STUDENT CONTRIBUTIONS
Excerpts of pages 273-277 in Herpetology: An Introductory Biology of Amphibians and Reptiles by George R. Zug, 1993.
Life-history theory "...offers explanations of an individual's survival and fitness (success in placing progeny in the reproductive pool of the next generation) relative to the individual's abiotic and biotic environment and links life-history traits."
"Four life-history traits (age of maturity, clutch/litter size, reporductive effort, and longevity) are used..." to divide populations into two groups; short-lived populations and long-lived populations. "Short-lived populations are characterized by individuals with short lives, early sexual maturity, large broods, and high annual reproductive effort (effort estimated by the proportion of egg-clutch mass to females's mass). Long lived populations are characterized by individuals with long lives, delayed sexual maturity, small broods, and low annual reproductive effort.)" The variations in the two populations could have arose from either adaptation to present habitat (predation, comptetition, climate, soil acidity or hardness, or to other abiotic and biotic conditions) or a response to a current disturbance in local conditions (food abundance, unusually wet or dry weather, or other conditions that improve or degrade the health of individuals). Frogs and lizards are examples of short-lived populations and turtles and crocodiles are examples of long-lived populations.
A disturbing wildlife phenomenon has turned up throughout the Great Lakes region, from Minnesota to Quebec: deformed frogs. Since August of 1995, researchers have found malformed frogs at more than 174 sites in 54 of Minnesota's 87 counties, prompting researchers to try and solve the mystery.
Last summer, after record reports of leopard frog abnormalities by Le Sueur elementary school students studying wildlife on nearby wetlands, University of Minnesota researchers and state pollution officials conducted some of their own studies and found "hideously deformed" frogs with missing eyes and legs, misshapen or extra legs, or rearranged, organs or extremities. Some specimens were found with eyes in their throat, webbed legs or legs which split into two sections midway down. Early evidence is pointing toward water contaminants located near frog breeding sites. Were chemicals responsible?
Dr. Robert McKinnell of the University of Minnesota, who's been studying frogs since 1958, says the discovery of such deformities is important to humans because frogs are a "sentinel" species, meaning they have metabolic functions similar to our own and may be indicating potential trouble ahead for humans.
According to hepetologist David Wake of the University of California, Berkeley, " Amphibians are excellent indicators of environmental stress. Since they live in both aquatic and terrestrial systems, they might tell us faster that something is wrong. They're an early warning system."
Overall, frog populations have been dwindling at an alarming rate in the last 10 years, due mainly to decreasing habitat, ozone depletion (frog skin is acutely sensitive to ultraviolent rays), acid rain and an increase in predators. But scientists say these factors cause death in the species, not deformities, forcing researchers to ponder which of the numerous chemicals or heavy metals polluting American waterways are the culprit.
So far, scientists have yet to pinpoint a cause. Some biologists, like Dr. Stan Sessions of Hartwick College, attribute the deformities to a reptilian parasite which burrows into tadpole bodies while they're developing, sometimes causing multiple legs to form. But researchers like Judy Helgin, lead researcher at the Minnesota Pollution Control Agency, doubt the explanation, saying that the abnormalities were inconsistent and too numerous and varied to be attributed to parasites. Mike Lanoo of the Declining Amphibian Population Task Force suspects methporene, used in mosquito control.
McKinnell wasn't alarmed by early reports, because he says frogs have a small number of birth defects naturally until he found one site with a 96 percent frog deformity rate. Herpetologists like McKinnell agree that frogs are enormously vulnerable during early life cycle changes to hormonal variations, which may be triggered by pesticides and other toxins, creating an estrogen-mimicking effect which disturbs a frog's development.
The students who discovered the deformities near the Minnesota River have since created A Thousand Friends of Frogs, and are trying to get others involved in documenting frog deformities in hopes that researchers will find an answer.
TECHNOLOGY ADVANCES-Frog Antibiotics:
As Dr. Michael Zasloff watched an African clawed frog in its tank, he noticed something unusual. A cut on the frog's belly, made a few days earlier, was clean and healing perfectly. How could this happen in water with bacteria that should have caused an infection?
Dr. Zasloff realized that the frog had an abitity to heal itself, even when surrounded by bacteria. In a few months, he had discovered a family of powerful natural antibiotics called magainins. Magainins kill a wide range of organisms including bacteria, fungi, and parasites such as those that cause malaria. There is a possibility they will also work against some viruses and cancers. Dr. Zasloff believes magainins might help to explain why amphibians have been successful in surviving on earth. Synthetic magainins have now been manufactured. They will be tested as drugs. Their discovery holds hope of new treatments for human infections.
Taken from a Life Science Textbook for children. Focus On Life Science. Merrill Publishing Co. Colombus Ohio. 1989. This little article could illustrate to students that even though frogs have a great ability to fight off diseases, parasites, bacteria, etc. they can not fight off the chemicals that we are polluting their bodies with. The article also explains one reason why frogs are important to us.
"Since evaporation consumes body heat, no amphibian has a high body temperature or a rapid rate of metabolism and the behavioral repertoire that goes with a warm body."
Wessells and Hopson. (1988). Biology. (p.621). Random House
Resource Links
Research in Endocrine-Disrupting Chemicals in Nature
The American Observer-article on frogs
Great Lakes Declining Amphibians Conference
References:
Kaufman, CL and Mallory, CK. The Last Extinction, 1986.
Odum, C. Eugene. Ecology and Our Endangered Life Support Systems, 1989.
These two books are excellent resources in researching terminology and ecological concepts. While they do not specifically refer to our topic this week, they provide the basic groundwork for the various concepts we will discuss.
