June 24, 2008

Sunbathing Frog May Hold Clue To Deadly Disease

A type of Central American tree frog could provide scientists with a major clue about how a deadly fungus is destroying the world's amphibians. Some of the tree frogs have been able to escape the deadly fungal disease, and experts are now implementing a non-invasive imaging technology to understand the phenomenon. 

The scientists believe that the frogs' atypical skin allows the animals to bask in hot sunlight to the extent that the high temperatures kill off the fungus, according to a report by BBC News.

The sunbathing tree frogs act differently than most other frogs, who typically avoid prolonged exposure to sunlight that often dries out their skin. 

But some tree frogs from Costa Rica apparently thrive in these conditions.

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

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

He said the challenge was finding ways to conduct an analysis of the frogs' skin in sufficient detail without causing harm to the frogs, some of which are extremely endangered.  Gray decided to partner with physicists from University of Manchester's Photon Science Institute.

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

"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," he told BBC News.

Using the OCT, the researchers discovered the frog skins contained an unusual pigment, called pterorhodin, that allowed the creatures to reflect infrared light instead of absorb it.   Typically, a skin pigment called melanin absorbs infrared light. 

Some hypothesize the frogs reflect light as a way to blend in with the leaves they sit upon, which also reflect at these infrared wavelengths, or to conceal themselves from predators that see images only in the infrared range.

"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," said Mr. Gray, adding that some of the frogs even exhibit a somewhat metallic sheen as they sit in the sun.

Gray believes the unusual reflective skin structure of these frogs might help scientists better understand how the chytrid fungus, known as Batrachochytrium dendrobatidis, is wiping out the world's amphibian population.

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

"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 defense against the fungus?'"

If the chytrid fungus is linked to temperature regulation, recent climate changes in the area the frogs inhabit might have affected their ability to battle the infections, which could be responsible for the dramatic reductions, 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 technology to examine different frog species whose skin contains the special pigment that reflects light, and to study the skin structure of frogs that do not carry the pterorhodin pigment.  The researchers believe the differences among frogs in their ability to reflect light may explain the varying responses to chytrid infections.


On the Net:

VIDEO: Andrew Gray reveals the secrets of the frogs' skin

Manchester Museum

University of Manchester

Photon Science Institute, University of Manchester