Fertility is harder to calculate. There was hardly any information on litter size calculated from embryos, and none at all on nestlings. The whole study turned up only 13 pregnant females, of the 641 collected, and these had from six to 13 embryos, average 8.8. Losses from ovulation (average 9.7) to implantation (8.8), which are common in mammals (Asdell 1964), were modest.

Far greater losses were likely between implantation and birth. In nine of 11 pregnancies where all corpora lutea could be counted, the number of fetuses was fewer than the number of corpora lutea. In addition, some embryos had died and were being resorbed; in 5 out of 12 pregnancies where at least some embryos could be weighed, at least one embryo was resorbing; in one, seven of the eight embryos were reduced to simple swellings, leaving only one normal embryo almost at full term.

If things get very bad, a female can resorb her entire litter, and then appear in the spring with no sign of having produced any young at all. The season of births is quite well synchronized by day length (see Figure 9.6), and more than 99 % of females are fertilized each season, so it is possible to predict the stage in the reproductive cycle that each female should be in at any given time. Females that are not pregnant or lactating at the expected date can be assumed to have lost their litters entirely, either by total resorption or in the den soon after birth. Some are already fertilized for the next season, months before the successful females. These were nearly always found in beech forests in the crash seasons that follow two summers after a good seedfall (Figure 10.4). In one sample taken two summers after a huge peak in numbers of mice, every single one of 28 adult females collected had lost her entire litter: The sample (total n = 63) included not a single young one born that season (King et al. 2003b). The mortality of embryos is not always so drastic, but the net result is that fertility is almost always lower than fecundity. Deaths among the small nestlings, which we know nothing about but which surely must happen, must add to these losses.

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