The final step in the long process of producing the annual crop of young stoats is the rearing and training of the young for independence. This phase takes 3 to 4 months, and there is practically no information about it. Biologists studying other kinds of predators, especially raptors, do well at this time, since raptor nests are conspicuous and the young are easily observed. Those who choose to study stoats have, until recently, had to wait until the young are ready to show themselves. The advent of radio tracking and specially trained dogs (Theobald & Coad 2002) have made it easier to find stoat breeding dens, and occasionally the fate of a whole litter can be documented.
In midsummer, young stoats start moving about and exploring the world, including the tempting wooden tunnels that cover traps. The number of young stoats caught per 100 TN is an indirect measure of productivity, not per female but of the population sampled. In poor years, fewer than one young stoat is caught per 100 TN, or even none at all (Murphy & Dowding 1995; King et al. 2003b); in good years, the crop of young can reach more than 6C per 100 TN.
Do these differences reflect real changes in fertility from year to year and not, for example, changes in trappability among the members of a population producing a constant number of young? Yes, because the number of adults caught varies less between years than does the number of young produced (King 1981a). The good years are those when a high proportion of the fertilized ova survive to become live, independent young stoats. Since the energy demands of a lactating female stoat may increase by 200% to 300% (Müller 1970), the chances of the young surviving increase in direct proportion to the mother's chances of finding enough food and still having time to keep her young warm. Those chances are highest when mice are abundant (King 2002).
Conversely, in the bad years, about the same number of potential offspring on average start as ova but few get all the way to independence. Because many females get only one chance to breed, and never "know" whether conditions in the following spring will be good or bad, it makes sense always to start with a large number of fertilized ova and defer adjusting that number for as long as possible. If, by the spring, things look unpromising, a female may be better off saving energy and producing fewer young, rather than taking a chance and losing all of them. Sometimes the chances of raising a few offspring to independence look good to start with but then rapidly deteriorate; then, most of the young will die in the den.
For example, one female radio collared by Dowding and Elliott (2003) on October 9, 2000 was heavily pregnant, and by October 11 had given birth in a rabbit burrow. By October 20 she had moved the kits to a new den under a sheet of roofing iron, and was visited by a male whose home range overlapped with hers, amid much squealing. On October 24, while she was away, Dowding and Elliott lifted the roofing iron and found 10 live kits. On October 26 there were eight live kits and one dead; on October 28, there were six live and one dead; on October 31, there were no kits in the den but possibly one was seen outside with the female on November 1. Rabbits were relatively scarce at the time, and it seems likely that when hunting is bad, the reduction of a litter from 10 to six in a week is not at all unusual. The same process applies to other mustelids with delayed implantation, such as martens; when food is short, young females do not mate, and older ones mate but fail to produce young (Thompson & Colgan 1987).
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