Reproduction in Domestic Animals

Dietary Pollutants as Risk Factors in Reproduction 19effects at least equivalent to, and sometimes higher than,those induced by much higher doses administered overshort periods of time (Borman et al. 2000).Individual variation in exposureIn species as diverse as sheep (Rhind et al. 2005b) andinsects from lake sediments (Kovats and Ciborowski1989) differences in tissue concentrations of variouspollutants of at least 10-fold between individuals fromthe same source are commonly observed. Similar environmentalvariation was reported in soil samples (foodcontaminant and invertebrate habitat) collected fromwithin experimental plots (Rhind et al. 2002). Thisfurther complicates the prediction of responses ofindividual animals to pollutant exposure.In summary, there seems to be little doubt that thediet is a major source of exposure to pollutants in manyvertebrate species and while the effects on tissueconcentrations are greatest in carnivores near to thetop of the food chain, significant tissue accumulationcan occur in herbivores. However, the relative contributionsof other factors such as uptake from thesurrounding media (air, water, soil or sediment) are lessclear and, in certain circumstances and species, thesesources may be more important than dietary ones,particularly in small, soil-, sediment-, or water-dwellingspecies.What Risk is Associated with Pollutants inFood?Since many studies of effects of pollutants on reproductivefunction frequently involve transient, unusuallyhigh levels of exposure of wild or laboratory species ofanimals to a single chemical, it is sometimes concludedthat normal, low, levels of exposure to pollutants areof no biological consequence. This argument is reinforcedby the fact that environmental concentrationsof some, but not all, pollutants are declining (Darnerudet al. 2001; Norstrom 2002), as are milk (Solomonand Weiss 2002) and tissue concentrations (Noakeset al. 2006). In fact, even tissue concentrations thatare at or near to background environmental concentrationsmay exert biological effects, particularly whenthey act in conjunction with a mixture of otherchemicals; ‘real-world’ exposure normally involvesprolonged exposure to a mixture of compounds at lowconcentrations. Effects of such exposure patterns havebeen demonstrated by long-term studies of sheepmaintained throughout their breeding lives on pasturesfertilized with either conventional inorganic fertilizeror sewage sludge. The latter contains large amounts ofboth inorganic and organic pollutants, relative toenvironmental levels (Smith 1996; Stevens et al. 2003)and, when applied to pasture as a fertilizer, results invery small increases in environmental levels of thesechemicals (Rhind et al. 2002) and in the tissue levelsof sheep grazing the pasture (Rhind et al. 2005a,b,2007b). However, these small increases are associatedwith disruption of reproductive and other functions(Erhard and Rhind 2004; Paul et al. 2005; Fowleret al. 2008).Such studies clearly demonstrate that pollutants infeed can represent a risk to reproductive function but donot allow the quantification of that risk with respect toeither individual chemicals or individual animals. Thatwould require determination of dose response relationshipsbetween the concentrations of each chemical, ineach relevant tissue or organ and for each observedchange in structure or function. It is possible to achievethis, empirically, for a small number of individualchemicals but, at present, virtually impossible to do sofor the thousands of chemicals to which animals areexposed. Understanding of mixture effects, will requireintegration of the observations from the wide range ofempirical approaches that are available using powerfulpredictive computer models (Suk et al. 2002).ConclusionsThere is no doubt that exposure to pollutants canadversely affect reproductive function in animals andthat food is likely to be the most important route ofexposure for many or most species, particularly forthose near the top of the food chain. However,assessment of the risk is complicated by the fact thatin the ‘real world’ food contains many different classesof pollutants, each at low and variable concentrations,and exposure to them occurs throughout life, includingduring developmental stages which are particularlysensitive to their effects. 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