Looking to the Future

One new approach that is only first beginning to be explored, is that of directly assessing the field-exposed animal for its own reproductive health condition. Although researchers may be disinclined to venture outside of the laboratory and into highly variable environmental settings to conduct unusual and possibly labor-intensive work, the potential gains can far outstrip any complications. Direct health assessment for reproductive effects is predicated upon an entirely different understanding of the potential for contaminated areas to pose health risks. The new approach recognizes that at virtually all contaminated sites where reproductive and other assessments are to be made, multiple decades have elapsed since the contamination was released, and consequently tens, and in some cases hundreds, of generations of ecological species (e.g., small rodents) have already cycled through by the present day. Thus, it is really too late to be endeavoring to project or forecast the likelihood of reproductive effects first arising; effects, if they were ever to occur, should have already been expressed. Additionally, with almost no documented cases of ill-health ever having been reported for terrestrial receptors at contaminated sites, the anticipation is that such a direct assessment method would reveal the same. Finding this supposition to be true would have vast ramifications for environmental management; we would then know that although chemicals remain in environmental media, cleanups are not necessary because the chemicals are not impinging on an animal's ability to survive and perpetuate its own.

Evaluating the sperm parameters of count, motility (the percentage of properly swimming sperm), and morphology (the percentage of normally/abnormally shaped sperm) in adult male small rodents collected at Superfund-type terrestrial sites (for a comparison with the parameters of rodents from nearby noncontaminated sites) has substantially advanced the field of reproductive toxicity for ecological receptors. This approach keenly recognizes that each of the sperm parameters is a barometer of reproductive success, a situation much akin to the case in humans, thereby explaining today's rather routine clinical studies investigating the causes of infertility. For the small rodent grouping, which is for all intents and purposes the maximally exposed terrestrial receptor (given the group's nonmigratory nature and high degree of site contact), it is known rather precisely how much of a reduction in either count or motility, and how much of an increase in morphology, is needed to compromise reproduction. For nearly all other biological measures that can be collected in exposed and nonex-posed animals, whether in the laboratory or the field, it is not known how much of a difference equates with a demonstration of impact, and consequently, only absolute measurement differences can ordinarily be reported. Where none of the sperm parameter thresholds are exceeded, it is logical to conclude that the larger, higher-trophic-level and wider-ranging receptors are also not experiencing reproductive impairment. Although this testing scheme would appear to be somewhat lacking, in that females are not evaluated, the US EPA, among others, find that there is abundant information to reliably conclude that the sexes respond similarly. Thus where demonstrated reproductive effects occur in one sex of a species, the response (or lack thereof) in the other sex should be the same.

There is promise too for the development of a female-based homolog of the above-described sperm parameter reproductive assessment scheme. From laboratory-based studies with mice and rats, it is clear that certain chemical exposures (e.g., the pesticide methoxychlor) can arrest the normal development of ovarian follicles. In theory, ongoing empirical research will lead to a chemical-specific understanding of the degree to which follicle development needs to be reduced or arrested such that fecundity is offset.

Given the ever-increasing contaminant releases to the environment from our highly industrialized society, both in volume and in the number of constituents involved, the argument can be made that our ability to assess the potential for reproductive effects by employing calculations that involve dose estimation, uptake, biotransformation, and metabolic rates, is being far outstripped. Potentially, the direct assessment of reproductive capability provides a way around the problem. Care must be taken though to conduct the empirical research necessary for the accurate identification of reproductive impacts should they exist. By way of example, one may detect distinct reductions in the gonadal mass of largemouth bass that have been exposed to dietary mercury under controlled laboratory conditions, but the extent of the reduction needed to compromise reproduction must be known. Conceivably, a gonadal mass reduction of some order could coincide with improved reproduction, and it could be also that reproductive impacts only occur when there are drastic reductions in gonadal mass. In the case of the rodent sperm counts, this is precisely the case; an 80-90% reduction is first needed for there to be compromised reproduction, a figure very much contrary to what would be expected. In a similar way, the brood patches of birds and the placental scars on mammals could be routinely examined for an assessment of reproductive health, but needed are rather iron-clad thresholds for effect, as in ''how many fewer placental scars in a certain age mammal is indicative of a certain percentage reduction in fecundity?''

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