To the high school biology student, the northern leopard frog is the dissection specimen that first revealed the secrets of animal anatomy.
To the modern scientist, it can be a sentinel, a harbinger of poor environmental health— especially in the river regions that flow into the Great Lakes. In Dr. William Karasov's lab, the frog is front and center in a series of research experiments, funded in part by the UW Sea Grant Institute, designed to explore how environmental toxins may be affecting the frog's immune system, growth and development.
Polybrominated diphenyl ethers—PBDEs for short—are toxic compounds used as flame retardants that have been found in significant concentrations in the Great Lakes region. Karasov, professor and chair of the UW-Madison's Department of Forest and Wildlife Ecology, and Tawnya Cary, a first-year PhD student, have fed northern leopard frog tadpoles a prepared diet that includes a static level of PBDEs from birth until metamorphosis, the point at which the frog's forelimbs begin to emerge. Their goal is to measure the potential effect of PBDEs on survival, growth rate and the amount of time it takes for the tadpole to develop into a frog.
"These contaminants can create a situation where animals can suffer direct effects, and also weaken their immune systems, making them vulnerable to other pathogens in the environment," explains Karasov.
He points to situations where large groups of dead frogs have been found by naturalists in the wild. "We've never really known if a weakened immune system is the problem."
While there's been plenty of research on the dangerous environmental effects of the more commonly known polychlorinated biphenyls (PCBs), PCBEs have gone almost entirely unexamined.
"That's where we began. We wanted to get a very fast start on this, because nobody has looked at it yet," says Karasov.
As it turns out, it's a good thing Karasov and Cary did. Their initial findings, published in a recent issue of Environment Toxicology and Chemistry, show that the PBDE-tainted diet both increased mortality and stunted growth among the tadpoles. Other experiments have shown that the toxin doesn't appear to damage the frog's immune system or make it more susceptible to infection from common funguses and bacterium. That's a question Karasov and Cary will be exploring in greater detail in the coming year.
Karasov and Cary's work is on the front lines of a new research discipline called immunoecology, aimed at creating a better understanding of the ways the immune functions of several animal species develop and protect them in their natural environment. Karasov is also studying development of immune function in nesting birds.
"Frogs play an important role in our ecosystem—they're a key part of the food web," says Cary, "It's a heads-up as to why we should be concerned about this, and how it could affect humans."
By Aaron R. Conklin