Common and Least Weasels

The breeding cycles of least and common weasels are broadly similar. The gestation period is about the same (34 to 37 days); both have direct implantation; young females of both subspecies can breed in the summer of their birth when conditions permit (Sundell 2003); and the length of the breeding season is very variable (Figure 9.4A). In Europe, male common weasels can have enlarged testes and are fecund from February until at least early August, occasionally to the beginning of October. The testes regress in autumn, but never relapse into complete quiescence, since the early stages of spermatogenesis can be found in the cells throughout winter. On the other hand, there are no spermatozoa in the epididymis from November to January inclusive, so winter is definitely an infertile period for common weasels. Adult females start coming into heat in February, though they may not necessarily conceive then. They become anestrous in September, or earlier when they are in poor condition.

Implantation in common and least weasels is direct; that is, the fertilized zygote appears to proceed straight through the stages of development without a detectable pause at any stage. The gestation period is, therefore, about the same as the time it takes the embryos to develop, and the ovaries contain corpora lutea only when the female is carrying actively developing young. Pregnancies may be observed at any time from March to early August, and the first litters are born in April. Rearing takes at least 9 weeks, so one full breeding cycle takes 3 to 4 months (see Table 9.3).

When voles are numerous, well-fed adult females may come into estrus again when their first litter of the year has been weaned, at earliest by the end of May. The corpora lutea of the first pregnancy persist well into the second, so females may be found with two sets of corpora lutea in the ovaries from June onward (Deanesly 1944; King 1980c). Second litters are born in July or August. In exceptionally good years some older females may still be suckling in October (Delattre 1983), while the earliest-born of the young females of the season produce their first litter in the year of their own birth (King 1980c; McDonald & Harris 2002).

Common and least weasels can respond to a glut of food by producing two litters in one reproductive season. One group of captive females fed year-round on as many live, breeding voles as they could eat was even persuaded to produce three litters in a season (Frank 1974). In the wild, two litters in a season is about the most any female can manage, and then only when hunting is exceptionally easy. More often, late litters are lost. For example, among 77 female common weasels collected in Britain by McDonald and Harris (2002), the last pregnancy was observed on October 5, and the last postpartum uterus on

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Figure 9.4 The reproductive cycles of weasels in the northern hemisphere. (A) In most years, least weasels and common weasels can rear only a single litter in spring. They remain fertile until midsummer and may mate again but without producing surviving offspring. In years when rodents are abundant, common weasels can rear additional litters in summer and autumn, and least weasels can continue breeding under snow all winter. (B) The reproductive cycles of stoats and long-tailed weasels are fixed by day length, regardless of food supplies.

Figure 9.4 The reproductive cycles of weasels in the northern hemisphere. (A) In most years, least weasels and common weasels can rear only a single litter in spring. They remain fertile until midsummer and may mate again but without producing surviving offspring. In years when rodents are abundant, common weasels can rear additional litters in summer and autumn, and least weasels can continue breeding under snow all winter. (B) The reproductive cycles of stoats and long-tailed weasels are fixed by day length, regardless of food supplies.

Figure 9.4 (continued)

October 13, but no female captured after August 27 was lactating, suggesting that few late-born young survived.

It is not correct to assume, as in some population models (Chapter 14), that common weasels can successfully produce two litters every year. On the contrary, when food is very scarce, adult females in the wild probably mate as usual but are unable to produce any surviving young even from their first litters. According to Erlinge (1974) and Tapper (1979), common weasels cannot rear young at all unless they have access to a minimum density of some 10 to 14 voles per ha, or about 400 rodents per territory (Jgdrzejwski et al. 1995). So pregnancy rates and the lengths of breeding seasons vary enormously in common weasels, from 7 to 8 months in vole peak years to total failure in crash years.

Reproduction by least weasels differs from that of common weasels in two minor, but very interesting, ways. First, when prey are abundant, least weasels in North America can be found pregnant or with small young in most months of the year (Hall 1951; Heidt 1970), whereas common weasels never continue breeding into the winter. In the tundra, least weasels breed under the snow well into the winters of peak lemming years (Fitzgerald 1981). This remarkable ability is actually less surprising than it sounds. The conditions in the far north for breeding of both small rodents and of weasels are, in fact, much more favorable in winter than in spring (Chernov 1985). Once the snow pack is established, the subnivean space provides near constant conditions and a reliable refuge from large predators (Chapter 1). The temperature is near freezing, so female least weasels must keep moving if they leave the safety of their fur-lined dens. Yet, spring conditions are worse. Then, the melting snow often floods the burrows and nests, drowning nestlings, blocking access to food, and exposing small animals to wind chill, late frosts, and hungry predators. The effort these tiny hunters expend on reproduction conveys some idea of the urgency and importance of breeding success to small, short-lived animals.

The second difference between least and common weasels was noted by Frank (1974), who kept both in his lab at Braunschweig, in West Germany. In each of three successive years, a wild-caught least weasel produced three litters, mostly common-least hybrids. The periods of pregnancy (5 weeks) and of rearing (8 to 9 weeks) were the same as he recorded in purebred common weasels kept in the same conditions. The difference was that the female least weasel came into estrus again only 5 weeks after the previous litter had been born, whereas none of the common females did so until 9 to 10 weeks after the births.

This means that the female least weasel was able to start the gestation of a second litter during the rearing of her first, and she completed the production of two litters in 5^ months instead of the 7 to 8 months needed by the female common weasels. Frank emphasized that this happened regularly, and suggested that it might be an adaptation to give least weasels maximum productivity in the short summers of their northern home, especially in periods of rapidly rising numbers of voles. Alternately, if the breeding cycle of least weasels is less closely controlled by season than in other weasels, their ability to breed during lactation would allow female least weasels to respond to a vole peak at any time of year, and to respond more rapidly than other weasels can. The speeding up of the reproduction process in least weasels could be more a matter of competitive advantage (Chapter 14) than of adjustment to short northern summers.

The failure of common weasels to produce surviving young in bad rodent years was already well known when Janne Sundell, a graduate student at the University of Helsinki, Finland, became interested in whether least weasels are also vulnerable to losing their litters in poor years, and, if so, how. The information available did not show whether females attempted to produce young but lost them, or did not bother to try. Not bothering is a strategy common in vole-eating raptors and owls (Southern 1970), but these are longer lived than weasels so have more opportunities to make up for bad years later.

Sundell completed a 5-year study of captive least weasels, following 65 litters of young least weasels averaging five kits per litter (53 litters counted: Sundell 2003). In 1998, some breeding females were kept on a restricted diet until after mating, but were restored to normal rations once pregnant. The four females kept on short rations mated as often as the five fed to excess, but produced fewer young (averaging 5.0 vs. 6.6), which also had a higher mortality rate (31% vs. 2%). Sundell concluded that short-lived species such as least weasels always try to breed at every opportunity, because the distribution of rodents is patchy and the chances of surviving to the next season are poor.

Common and least weasels are easier to breed in captivity than stoats and longtails. They are not strictly confined to one litter a year, and, because they are smaller animals, perhaps they are less likely to be stressed by confinement in small spaces. Experiments on breeding them have illustrated the point, which probably applies to all species of weasels, that the quality as well as the quantity of food is a significant determinant of breeding success.

The most likely reason that Fritz Frank was able to breed both subspecies and their hybrids so easily was that he fed them entirely on live Microtus from an adjacent breeding colony. The weasels, guaranteed regular supplies of their favorite food, plus the stimulus of killing them, produced 94 young over 10 years (Frank 1974; King 1980e). In the experiments by Sundell (2003), five female least weasels kept on a diet of rodents (85% voles, either live or freshly killed, and the rest defrosted lab mice) produced larger litters (averaging 6.2 kits vs. 4.3) with lower juvenile mortality (17% vs. 58%) compared with four females fed only on defrosted, domestic chicks. These experiments are consistent with the view that, when rodents are scarce, the weasels' chances of successful reproduction are reduced, even if other foods are available.

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  • aliisa
    Where are least weasels held in captivity?
    7 years ago

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