Age and Size at Puberty

Young common weasels born early in the season, especially when food supplies are good, grow very rapidly. Early-born young of both sexes can be physically and sexually mature by the age of about 3 or 4 months. By mid- or late summer (July to August), they may be taking full part in reproductive activities on equal terms with the adults, and in vole peak years their contribution is extremely important (Chapter 10). In most seasons, though, young-of-the-year females either fail to conceive or lose their first litter at a very early stage. The average young common weasel does not breed successfully until its second year.

By contrast, in young stoats and longtails the development of young males and females is radically different (Figure 9.9). Young females reach puberty as nestlings (stoats) or when newly independent (longtails), and then grow to their full adult size by the autumn, when they are about six months of age. Young males stop growing in autumn, and remain immature and distinctly smaller than adults throughout their first winter. In spring, last year's young males reach puberty, and suddenly put on another spurt of growth. They are indistinguishable from older adults by the time they are 12 to 14 months old and the breeding season is well under way.

The precocity of the juvenile females is the most extraordinary thing about the development of young stoats. While they are still in the den, in fact while they are still blind, these tiny, unweaned babies become reproductively mature, and are fertilized by an adult male, probably the same one that mates with their mother. It passes belief that the large and aggressive males can mate with sexually mature but developmentally infant females without damaging them, yet that is exactly what happens.

Many observations of captive matings confirm that adult males do mate with nestling female stoats—not just occasionally but as typical and normal breeding behavior, which leads in the following year to a litter of typical and normal young. Ternovsky (1983) observed 58 young females mating at between 17 and 134 days old, and all of them produced young in the following year, including this remarkable case:

The youngest female was 17 days old on the day of mating (25 May 1980), was 112 mm long, was helpless, deaf, blind and toothless; she was feeding on her mother's milk and could move only slightly by

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Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

First Year

Figure 9.9 The growth patterns of male and female stoats in New Zealand during their first year in life. The two sexes reach both puberty and full size at different ages. (Note: November is the austral late spring, and March is autumn.) The same seasonal pattern applies to stoat populations everywhere, although these body weights are too high for most other stoat populations. (Redrawn from King & Moody 1982.)

crawling. Her body mass (18 g) was 13% of her mother's body weight and only 6% of the body weight of the male. Coitus lasted only for one minute. Spermatozoa were present in a vaginal smear. On 22 May 1981, after 337 days, this female gave birth to 13 young and fed them successfully.

Considering the enormous difference in size and strength between an adult male and a juvenile female, one might regard this behavior as an animal version of rape, but that idea would be quite wrong. Juvenile females are not only willing to cooperate with the male, but they are also positively eager. The arrival of a male (or perhaps more specifically, his scent) stimulates a sexual reaction in blind and deaf female young. They call to him with high-pitched trills and chuckles, grading into the typical adult nuptial cooing signal. They grab onto him as he passes by, or crawl after him, interfering with his mountings with both their mother and sisters (Müller 1970; DonCarlos et al. 1986).

This does not sound like coercion, and, indeed, it is not. It is in a young female's interests to mate with a male that is strong enough to overcome her mother's defenses and enter the den, or careful enough to call when she is absent. The lack of a year-round pair-bond and the short average life span and rapid replacement of locally resident males all minimize the chances that a young female might be mating with her own father.

Adult male stoats can tell the sex of a juvenile as soon as they have grabbed its neck, and they usually drop the males at once. This reaction must presumably be a response to a scent or taste signal. Young stoats develop prominent manes of brown hair on the napes of their necks when about 3 weeks old (Table 9.3; Figure. 9.10). This juvenile mane was first described many years ago (Bishop 1923; Hamilton Jr. 1933), but its function has never been explained. Presumably it is associated with glands that produce a scent or taste distinctly different in young males and females.

Ternovsky (1983) described one case in which the adult male mated with all four young females in a litter of eight, and killed all four males. One might ask why the adult males do not make a habit of this, since, presumably, it would be in their interest to remove potential future rivals. On the other hand, if the old male lived that long, he would be well able to drive any youngsters away and force them to try their luck elsewhere. Meanwhile, restraint might be worthwhile on the off chance that he might be killing his own sons, or that his interference might provoke an aggressive reaction from the mother.

Juvenile female longtails also mature early, though not quite to such a dramatic extent. They are 3 or 4 months old, weaned, and almost full grown before their mother comes into heat between June and August and the adult males are permitted to approach the family. Only then do the young females mate. Young longtails do not develop a mane, as do young stoats, and this difference supports the idea that the function of the mane has something to do with helping adult male stoats to identify their infant mates. One might ask why the young female longtails do not also mate as nestlings; but remember, the two species are not as closely related as they look (Chapter 1).

By the time they leave the family group, virtually all juvenile female stoats and longtails are already fertilized. Even in very large samples of trapped stoats, it is unusual to find even one young female without corpora lutea. The energy required to maintain the blastocysts is slight, so this stage of pregnancy does not add much extra burden to the young females as they complete their own

Figure 9.10 Juvenile stoats of both sexes develop a transitory mane when about 3 weeks old, which seems to be correlated with the extreme sexual precocity of the females. Adult males apparently use a cue located in the mane to distinguish infant females from males, and to mate only with the females.

body growth. The young males of both species are more conventional—they breed first when they reach adult size as yearlings.

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