Sun-loving frogs aid fungus fight

Rebecca Morelle, BBC News 24 Jun 08;

Sunbathing tree frogs may hold the key to understanding how a deadly fungus is wiping out amphibians around the world. The chytrid fungus has been implicated in many amphibian extinctions.

Now scientists are using non-invasive imaging technology to find out how some frogs from Central America may be able to beat this deadly disease.

They believe that the frogs' unusual skin is allowing the animals to bask in hot sunlight, possibly boosting their temperatures to kill off the fungus.

Most frogs avoid prolonged exposure to sunlight; the light and heat dry out their skin. However, some tree frogs from Costa Rica thrive in these conditions.

Andrew Gray, curator of herpetology at Manchester Museum who keeps a large collection of frogs from this area, said: "They sit in the Sun and bask for long periods without doing themselves any harm.

"However, until now, nobody has really looked at how they do this."

The challenge, he said, was to find ways of examining the frogs' skin in detail without harming the creatures, some of which are extremely endangered.

So the researcher teamed up with physicists from the Photon Science Institute at the University of Manchester.

Dr Mark Dickinson said: "I had been working on a new imaging technology called Optical Coherence Tomography (OCT) for medical imaging.

"But when Andrew approached me, I thought that this would be perfect for the frogs - it can show us what is happening in the frogs' skin but it is non-invasive."

Hot stuff

The OCT revealed that the frogs had an unusual pigment in their skin, called pterorhodin, that was allowing the creatures to reflect light in the infrared spectrum rather than absorb it. Melanin, the pigment typically found in skin, absorbs light.



Some believe the frogs could be reflecting light so they can blend in with the leaves they sit on, which also reflect at these wavelengths, to hide from predators that can only see in the infrared range.

But Mr Gray said: "We believe that the frogs are also reflecting the light and heat for thermoregulation - to cool themselves down. The surface of the skin is hot, while the body stays cool."

Some of these sunbathing frogs even take on a slightly metallic sheen as they bask in the sun, he added.

He believes that the unusual reflective skin structure revealed by OCT could help scientists to better understand how the chytrid fungus ( Batrachochytrium dendrobatidis ) is affecting frogs.

Mr Gray said: "The chytrid fungus lives in the skin of the frog, but it can only live at certain temperatures.

"It has been shown with frogs in captivity that if you elevate the skin temperature for short periods, you can clear them of the fungus.

"We thought: 'what if the sunbathing frogs are doing this naturally?'; is this their natural defence against the fungus?"

If temperature regulation is linked to the chytrid fungus, recent climate changes in the regions where the frogs live could have affected their ability to fight off infections - causing the recent dramatic declines, said Mr Gray.

"In Costa Rica, in the Monteverde rainforest, conditions have changed a lot in the past 10 years.

"There is now much more cloud cover, which leaves the frogs with less opportunities for sunbathing, and for possibly clearing themselves of the fungus."

The team is now using the OCT technique to see how different species of frogs that carry the special pigment reflect light, and also to study the skin structure in frogs that do not carry the pterorhodin pigment.

They believe that the amphibians' differences in ability to reflect may explain why some species are coping better with chytrid infections than others.