Scientists from the Natural History Museum have led a new study reconstructing the evolution of the frog skull, to understand how habitat and reproduction influence the diversity and evolution of frogs.

The research team measured how diverse the skull shapes are across different kinds of frogs and how fast they evolve. They found that where a frog lives as an adult is by far a greater influence on skull shape than whether or not they feed as tadpoles, with aquatic and burrowing frogs showing the fastest evolution, likely related to evolving specialised ways of feeding. However, the study also shows that whenever frogs lose the active feeding larval stage as tadpoles, they show a burst in rate of evolution across the skull. This work helps us understand how metamorphosis impacts evolution, and why it is so common among animals today, with more than half of animal species living this "double life."

Prof Anjali Goswami, research leader, vertebrates at the Natural History Museum said: "We found rather unexpectedly that the frog skull shape is hugely driven by where they live as adults, if they live in trees, or in the water, it's a big factor affecting their skull shape. When looking at their development, what really differs in frogs, is that their larvae forms, or tadpoles, can be feeding or non feeding. We found whether they feed or not doesn't impact on how their skull is shaped, but every time they lose that feeding larval stage as tadpoles—there is a burst in evolution and diversification."

The reason scientists thought that what larvae do could impact how skulls evolve is because organisms have to adapt to the environments they inhabit in order to survive. If you have a very different lifestyle as a baby or juvenile than you do as an adult, then you may have to adapt to two very different kinds of environments, and that may mean you end up being not as good at either one. Feeding as an aquatic tadpole is quite different from feeding as an adult on land, it might be expected to affect how the adults evolve, and it turns out the answer is both yes and no. What the skull looks like doesn't seem to be affected by what tadpoles do, but how fast it evolves does. The loss of feeding in tadpoles consistently resulted in a burst of faster evolution, and adults without feeding tadpoles show a wider variety of skull shapes than those that feed as tadpoles. So, for frogs, it's not so much what you eat, but when in your life you eat it that seems to matter for evolution. This could be important for understanding which frogs will be most affected by climate change, because being able to evolve quickly is a going to be very helpful when environments are changing at an unprecedently fast pace due to human activities.

This is the first study of this scale which looks at the whole diversity of frogs and captures the whole of their skull shape in detail, whereas previous studies have focused on one family of frogs, or frogs which live in a specific habitat. Prof Goswami continues: "When selecting the species of frog for the study, we wanted to include a full range of diversity. We took the evolutionary tree of frogs and picked a representative from every family and subfamily, including the diversity of ecology and different ways of developing as part of the sample."

The study also found that the way the frog skull changes with size, getting relatively wider in big frogs, is similar to patterns in salamanders and caecilians—suggesting a new amphibian pattern of skull growth. Similar patterns have been identified in mammals, with the skull getting longer as it gets bigger, which has never been identified in another vertebrate class.

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