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4. Causes
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four CAUSES And now remains That we find out the cause of this effect— Or rather say, the cause of this defect, For this effect defective comes by cause. Thus it remains, and the remainder thus. WILLIAM SHAKESPEARE1 The year 2006 marked the 300th anniversary of known published observations of malformed frogs. The recognition that malformed frogs preceded the beginning of the Industrial Revolution by about 150 years and preceded modern agricultural techniques (including the application of pesticides) by about 250 years has led many people to deduce that there are natural causes of frog malformations. Indeed there are. NATURAL CAUSES In his treatise on the history of malformed amphibians, Martin Ouellet2 lists several natural causes of malformations. Wounding Wounds from failed predation attempts (“bites and mutilations ”) and leech (Erpobdella octoculata) parasitism can lead to either missing limbs or parts of limbs, or polymely through “hyper-regeneration following wounding.” See also the comments of Johnson and colleagues,3 and the discussion of predation in Chapter 5. Fish excrement In the European frog Rana esculenta, “anomalie P” consisting of bilateral brachymely, polymely, and other types of limb malformations can be induced by rearing tadpoles in the presence of excrement 105 from eels (Anguilla sp.) and/or minnows (Tinca sp.).4 Other amphibian species appear to be resistant to these effects. It is unknown whether chemicals contained in fish excrement or microbes transmitted through fish excrement are the cause. Extreme tadpole densities Some Rana esculenta raised in high densities of 3.5 to 11.1 tadpoles/liter exhibit forelimb ectromelia and hindlimb hyperextension thought to be caused by teratogenic properties of chemical(s) released by the crowded tadpoles. Extreme tadpole densities may also produce wounding. Lathyrogens Extracts of sweet pea (Lathyrus odoratus) seeds have teratogenic effects on both salamander larvae and frog tadpoles. When young embryos are exposed to lathyrogens, notochord and tail malformations are produced; when older tadpoles are exposed, joint and limb dislocations occur. Nutritional deficiencies Malformations produced by nutritional deficiencies are often skeletal malformations, including decreased bone density, scoliosis , joint dislocations, mandibular malformations, and folding fractures of long (limb) bones. Paralysis may also be present. Nutritional deficiencies are most often seen in captive animals. Because most frog tadpoles are herbivorous, and aquatic plants are abundant in healthy, established wetlands , it is difficult to envision food limitation in these ecosystems. Ultraviolet-B radiation Andy Blaustein and his colleagues5 suggest that UV-B can cause severe structural malformations in early-stage embryos of native amphibian species. In the laboratory, Gary Ankley and his colleagues at the EPA found UV-B effects that included bilaterally truncated hindlimbs.6 The effects of UV-B acting either alone or in association with other factors are interesting and potentially important. I discuss the role of UV-B in producing amphibian malformations in more detail below and in Chapter 5. Disease Ouellet cites evidence for production of bilateral posterior ectromelia and hemimely by disease. The potential role for disease in producing malformations must be pursued more ambitiously.7 106 cause s [54.147.17.95] Project MUSE (2024-03-29 05:55 GMT) Temperature extremes Amphibian malformations have been induced by artificially high temperatures (30ºC) in the laboratory and naturally low temperatures (cold spring water) in nature. It is unlikely that severe extremes in temperature are experienced by tadpoles in most natural wetlands . However, surface pockets of hot water are present at the NEY pond in Minnesota (Fig. 3.2), and several of the Minnesota wetlands we sampled were spring fed and noticeably cold (CBA, Fig. 3.7). Hereditary factors Ouellet notes that some types of malformations are due to genetic mechanisms. However, both Ouellet and I question how similar genetic mutations can arise simultaneously across large geographic regions, or arise simultaneously in several species from the same wetland when animals in the same species from adjacent wetlands are normal. Parasitic cysts Pieter Johnson, Kevin Lunde, and their colleagues8 report a range of malformations induced experimentally by the trematode parasite Ribeiroia ondatrae and malformations associated with encysted Ribeiroia metacercariae in field-collected animals. Understanding the role of parasites in inducing malformations is critical to understanding the recent frog malformation phenomenon, and with this in mind, I discuss this role of parasites in detail below and in Chapter 5. MANMADE CAUSES In addition to these natural causes of amphibian malformations, Martin Ouellet2 lists several manmade causes of amphibian malformations. Acidification Ouellet notes that skeletal malformations such as brachymely and hyperextension may occur when tadpoles are raised...