Variation at high latitudes: the geography of body size and cranial morphology of the muskox, Ovibos moschatus

Abstract

Aim Bergmann's rule, one of the most studied and controversial ecogeographical generalizations, has rarely been tested with observations from high latitudes. We tested the rule using cranial measurements of the muskox [Ovibos moschatus (Zimmerman)], a homeotherm with an extremely northern distribution. We also used these data to describe geographical patterns in the species' dental architecture, an extension of the framework developed from interspecific comparisons. Location Specimens were compiled from arctic Canada, Alaska and Greenland, a latitudinal range of 60° N−83° N. Methods Body size was estimated from principal components analysis (PCA) of five cranial characters from 128 specimens. Mean scores on the first principal component from each locality were regressed against latitude and mean temperature to identify geographical variation in body size; scores on the second principal component were regressed against latitude to assess patterns in dental architecture. Regression analyses of the individual characters were performed as a complement to PCA. Results No latitudinal or climatic trend in body size was observed in either sex. On the other hand, for males, significant latitudinal variation was found for the second PCA axis (r = −0.434), and the feature which loaded most heavily on it, maxillary tooth row length (r = 0.429). For females, this dental structure also tended to increase with latitude (r = 0.423), but the trend was only marginally significant (P=0.12), perhaps owing to a smaller sample size. Main conclusions The geographically invariant body size of muskoxen failed to support current hypotheses of size variation. Behavioural and physiological adaptations may exempt the muskox from selective pressures underlying these hypotheses. We interpret latitudinal variation in dental architecture as a reflection of a cline in diet, dominated by graminoids at the expense of willows at higher latitudes. This intraspecific geographical trend is a recapitulation of the interspecific framework for large mammalian herbivores.

Morphological variation of the Japanese raccoon dog: implications for geographical isolation and environmental adaptation

“…According to Bergmann's rule (Bergmann, 1847;Mayr, 1956;James, 1970), within a given species, the body size of a northern race tends to be larger than that of a southern race. Bergmann's rule is a valid ecological generalization for birds and mammals (Lindsey, 1966;Brown & Lee, 1969;Meiri & Dayan, 2003), although exceptions are common (Abe, 1967;Zink & Remsen, 1986;Kaneko, 1988;Smith, Schaefer & Patterson, 2002). Among the mammals of Japan, we expect the body size of northern races, those distributed in Hokkaido, to be larger than those of southern races, those occurring in Honshu, Shikoku and Kyushu.…”

Biodiversity, Distributions and Adaptations of Arctic Species in the Context of Environmental Change

“…The body size within some vertebrate taxa increase towards the north (Bergman's rule), but there are several notable Arctic exceptions to this (e.g. reindeer (131); muskox (132)) There are few physiological adaptations in homeotherms (i.e. warm blooded) that are unique to Arctic animals.…”

Sensible Heat Loss from Muskoxen (Ovibos moschatus) Feeding in Winter: Small Calves Are Not at a Thermal Disadvantage Compared with Adult Cows

“…Selection may favor much greater thermal tolerances at lower temperatures and greater wind strengths than we could test in captivity at Fairbanks. Interestingly, Smith et al (2002) found no clinal-or temperature-related variation in body size for adult muskoxen across a latitudinal range of 60Њ-83ЊN. This is contrary to Bergmann's rule (see Ashton et al 2000;Smith et al 2002) that endotherms should be larger nearer the poles because selection should favor lower SA : vol ratios that minimize heat loss.…”

“…Interestingly, Smith et al (2002) found no clinal-or temperature-related variation in body size for adult muskoxen across a latitudinal range of 60Њ-83ЊN. This is contrary to Bergmann's rule (see Ashton et al 2000;Smith et al 2002) that endotherms should be larger nearer the poles because selection should favor lower SA : vol ratios that minimize heat loss. Lyberth et al (2007) did report spatial differences in body sizes for mature muskoxen, but the effect was most pronounced for males and was attributed to the interplay between resource availability, density dependence, and sexual selection rather than to weather-induced phenotypic variations.…”