Of the 451 female stoats collected from all around New Zealand in good condition during the main period of delay in implantation, December to July inclusive, the ovaries of all but two contained corpora lutea of delay. That is, all but these two had mated. The two odd ones had not merely failed to find a mate, because they were not still in heat (Chapter 9). They were, instead, the exceptions that prove how rare any kind of reproductive inefficiency is in stoats. In the rest, the number of corpora lutea, each representing one ovum released during the previous mating season, were counted by serial sectioning. We did not know at the time that it would have been easier to count the free blasto-cysts, which can be seen in fresh uteri.
The number of corpora lutea per female varied from 0 to 19, but the average number per sample was remarkably constant through the year, in females of all ages (Powell & King 1997), in different years, and in different areas. The local averages varied only from eight to 10, and the general average was 9.7. The number of corpora lutea in the two ovaries of one female were inversely related— when one had more than the average, the other had fewer.
In each area there were some individuals with very high counts in some years, usually in the breeding seasons when mice were abundant, and perhaps this was an effect of good eating. But this higher fecundity had no effect on the number of young produced by those individuals in the following season, which is con trolled by food supplies at the time of implantation and onward. In some animals, fecundity and productivity are linked, but in stoats they cannot be.
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