What is the evolutionary advantage of metamorphosis

Metamorphosis: Why some species swap their bodies for another

Vienna - In four out of five species, the animals go through a metamorphosis in the course of their life: caterpillars transform into butterflies, tadpoles become frogs and symmetrical fish larvae become one-sided, flat flounder. This ability to change physically has arisen only a few times in evolution, but where it has happened it has never disappeared. Scientists have long been puzzling over what the advantage of such a metamorphosis is, and what benefits it ultimately brings to animals. Researchers at the International Institute for Applied Systems Analysis (IIASA) have now found that the cause of this is probably the expansion of the nutritional spectrum in many cases.

Food as the decisive criterion

With the help of metamorphosis, the animals can develop new sources of food that were denied them in the other form, according to Ulf Dieckmann and Hanna ten Brink (now at the University of Zurich) from the IIASA in Laxenburg near Vienna. The researchers created a mathematical model that included, for example, how much the animals eat, how quickly they grow and how much offspring they produce. They used this to calculate the circumstances under which the species would benefit from a metamorphosis, and when they would be better off without metamorphosis. The type of food turned out to be a decisive criterion.

If the animals are in the same body all their life, they are usually well adapted to a certain food source. In the case of caterpillars, for example, these are leaves and in the case of tadpoles, algae. If there is another form of food in their habitat, it is difficult for them to develop it, even if it occurs in large quantities, because evolution prefers specialists. If they learned to use the second source of food, the efficiency with which they get their main meals deteriorates and they can no longer reproduce as successfully, the researchers explain.

New body, new food

Animals can solve this dilemma through a complete transformation. As larvae, the butterflies and frogs stay with their main food, i.e. leaves with the caterpillars and algae with the tadpoles, but as soon as they get stuck in the new body, they only consume nectar or flies.

Of course, they have to pay a certain price for the metamorphosis: it costs many calories to completely reshape the anatomy of each individual. Sometimes things go wrong and they die. It also takes time, and while a butterfly hangs under a leaf as a pupa, it cannot escape dangers such as predators. In many cases that price was too high, so the metamorphosis only occurred a few times in evolution, the researchers say.

However, natural selection favors the ability to metamorphose when the second food source is in excess, they explain. In the beginning the new adult stages are not yet well adapted to this, but due to the abundance of offers they still have more offspring than before and over time they also learn to use the new food better.

Evolutionary impasse

The fact that the ability to change is so widespread in the animal kingdom, although it is not so easy to acquire, the researchers attribute in their study presented in the journal "The American Naturalist" to the fact that no one can get rid of it. It is evolutionarily a dead end. If the second food source becomes rarer, the animals have to use it more efficiently or die out. For example, because the youth stage is not capable of reproduction, they cannot simply do without metamorphosis.

In some animals, the metamorphosis can hardly be seen, but it can never completely disappear, commented the biologist Vincent Laudet from the University of Paris, who was not involved in the study the results in the "New York Times". He also includes people. "When a baby leaves the womb, its tissues undergo important changes that are regulated by some hormones that also trigger metamorphosis in frogs and other animals," he said. (red, APA, 3.4.2019)