The only method of determining the age of a weasel that can be truly accurate is continuous observation of a marked individual from the time it was recognizably juvenile. Such long-term observation is done best in captivity or in conjunction with a successful live-trapping and radiotelemetry study. For unmarked dead animals collected from the wild, the only methods that are reasonably dependable are those that have been calibrated against a set of specimens of known age.
To our knowledge, two such sets have been collected. The first, the property of Fritz Frank of West Germany, is a set of skulls of 26 male and 18 female common and least weasels and their hybrids bred under seminatural conditions in his laboratory. The other set comprises 22 skulls of stoats from New Zealand, some marked and recaptured from the wild and some held as wild-caught captives. From these two sets of material, two rather different methods of age determination have been developed. One is based on observation of changes in the shape of the skull and the baculum, and the other on the sectioning of canine teeth. Each has its usefulness and complements the other to some extent.
The date-skull-baculum (DSB) method, developed by King (1980e) from Frank's material, is simple, requires no special equipment, and is most useful for distinguishing young animals, not yet full grown, from among a set of skulls and bacula that must not be damaged (see Figures 4.1 and 9.1). The development of the postorbital constriction, allied with features of the baculum and other anatomical features known to be related to age, allow classification of wild-caught common weasels into two, or at most three, year classes. It is very reliable for the first 6 to 8 months after the young begin to appear in the population, though less so in late winter and spring. In Pascal et al.'s (1990) modified version of this method for common weasels in France, the first year class is divided into three stages, and all weasels older than 12 months are grouped into a single class. The skulls of stoats (King 1991a; Sleeman 2004) and longtails (Hamilton Jr. 1933) show the same developmental changes, although they span a longer period than in common weasels, because of the slower development of larger animals (see Table 9.3).
The disadvantage of the DSB method is that it cannot distinguish between the year classes of full-grown animals. This does not matter much for common or least weasels, but for stoats and longtails, if histological machinery is available and destructive processing is permitted, the more complicated but more reliable method of counting cementum layers in the teeth is better.
The teeth of mammals are anchored in their sockets by cementum, a hard substance that is strengthened with new layers year after year. Thin sections of the teeth can be stained to show the cementum lying around their roots. Within the cementum, the edges of the annual layers show up as dark lines in a paler field (Figure 11.1). The technique of counting the cementum lines to estimate an animal's age in years has become routine, provided that it can be proved first that the lines really are annual. For example, the equally obvious lines in the superficial bone of the jaw are also annual in some animals, but not in common weasels (King 1980e).
Young stoats are relatively easy to catch and to distinguish in summer, so King and McMillan (1982) marked and released a cohort of young known to have been born in the southern summer of 1979-1980, and retrieved them over
the next few years. By August of 1981, 22 stoats of known or part-known age had been recovered: In every case the number of lines in the cementum corresponded exactly to the number expected (Grue & King 1984). The lines are always formed in autumn and winter, so the date of death must be known before the age of a specimen can be estimated. When that information is available and large samples are classified into year classes, the reliability of many other methods of age estimation can be calibrated (King 1991a). The same method can be applied to common weasels, though for them it still lacks calibration.
The ages of living stoats and longtails can be classified in the summer months when the young are still visibly immature (males have only very small testes, and females have no visible teats), but the ages of living common and least weasels, other than kits, are impossible to estimate with any confidence.
The problems of deciding how to classify the ages of common weasels are a little different from those of classifying stoats and longtails, because of the different reproductive cycles of species with and without delayed implantation. In common weasels during a good year for rodents, the annual cohort of young can be added to the population over a long period. The earliest-born young of the year can be up to 6 months old and reproductively mature while their later-born part-siblings are still helpless nestlings. The physical distinction between young and adult blurs quickly in common weasels, and never can be defined in reproductive terms.
By the end of a productive breeding season the generations of common weasels overlap, although in a big enough sample, a bimodal frequency distribution of age classes may appear (Pascal et al. 1990). The compensation is, however, that these small weasels are short-lived. Young common weasels are usually vastly in the majority, and for most of the year can be distinguished from the smaller numbers of second-year and older adults. In many studies, division into two age classes, while not ideal, may be enough.
By contrast, the annual production of young stoats is closely synchronized by day length, so each cohort is distinct. The young of both sexes can be separated from the adults with confidence until well after the end of the breeding season, and the young males are clearly recognizable until shortly before the next season (see Figure 9.9).
On the other hand, stoats are relatively longer-lived, and for them, separation of the young animals from adults into two simple age classes by cranial features is often not enough. Tooth sectioning of the adults into year classes is usually necessary. This time-consuming and technically demanding operation is best done by someone with experience in cutting and reading the sections.1
The results are worth the effort, but it saves money if skull and baculum characters are used to exclude the young of the year from the list of specimens to be sectioned.
1For example, Matson's Laboratory, P.O. Box 308, Milltown MT 59851 (http://www.matsonslab.com/).
We assume that ages of longtails can also be estimated accurately via cemen-tum annuli, but we know of no one who has done it.
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