Problems Of Measuring Size

Study of the geographical variation in body size of animals is a hazardous business, especially when weasels are the subject. The first problem is to decide which measure of body size to use. The two most obvious are head-and-body length

Table 4.1 Some Representative Examples of Local Variation in Mean Body Size of Weasels

M. nivalis n. nivalis n. vulgaris M. erminea M. frenata

Alaska, Yukon and Northwest Territories (Holmes unpubl.)

Alaska, Kodiak Island (Holmes unpubl.)

Alaska, mainland Southeast (Holmes unpubl.)

Boreal Canada (Holmes unpubl.)

Vancouver Island, British Columbia (Holmes unpubl.)

British Columbia island population (Holmes unpubl.)

Rocky Mountains, British Columbia, and Montana (Holmes unpubl.)

Great Plains, U.S. and Canada (Holmes unpubl.)

Great Basin and Rocky Mountains, U.S. (Holmes unpubl.)

Northeastern Oregon (Holmes unpubl.)

Coastal Oregon and northern California (Holmes unpubl.)

Coastal Central California (Holmes unpubl.)

Sierra Nevada (Fitzgerald 1977)

Upper Midwest U.S. (Holmes unpubl.)

Table 4.1 (Continued)

M. nivalis n. nivalis n. vulgaris M. erminea M. frenata

Southwestern Midwest, U.S. (Holmes unpubl.)

Female

Indiana (Gehring & Swihart 2000)

New York State (Hamilton Jr. 1933)

Northeastern U.S. and Southeastern Canada (Holmes unpubl.)

Male Female

156 154

Female BWT Male

Female

Britain (Corbet & Harris 1991) HBL Male —

Female

Ireland (Fairley 1981) HBL Male —

Sweden (Stolt 1979; Erlinge 1987) HBL Male 166

Female 148

BWT Male 54

Female 35

Switzerland (Güttinger & Müller 1988)

Male Female Male Female

167 144 53 31

Italy (C. Magrini, unpubl.) HBL Male

Female BWT Male

Female

202-214 173-181 106-131 55-69

189 154

73 36

168 142 54 32

222 180 193 82

201 181 81

217 181

217 134

297 264 321 213

252-278 208-230 233-334

184-230 98-137

257 231

195 111

270 218 225 102

253 209

274 256

(continued )

74 | The Natural History of Weasels and Stoats Table 4.1 (Continued)

M. nivalis

74 | The Natural History of Weasels and Stoats Table 4.1 (Continued)

M. nivalis

n. nivalis

n. vulgaris

M. erminea

M. frenata

Egypt (Osborn & Helmy 1980)

HBL Male

289

Female

242

BWT Male

390

Female

209

Siberia (Heptner et al. 1967)

HBL Male

160

260

Female

?

212

BWT Male

53

166

Female

41

?

New Zealand (King 2005b)

HBL Male

217

285

Female

183

256

BWT Male

127

324

Female

58

207

—, species absent or rare in that area; ?, data missing

Mean head-body lengths (HBL) in mm, weights (BWT) in g. Measurement ranges given are between, not within, sample means. For continental-scale geographical variation in skull lengths, see Figures 4.2 to 4.4. Note that sexual dimorphism is more pronounced in weight than in length. There is extensive north-south variation in Irish and Swedish stoats (larger in the south) and in British common weasels (larger in the north).

—, species absent or rare in that area; ?, data missing

Mean head-body lengths (HBL) in mm, weights (BWT) in g. Measurement ranges given are between, not within, sample means. For continental-scale geographical variation in skull lengths, see Figures 4.2 to 4.4. Note that sexual dimorphism is more pronounced in weight than in length. There is extensive north-south variation in Irish and Swedish stoats (larger in the south) and in British common weasels (larger in the north).

and total body weight, but records of these are scarce. Body weight is easy to measure but not very reliable, as it is affected by how recently a weasel last had a decent meal (King 1975c).

Skulls are easy to clean using a simple chemical method (McDonald & Vaughan 1999). Clean, dry skulls are small and inoffensive, and can be stored indefinitely in air. Large numbers of skulls of weasels can be kept in a few drawers; some natural history museums have collections going back for a hundred years. The condylo-basal length of a skull (defined in Figure 4.2A) is easily measured from existing material, so it is, therefore, the most readily available estimate of size. The simpler phrase "skull length" used here refers only to this measure of skull length.

Skull length has the slight disadvantage that it gives a less immediately obvious picture of body size than body weight or length, but it is easy to measure accurately, and in adults is immune to minor variations in environmental conditions. Other potential measures of body size in carnivores, such as the diameter of the canine teeth, vary more with the technique of attack, and the mechanical stresses of making a kill, than with body size itself (Biknevicius & van Valkenburgh 1996).

A second problem is that, since variation in size of a species as a whole must

Figure 4.1 Skulls of male common weasels and long-tailed weasels of known age, showing the changes in skull shape with age. Skulls are drawn at actual size. (Redrawn from King 1980e [common weasels bred in Germany by F. Frank] and Hamilton 1933 [long-tailed weasels bred in New York].)

be studied from adult animals, it is important to reduce the amount of error due to the accidental inclusion of young animals, not yet full-grown. Fortunately, this can also readily be done from skulls. The skulls of all young animals have open sutures between the growing skull bones, whereas in the skulls of older animals these suture lines are closed or even invisible (Figure 4.1).

The skulls also change in shape with age. The fully adult form is quite distinct from the juvenile form, especially in the larger species in which the sequence of changes continues through most of the first year of life. The single feature that best expresses these changes is the development of the postorbital constriction. The ratio between interorbital and postorbital widths, the postorbital ratio (King 1980e, 1991a), is a reliable method of identifying young animals and, unlike the best alternative method, tooth sectioning, it can be used on museum material, which must not be damaged.

A third problem is that of comparing measurements taken by different people at different times, perhaps in different ways, and often on very small samples. Potential errors thereby introduced are less troublesome with skulls of small animals, which can be measured with a micrometer between easily definable points. In recent years, the data on cranial variation in European and North American weasels have greatly improved. Many recent studies present much larger datasets than had ever been available before, controlled for individual variation in measurements, and use sophisticated new techniques to analyze them (Eger 1990; Meia 1990; van Zyll de Jong 1992; Reig 1997).

Yet a fourth problem arises from natural variation. This is particularly significant in weasels, in which pronounced sexual dimorphism in body size is very marked and variable both with locality and in time. Males are nearly always better represented than females in samples of weasels but, fortunately, there is a general covariation in size of the males and females of one species. If the objective is only to map the variation on the scale required to make comparisons across whole continents, it is fair to take the males as representing each local population as a whole.

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