These physiological effects combine with environmental disruptions to make the life of a frog seem like something out of a horror movie and are likely among the factors causing a decline in amphibian populations worldwide, the researchers said.
"If you look at one of these frogs, it's probably a hermaphrodite - plus, it metamorphoses late, which means it is subject to its pool drying up before it can become a frog," said lead researcher Tyrone Hayes, professor of integrative biology at UC Berkeley. "It's also smaller, if it metamorphoses at all, which increases the likelihood it will be eaten and decreases its ability to eat. Plus, it's immuno-suppressed, so more prone to die from infection."
The stress on the frogs is increasing stress hormone levels, he found, which in turn create holes in the thymus gland that likely cause the impaired immune response.
"It's not the pesticides alone or introduced predators or ultraviolet light or global warming that's causing this decline, but the interaction between these on an animal that is pretty sensitive to its environment," said Hayes.
In the new paper, published online last week in the journal Environmental Health Perspectives, Hayes and his colleagues report four years of experiments showing that, while some of the pesticides, herbicides and fungicides used on corn fields may not by themselves have a noticeable impact on frogs, in combination they create significant effects. Among these are delayed maturation - the tadpoles take longer to metamorphose into frogs - retarded growth and an increased susceptibility to meningitis caused by normally benign bacteria.