A row of frog hanging on a branch looking at the camera, they have orange bellies and a green back

It's not what you eat, but when in your life you eat it that seems to matter for frog evolution © Kurit afshen/Shutterstock.com

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Not feeding as a tadpole speeds up frog evolution

Frog skulls have revealed that a frog's environment is the main driver of its skull shape, but that tadpole feeding behaviour influences how fast frogs evolve. 

Researchers have pieced together a picture of frog evolution in the biggest study of frog skulls to date.

Prof Anjali Goswami, Research Leader at the Natural History Museum said, 'We found rather unexpectedly that the frog skull shape is hugely driven by where they live as adults, so if they live in trees or in the water, that has a big impact on what their skulls look like.

'We also found that while skull shape isn't impacted by whether or not they feed as tadpoles, every time they lose that feeding larval stage there is a burst in evolution and diversification.'

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. If the tadpole experiences functional pressures from feeding at that early stage in its life, it can limit how much the adult can evolve. 

frog skulls arranged in a pattern against a black background

The team looked at the skulls of 173 different frog species and used 3D methods to capture the shape of the skull with a high level of detail © Anne-Claire Fabre

The diversity and double life of frogs

There are more than 7,300 species of frog found all over the world and they show a remarkable diversity in life history and ecology. Frogs can live in water, in trees, under the ground and on top of the ground.

There are two distinctive stages in a frog's life. The first is its larval form, which can look like eggs or tadpoles, before they change into their adult form through a process called metamorphosis.

In different species of frog, the tadpole will either stay in their eggs for the entire larval stage and go through metamorphosis within the egg, or the larvae can hatch and feed themselves as free-swimming tadpoles.

The team of researchers thought that because frogs have needed to adapt to specific environments to survive, the behaviour of their larvae might have impacted the evolution of their skulls. This is because feeding as an aquatic tadpole is quite different from feeding as an adult on land and the team wondered how this might impact how the adult's skull would develop.  

The loss of feeding in tadpoles consistently resulted in a burst of faster evolution and a wider variety of skull shapes than those frogs that hatch and feed themselves as tadpoles. Precisely why this is the case is still not fully understood.

This information 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.

In the study, the team collected micro-CT scans of lots of different kinds of frogs with different lifestyles as adults and as tadpoles. 

Anjali says, 'When selecting the species of frogs 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, and we also included diversity of ecology and different ways of developing when choosing species.'  

In total, they looked at the skulls of 173 different frog species and used 3D methods to capture the shape of the skull with a high level of detail.  

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 (legless amphibians). This suggests a new amphibian pattern of skull growth. Similar patterns have been identified in mammals, where the skull gets longer (rather than wider) as it gets bigger, but a similar pattern has never been identified in another vertebrate class.  

A green frog sits on a mossy log

European green tree frog (Hyla arborea formerly Rana arborea) © Manuel Findeis/Shutterstock.com

Understanding animal diversity

This work is part of a bigger study of skull evolution across all vertebrates (fish, amphibians, reptiles, birds and mammals) led by Anjali.

The research group within the Museum are studying life on the planet to understand how factors like life cycles, ecology, habitat and extinction drive the diversity of species in the past, present and future. 

Anne-Claire Fabre, postdoctoral researcher and co-author of the study describes how they conducted the study, 'We use micro-CT scanning and laser scanners to build gigantic 3D data sets of species.'

In another study, the researchers found that salamander evolution is impacted by metamorphosis