show, which includes a reconstruction of a small shrew-like mammal from about 50 million years ago called "batanoides" that weighed a gram.
"That's less than a dollar bill [weighs]," says paleontologist and exhibit curator John Flynn. "… And in contrast, you have the blue whale, which weighed over 10 million grams - the equivalent of roughly 40 full-grown elephants. Those are the largest animals ever to have lived."
What makes a mammal?
The first of several interlocking exhibits at the show addresses the question of what a mammal is. Most people focus on characteristics when answering this question. They say mammals have hair, for example. But that characteristic is far from universal.
"Some mammals, like whales, don't have hair because they live in an aquatic environment," says Flynn. "They've lost that hair. Other mammals modify hairs into very specialized structures like the quills on a porcupine."
One display case features stuffed armadillos that show the bony plates that grow within their skin to form a protective covering. And there are odd-looking mammals called "pangolins." These mammals, which inhabit the tropical forests of Asia and Africa, are covered in "scales" made of a fingernail-like substance called keratin.
Babies in all shapes and sizes
One popular section of the exhibit deals with mammalian sexual reproduction. But even here, the mechanics vary widely among the 5,000-plus known mammal species living on Earth today. Like us humans, most mammals develop inside their mother's womb over a relatively long time and are born live.
But other mammals, like the platypus, lay eggs. And marsupials, including kangaroos and koalas, are born tiny and undeveloped, but feed on mother's milk for long periods, often inside a specialized pouch on the mother's body. That pouch protects the baby and also its marsupial mother.
Since marsupials, they don't invest a lot of energy and resources into their young during the gestation phase, it is easier for mothers to abort their young when there is a scarcity of food or in other adverse conditions.
"And it's a lot easier to get rid of them if it's already been born, and it's in the pouch, [and] it's in the external environment," says post-doctorial researcher Ted Macrini, who helped curate the show.
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Yet there is one characteristic that all mammals share: Human and elephant, mouse and porpoise, and all other mammals have three small bones in their middle ear - the hammer, the anvil and the stirrup.
"And fascinatingly enough, two of those bones are actually bones that migrated from the lower jaw of our early vertebrate ancestors," says paleontologist and museum official Michael Novacek.
This assemblage helps refine the hearing, a sense which is extraordinarily acute in mammals. Bats, for example, hear at ultra-high frequencies that humans can't detect, and whales hear at frequencies lower than the human audible range.
"And those ear bones and the design of the ear is very important in that," adds Novacek.
Mammals have shared ancestors - and a shared future
Rather than defining animals by their attributes, evolutionary biologists group animals according to their ancestry. They say all mammals share a common ancestor with a group of ancient "sail-backed" animals resembling reptiles or dinosaurs that lived 250-300 million years ago. These animals are considered proto-mammalian because of features of the skull, the way the bones fit together in the brain case, and the relative sizes of the bones themselves.
"So we see a long transition there," says Novacek, "a kind of a series of links between very reptilian-looking forms and very mammalian-looking forms over a period of, say, 50 to 100 million years."
And there are more transitions ahead, many of them potentially catastrophic. Many zoologists fear that most large mammals may become extinct in the 21st century, and if man-caused climate change is not checked, the survival of many other animal species, including our own, could be at risk. One hopes that the "Extreme Mammal" show at New York's American Museum of Natural History, and exhibits like it, will inspire us to do what our species can do best - think hard and act wisely.