Northern Prairie Wildlife Research Center
Second, the "true" rate of malformation should be based on very young animals, recent metamorphs, for example. If, as seems very likely, malformed animals are likely to die sooner than their normal cohorts, the incidence of malformations among populations of older animals will be biased low.
Third, despite carefully prescribed protocols and instructions, it is not always easy to identify malformations with certainty. Missing limbs or digits may represent injuries, rather than malformations. While every effort was made to exclude likely injuries, it cannot be assured that all such reports were distinguished.
Fourth, the incidence of malformations can vary dramatically in space and time. Research by the Minnesota Pollution Control Agency has demonstrated that a site may harbor only normal-appearing amphibians on one occasion, but have amphibians with an elevated rate of malformations the very next year (J. Helgen, Minnesota Pollution Control Agency, pers. comm.).
Ideally, to gain a clearer understanding of the patterns and trends in the incidence of malformed amphibians, systematic surveys should be conducted in a variety of areas and repeated in time. Standardized protocols must be followed. Surveys timed to capture recent metamorphs are best, so that the incidence of malformations can be determined before any affected animals die. In some circumstances, surveys of larval amphibians can be used, although certain species are difficult to distinguish, especially by a nonherpetologist. In addition to conducting surveys specifically to evaluate malformations, it would be useful for scientists involved in capturing substantial numbers of amphibians to take note of any malformations that occur, and to report those data. Even if no malformed animals are encountered, the information can be used to evaluate the true extent of the problem.