In a recent post, I discussed some of the factors implicated in the current startling global decline of frogs. In two papers in the February edition of the online journal Environmental Health Perspectives, Tyrone Hayes and his colleagues, of UC Berkeley, have added some interesting new pieces to the puzzle.
The first paper provides strong evidence that, while tiny concentrations of many chemical pesticides have deleterious effects on frogs, mixtures of the compounds may be far worse than the sum of their parts. In a four-year study which raised tadpoles of Northern Leopard Frogs (Rana pipiens) in water with various combinations of pesticides typical of waters near midwestern cornfields, mixtures showed far greater effects than did single chemicals. When a mixture of all nine chemicals (four popular herbicides, three insecticides and two fungicides) was present at 0.1 ppb (one of the lowest concentrations measured in the field), tadpoles took 30% longer to metamorphose, and a 35% mortality rate was measured. While the metamorphosis period was longer, the affected adult frogs were considerably smaller than average, making them dependent on new prey, and vulnerable to new predators. The pesticide cocktails also caused thymic damage, making the frogs particularly susceptible to bacterial meningitis. In a separate study of African Clawed Frogs (Xenopus laevius), the thymic damage was shown to be caused by elevated levels of the stress hormone corticosterone.
Hayes has studied the feminizing effect of minute concentrations of Atrazine, the most popular agricultural herbicide used in America, on male frogs for years. In his second paper, he shows further details of how the powerful endocrine disrupter blocks the effects of androgen, and stimulates estrogen production.
illustration: SOUTHERN LEOPARD FROG & TRICOLORED HERON (2000) acrylic 13.5" x 7"