One of the keys to success for opportunists is that they can respond quickly to any local improvement in living conditions. For example, a sudden surge in the population density of voles presents adult weasels with the opportunity to produce more young. Also, while the surge lasts, it increases the young weasels' chances of surviving to become adults, and the adults' chances of surviving long enough to breed again. When this happens, common and least weasels can adjust their breeding efforts almost immediately; hence, the breeding success of common and least weasels is very closely linked to the density of rodents.
Small rodents comprise ideally convenient packages of meat to feed to all young weasels; they are easy to kill, light to carry, and wrapped in ready-made waterproof packets. Populations of common and least weasels have developed their opportunistic way of life to such a fine art that they can survive the most dangerous conditions, such as in the fierce cold and regular famines of the far north. Local extinction is common, but there are always a few patches of habitat with enough rodents to support at least a few individual weasels, which in turn can survive and disperse to recolonize other areas vacated when times were bad.
Most adult stoats and longtails are large enough to survive short periods of scarcity of small rodents by turning to other prey, but their chances of breeding success are still highest when rodents are abundant. Hence, the rate of survival of the juvenile stoats and longtails and the productivity of the adults are also linked to the density of rodents or rabbits, just as they are for the smaller least and common weasels.
Stoats and longtails are, therefore, also classed as opportunists, even if not quite such highly specialized ones. But stoats and longtails appear to have one huge disadvantage that common and least weasels do not have: a compulsory period of embryonic diapause, which introduces a long delay between fertilization and implantation.
Delayed implantation is obligatory for stoats and longtails, regardless of environmental conditions, and it prevents the females of both species from responding immediately to fluctuations in the numbers of voles available. Their birth season is fixed by day length, and their fertility (maximum potential litter size) in any given year is set by their fecundity (ovulation rate) of the previous year. Any increase in fecundity in response to abundant food cannot be translated into larger litters until the following season, by which time the rodents may well have disappeared (King 2002).
This handicap must be particularly serious for the stoats of the smallest races living in the far north, which depend on small rodents as much as do common or least weasels elsewhere. If the capacity to respond rapidly to variations in food supply is important to common and least weasels, then surely it must also be important to stoats and longtails. How, then, can stoats and longtails survive in the face of such a serious penalty? Indeed, how did such an apparently disadvantageous arrangement become established in the first place?
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