Center for In Vivo Microscopy
http://www.civm.duhs.duke.edu/
Amazing images of animals at different stages of development, including those undergoing metamorphoses, from Duke University’s Center for In Vivo Microscopy.

Newt Limb Regeneration
http://www.luc.edu/depts/biology/dev/regen2.htm
See a salamander limb regenerate in time-lapse photographs on this developmental biology site maintained by Bill Wasserman of Loyola University.

Cannibalism Can Kill You
http://www.newts.org/~newtsweek/html/cannibalism_can_kill_you.html
Being a cannibalistic tadpole has its risks, as this article from Newtsweek notes.

The Slug Site
http://slugsite.tierranet.com/
Learn about the wonderful world of sea slugs from Michael D. Miller. Don’t miss the Nudibranch of the Week.

Cecropia Moth Life Cycle
http://www.geocities.com/RainForest/5479/page2.htm
Beautiful photographs and more from Pete Honl.

Books

Coen, Enrico. THE ART OF GENES: HOW ORGANISMS MAKE THEMSELVES. Cambridge UK: Oxford University Press, 1999.

McDiarmid, Roy W. TADPOLES: THE BIOLOGY OF ANURAN LARVAE. Chicago: University of Chicago Press, 1999.

Petranka, James W. SALAMANDERS OF THE UNITED STATES AND CANADA. Washington: Smithsonian Institution Press, 1998.

Shi, Yun-Bo. AMPHIBIAN METAMORPHOSIS: FROM MORPHOLOGY TO MOLECULAR BIOLOGY. New York: John Wiley & Sons, 1999.

Ulijaszek, Stanley J. THE CAMBRIDGE ENCYCLOPEDIA OF HUMAN GROWTH AND DEVELOPMENT. Cambridge UK: Oxford University Press, 1998.

If you are a tadpole, sometimes it pays to watch out for headhunters. That’s because the wriggler next to you may be a cannibal — and you could be its next meal.

As NATURE’s The Body Changers shows, some animals can undergo dramatic transformations when food or living space becomes scarce. The tadpoles of some toads and frogs, for instance, can morph from placid plant eaters to fierce cannibals if their pools threaten to run dry. By becoming meat eaters that snack on other tadpoles, the cannibals speed up their development, hoping to transform into toads before the water runs out.

Tadpoles can become cannibalistic.

Spadefoot toads, for example, live in dry areas. Adults wait to lay their eggs until temporary pools are formed by infrequent rainstorms. Though the tadpoles take only two weeks to develop into toads, sometimes the pools don’t last that long. The tadpoles get a tip that the pools are shrinking when they start bumping into each other and fairy shrimp, insect-like animals that also thrive in puddles. If a tadpole eats one of the shrimp, it can spark a transformation into a full-fledged cannibal.

Even the cannibals can be somewhat careful about their diets, however, as researcher David Pfennig of North Carolina State University has shown. In some species, the cannibals generally avoid eating their brothers and sisters, preferring to eat unrelated tadpoles instead. Exactly how they tell the difference between friend and foe isn’t known.

Tadpoles can also undergo other types of changes when conditions change. Some, for instance, change color and start producing a distasteful chemical when one of their friends is munched by a predator. The dying tadpole releases chemicals that warn other tadpoles that a predator is nearby. Within hours or days, nearby tadpoles turn bright colors, which, like a living billboard, warn predators that they don’t taste very good. It’s a change that helps ensure their survival.

It doesn’t take much imagination to see the similarities between a human child and an adult. But as NATURE’s The Body Changers shows, in other animals, the young may bear little resemblance to the grownups. It’s hard to see a butterfly within the fuzzy form of a caterpillar, for instance, or a sleek dragonfly in the squat features of its aquatic larva. Indeed, early naturalists sometimes believed that the larva of some animals, such as eels and frogs, were a totally separate species from the adults, because they looked so different.

Today, however, we realize that many creatures undergo startling transformations as they mature. But we are just beginning to understand the genetic blueprints that guide these changes.

Genes exert a powerful influence on body changes.

Over the last few decades, researchers have been homing in on the chunks of DNA, called genes, that hold the instructions that tell cells when, and where, to grow. Some genes, or sets of genes, for instance, control where limbs or wings sprout. Others determine the top and bottom of an organism, or its head and tail. By turning these genes on and off in sequence, organisms guide their development from just a single cell to a creature made up of billions of specialized cells. Eye cells, for instance, look and behave very differently from skin cells, though both are children of exactly the same original cell.

To create such differences, some genes exert especially powerful control over cells when turned on. In 1995, for instance, Swiss biologists showed that inserting and turning on a few related genes in fruit fly cells could cause surplus eyes to sprout virtually anywhere: on wings, legs, and even antennae. Researchers manipulating other genes have caused salamanders to grow legs in the oddest places, or not grow them at all. And turning off some genes by blocking their action can prevent some animals from maturing at all, leaving them permanently stuck in their larval forms. Such research is helping scientists understand genetic diseases, and may eventually lead to new treatments.

The research is also teaching us something about evolution. Researchers have discovered that many of these key developmental genes — which have been given whimsical names such as “sonic hedgehog” and “superfly eye” — appear in many animals, from insects to humans. Over time, they have been passed along as one species has transformed slowly into another.

But different animals may use similar genes in different ways. A gene that prompts an arm or a leg to bud off from a main body in a mammal, for instance, might prompt a sharp spine to bud off on the shell of a sea urchin. Despite the very different structures involved, the underlying genetic instructions are the same, suggesting that the two animals shared a common ancestor long ago. “A lot of evolution represents the commandeering of genes from one form to another,” David Jablonski, a paleobiologist at the University of Chicago, told the magazine SCIENCE in 1999.

Gene hunters are far from finding — or understanding — the millions of genes that play key roles in body changes. It is already clear, scientists say, that genes allow for a lot of creativity in organism design. It’s like having a set of building blocks you can snap together in an infinite variety of ways. According to one researcher: If you don’t like one design, you can change it.

“Change alone is unchanging,” the ancient Greek philosopher Heraclitus once wrote. But even such a wise man didn’t know the half of it. As NATURE’s The Body Changers shows, researchers have discovered that all kinds of animals — from sea slugs and caterpillars to songbirds and people — undergo constant and often remarkable physical changes during their lives. And scientists continue to discover that we can change our bodies in ways once thought impossible.

A butterfly will emerge from this body changer.

Each of us knows from personal experience that the passage of time is marked by constant variation and modification. From a single cell unable to live on its own, we multiply into creatures composed of trillions of cells able to move about freely. Our bodies grow taller, heavier, and hairier as we mature, then shrink and wrinkle as we age. Our hair may change color and our voices modulate from a howling cry to a quivery whisper.

But even these dramatic physical alterations are overshadowed by the extraordinary transformations experienced by other creatures profiled in The Body Changers. Fleet-flying dragonflies, for instance, start life as swimming nymphs that paddle about beneath the surface of a pond or river. High-leaping frogs take their first trips as awkward, wriggling tadpoles. And the elegant, fragile butterfly emerges from a capsule spun by a chunky, crawling, earth-bound caterpillar.

Still other animals are able to execute even more amazing tricks. Salamanders can regrow legs snipped off by hungry turtles, while lizards routinely rebuild tails that break away, by design, in the mouths of predators. Male deer grow magnificent antlers that are used for just one season and then discarded, like a wedding dress banished to the back of the closet. And some songbirds remold their brains every spring, adding and subtracting neurons as needed. When more brainpower is needed to sing and remember their courtship songs, their brains swell. But when breeding season is over, they conserve energy by scaling back.

Such modern-day adaptations are the product of millions of years of evolution — another process dependent on change. Many researchers, for instance, believe today’s birds began as dinosaurs, while people evolved from tree-dwelling apes. Over the eons, seemingly insignificant changes began to add up, separating new species from the old. The genetic flaw that produced feathers on some mutant dinosaur, for instance, may have helped keep it warmer and enhanced its survival. Later, the feathers might have helped its offspring become better hunters and eventually fliers. It was just a short flap, in geologic time, to modern birds, which bear just a fleeting resemblance to their forebearers.

We’ll probably never know for sure just how all these changes took place, or why they occurred. It is difficult to follow the biological path that brought us here back into the mists of time. But researchers continue to make surprising new discoveries about our ability to change. For years, for instance, scientists believed that it was nearly impossible for humans and related apes to grow new brain cells. Conventional wisdom held that the most important parts of our brains were pretty much set by the age of three or four, and would grow no more.

Are brain cells continually added to all primate brains?

But in 1999, in a finding that eventually could lead to new methods for treating brain diseases and injuries, Princeton University scientists discovered that new brain cells are continually added to the brains of adult monkeys. Brain researchers Elizabeth Gould and Charles Gross found that the monkeys add neurons to several regions of the cerebral cortex that are crucial for memory, high-level decision making, and for recognizing and learning about the world. The results strongly imply that the same process occurs in humans, because monkeys and humans have similar brain structures. “If what they have shown holds true for all primates, including humans, it means we really need to rewrite the book on brain development and the way that experience can affect the brain,” says William T. Greenough, director of the neuroscience program at the University of Illinois’ Beckman Institute.

At the time of the discovery, Gross asked the question shared by many: “If the cerebral cortex is important in memory, how could it change?” He went on to explain, “In fact, the opposite view is at least as plausible: if memories are formed from experiences, these experiences must produce changes in the brain.”

Practical applications of the discovery could be years, even decades away. But the results suggest that scientists may one day exploit natural repair mechanisms to treat brain injuries or diseases, such as Alzheimer’s and Parkinson’s diseases. The discovery also may require scientists to draw a less bold distinction between the brains of humans and other animals, says Fernando Nottebohm of Rockefeller University, who has pioneered the study of changing bird brains. “What you can say now,” he says, “is that the primate brain is more like that of songbirds.” That, indeed, is a change in thinking.

Evolution makes the surreal seem commonplace in NATURE’s The Body Changers.

In The Metamorphosis, Franz Kafka wrote about a hapless fellow who awakens one day to find he has changed into a cockroach. In the world of fiction, this is an example of surrealism; in the natural world, it typifies an everyday occurrence.

Many animals have a special ability to transform themselves, for the sake of survival, reproduction, or both. And, as anyone who has watched a caterpillar become a butterfly knows, some animals experience two very different existences, in effect, living for a time as one creature and then changing into another. NATURE probes the intriguing subject of physical transformation in The Body Changers.

The program explores a broad range of creatures with astonishing abilities, from the redwing blackbird, whose brain dramatically changes shape to accommodate the chants and songs of mating rituals, to a fungus that transforms itself into something resembling a stalk that plays a vital role in the proliferation of roundworms.

To order a copy of The Body Changers, please visit the NATURE Shop.

Online content for The Body Changers was originally posted May 2000.