Why would one animal change body for another?

In the Netherlands, Hanna ten Brink spent many days contemplating the pond in her family's garden, fascinated by the metamorphosis.

The tadpoles were born from eggs in the pond and swam all around, sucking up tiny particles of food with their mouths. After a few weeks, the tadpoles lost their tails, grew their legs and jumped to the mainland, where they were able to catch insects with their new tongue.

Over time, ten Brink has become an evolutionary biologist. Now science has brought back to this fascination of childhood.

About 80% of all animal species undergo metamorphosis, from frogs to flatfish to butterflies and jellyfish. Scientists are deeply intrigued to know how this has become so commonplace.

What evolutionary journey could lead a caterpillar – an incredibly adapted leaf-eating machine – to get rid of its body and rebuild it in the form of a butterfly?

In the May issue of American Naturalist, ten Brink, now a postdoctoral researcher at the University of Zurich, and his colleagues presented a map showing the evolution of metamorphosis. They argue that this has emerged as a way to allow a species to eat more food.

The path leading to this feast is sinuous and the metamorphosis has appeared only a few times in history. However, once this happened, scientists also discovered that it rarely disappeared.

Ten Brink, Andre M. de Roos of the University of Amsterdam and Ulf Dieckmann of the International Institute for Applied Systems Analysis (IIASA) in Austria have created complex mathematical equations reflecting some of the fundamental aspects of animal life: the amount of food they consume, how fast they grow, how many babies they have, among others.

The scientists badume that the animals have not metamorphosed, they have kept the same body throughout their lives and have adapted well to one type of food.

However, what happens if their environment contains a second food, which they could consume as adults if their anatomy evolved to another? The researchers found that natural selection would prevent animals from adding the second food to their diet.

In this case, the evolution favors the specialists: if the animals evolve and start to eat the second food, their offspring will have more difficulty to consume the original food when they are young. More of them will die before they are mature.

"The obvious solution to the problem is the evolution of metamorphosis," said ten Brink. The young animals remain well adapted to the original food, while the adults pbad to the new one with a renewed body.

However, animals pay a heavy price for metamorphosis. They burn a lot of calories to get rid of their old anatomy and develop a new one. This process may fail and leave them with defects.

Metamorphosis also takes time and exposes animals to predators and parasites. Ten Brink and his colleagues found that in many cases the price of metamorphosis is too high for natural selection to support.

"There must be a very good reward," he said.

Natural selection favors metamorphosis if adult animals are rewarded with an abundant supply of food, enough to offset the cost and allow them to have many small ones.

At the beginning of this change, adults are not well adapted to the new food. However, they find it in such large quantities that they still eat well.

"I like the concept, I like the fact that they have tried to find the definitive cause," said Joanna Wolfe, a postdoctoral researcher at Harvard University. But he wondered if food was the only reward that could propel the evolution of the metamorphosis.

Some species can benefit in other ways. Adults can adopt organs that allow them to mate more effectively, for example. Larvae in the ocean can change shape to be carried by more distant currents and thus expand their range.

"I would like to see them add other elements to their model," he added.

Ten Brink agreed that the new study serves as a basis for more detailed studies. "This work is really the beginning of something," he explained.

If the metamorphosis of animals evolves so infrequently, why is it so common? One of the reasons may be that once the metamorphosis occurs, it is very difficult for a species to lose it.

It is easy to imagine a situation in which it would be beneficial to give up the metamorphosis. Imagine an epidemic that destroys all the foods that adults eat. For this species, it would be useful for individuals to remain in the larval stage and survive with their remaining food.

However, the study of Ten Brink indicates that in most situations, the evolution does not meet our expectations. If adult food becomes hard to find, natural selection will favor adults who are better able to find the little food left.

"It's an evolutionary trap," said ten Brink. "If the conditions get worse, you're off."

Biologist at the Pierre and Marie Curie University in Paris, Vincent Laudet said that his own research on vertebrates supported the conclusions of Ten Brink.

The first fish species have undergone a metamorphosis, he said, and this has been going on for more than five hundred million years.

"In some vertebrates, the metamorphosis is camouflaged, but it's never lost," Laudet explained.

When we speak of "certain vertebrates", Laudet also speaks to us. When babies leave the uterus, their tissues undergo major changes, caused by some of the same hormones that trigger metamorphosis in frogs and other animals.

"In biological terms, our birth is a metamorphosis," said Laudet.

* Copyright: c.2019 New York Times News Service