Essential Questions

This exhibition uses both living and extinct mammals to trace the ancestry and evolution of this remarkable group of animals. It's a story full of surprises, with quite a cast of characters.

What is a mammal?
You might have grown up thinking that all mammals share certain traits, like fur and giving birth to live young—and most living mammals do. But some are hairless, like the river dolphin, while a few others, like the echidna, or spiny anteater, lay eggs. So what defines this diverse group of animals? They all are descended from a common ancestor shared with no other living animals. Mammals can look radically different from each other, but the underlying body plan is still strikingly similar.

What's "normal" for mammals?
The terms "extreme" and "normal" are relative, of course. In mammals, one way to define normal is in terms of traits that are inherited from ancestors, such as producing milk, warm-bloodedness, and four limbs for walking, or quadrupedalism. Another way to think about "normal" is what is typically observed in the group. For example, today the typical mammal weighs about one pound, although for most of their history they were generally even smaller. All mammals have both normal and extreme, sometimes unique, features.

How and why are some mammals "extreme"?

The six-banded armadillo is almost completely covered by bony, keratin-capped armor plates
© iStockpPhoto
(click to enlarge)
As species evolve over time, "extreme" traits can arise: characteristics that differ widely from those found in ancestors or from the most common condition. Examples include the tiny brain of the extinct, two-ton Uintatherium; the bipedal hopping of kangaroos; or the venom of species of Solenodon, large shrew-like mammals. Humans also have a mix of both normal traits inherited from early ancestors (we're warm blooded and have differentiated teeth and five fingers and toes) and specialized extreme features (upright bipedalism, and big brains relative to our body size).

How have mammals evolved in response to changing environments?
As environments change over time, living things must adapt or go extinct. Different traits are favored in different habitats and are passed on to future generations. For example, when various groups of mammals invaded the sea, adaptations like blubber and flippers—which are modified limbs—helped them survive. Gargantuan blue whales can grow far larger than any land animal because water helps support their bulk. Their ancestors lost their teeth but evolved baleen in the upper jaw: rows of flexible plates made of keratin that help them capture vast amounts of small crustaceans. Tails also evolved a wide array of forms to help mammals swim, balance, grasp tree limbs, or keep warm.

How have extreme or specialized traits helped mammals adapt and survive?
Over millions of years, every part of the mammalian body was shaped and reshaped through evolution, from snout to tail. Sometimes remarkable in form, these adaptations conferred evolutionary advantages, like the extremely long premolars of the extinct marsupial Thylacoleo carnifex or rodents' sharp incisors, which never stop growing. Echolocation evolved independently in a few specialized groups, including bats, cetaceans (dolphins and whales), and subterranean shrews. Evolving big brains relative to body size distinguishes mammals from other vertebrates, and humans from all other mammals. Much of the increase occurs in the cerebrum, the part of the brain largely associated with thought, memory, most senses, and information processing. Some of the most remarkable adaptations occurred when mammals evolved in isolated areas.

How do scientists study mammals?
Scientists observe mammals in their environments, and analyze modern and fossil specimens back in the laboratory. Combining physical and genetic data helps scientists recognize the relationships between species. Today, nearly 25 percent of living mammal species are threatened with extinction, even as new species are still being discovered. Scientists find them by exploring—and helping to protect—remote ecosystems in the ongoing quest to learn more about life on Earth.

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Extreme Mammals Online Educator's Guide