while other hexachi orobi phenyl s are relatively innocuous (Aulerich et al . 

 1985). Biologically modified PCBs are more toxic to mink than corresponding 

 technical mixtures. For example, tissues from cattle that had been dosed with 

 Aroclor 1254 and fed to mink at levels as low as 0.64 ppm fresh weight of diet 

 caused severe reproductive effects, possibly because cattle tissues retained 

 more of the highly chlorinated congeners of the PCS mixture once they have 

 been metabolized. But Aroclors 1016 and 1221, at dietary concentrations of 2 

 ppm, produced no adverse reproductive effects in mink over a 9-month period, 

 nor did Aroclor 1242 at 5 ppm during a similar period; the reasons for this 

 are unknown but may be due to the ability of mink to metabolize selected PCB 

 congeners. Mink, for example, easily eliminate 2,2' ,4,4' ,5,5'-hexachloro- 

 biphenyl, but in rats, domestic pigeons ( Columba livia ), and trout, this 

 congener remains almost indefinitely (Hornshaw et al . 1983). Placental 

 transfer of PCBs occurs in mink, as has been demonstrated for rats, rabbits, 

 cattle, rhesus monkeys, ferrets, and humans, and gives rise to the 

 embryotoxicity demonstrated by these species (Ringer 1983). A significantly 

 greater quantity of PCBs enters the growing offspring of mink from mammary 

 transfer than from placental transfer. This PCB transfer by the lactating 

 mink probably resulted in the high offspring mortality observed when Great 

 Lakes fish were fed to commercial ranch mink in the late 1960's. Aroclor 1254 

 residues in subcutaneous fat of adult mink was up to 38X dietary levels, and 

 some individual congeners accumulated up to 200X. The time for 50% 

 elimination of PCBs from adipose tissues was about 98 days, and about 199 days 

 for 100% elimination. Comparable data for other mammals indicate that PCBs 

 are eliminated more rapidly by these species than mink, with 50% half-life 

 times of 33 hours to 69 days for tissues of rat and dairy cows (EPA 1980; 

 Stickel et al. 1984). In general, PCB residues in fat of mink were highest in 

 winter when fat deposits were mobilized during cold weather and PCB residues 

 were concentrated in remaining fat stores; this is consistent with the results 

 obtained from pigeons subjected to low temperatures and starvation (Hornshaw 

 et al. 1983). 



The European ferret was significantly more resistant to PCBs than mink, 

 and demonstrates that interspecies sensitivity to PCBs varies widely, even 

 among taxonomically close species (Ringer 1983; Bleavins et al . 1984). In 

 ferrets, total reproductive failure was documented in 9-month feeding studies 

 of Aroclor 1242 at dietary levels of 20 ppm, but Aroclor 1016 at 20 ppm did 

 not affect reproduction during a similar period. PCBs in maternal body fat 

 stores of ferrets represent a reservoir that can be transferred to the 

 developing fetus and growing neonate (Bleavins et al. 1984). Placental 

 transfer of Aroclor 1254 to kits of the European ferret was about 0.01% per 

 kit of the female's absorbed dietary dose when exposure occurred during the 

 first trimester of pregnancy, and 0.04% when PCBs were administered during the 

 third trimester. Transfer of PCBs through the dam's milk was also documented, 

 and this route was 6 to 15X more effective than placental transfer. 



The rhesus monkey ( Macaca mulatta ) is extremely sensitive to PCBs 

 (Roberts et al. 1978; NAS 1979; EPA 1980; Safe 1984). Females fed diets 

 containing 2.5 to 5.0 ppm of Aroclor 1248 for 6 months showed altered 

 menstrual cycles, an increased frequency of stillbirths and abortions, a 



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