Head or egg: an evolutionary trade-off

The native planarian Crenobia alpina found in alpine streams (collage). Credit: Bernhard Egger/Fabian Oswald Would you rather lay lots of eggs - or grow your own head back? In the evolutionary history of flatworms, some species apparently had to choose one of the two abilities. This is what a team of scientists found out when they investigated the regenerative ability of planarians. Bernhard Egger, who heads the Regeneration Group at the Institute of Zoology at the University of Innsbruck and was involved in the study, reports on the amazing properties of these animals . Why can some animals regrow entire body parts, but not others? This question remains largely unanswered in biology. To test the evolutionary principles behind the ability to regenerate, a team of scientists studied planarian species from around the world. These animals from the flatworm phylum are usually only a few centimeters in size.