Bergmann's rule in shrews: geographical variation of body size in Palearctic Sorex species

Abstract

According to Bergmann's rule, individuals of a given species tend to be larger in colder (northern) climates. Traditional explanation points to the relatively lower surface-to-volume ratio in larger animals and, consequently, relatively lower costs of thermoregulation. We examined intraspecific covariation of body size with geographical location and climate in five species of Sorex shrews, animals that are among the smallest extant mammals. The condylobasal length of skull (CBL), compiled from literature data and measured on museum specimens, was used as an indicator of the overall body size of shrews. Surprisingly, in three out of five shrew species, the CBL was negatively correlated with latitude, and the same trend, although not statistically significant, was found in the fourth species. In general, shrews were smaller in colder areas, as evidenced by the positive correlations between the CBL and temperature. In two species, these positive correlations appeared when the effect of longitude was held constant in the partial correlation analysis. Characteristically, the strongest negative correlation with latitude and positive with temperatures was found in S. minutus , the smallest species under study. Shrews were in general larger in environments with high actual evapotranspiration. Body mass reviewed in S. araneus paralleled the pattern found in the CBL variation in this species, i.e. it decreased northward, both in summer-and winter-caught animals. In addition, contrary to the widely acceptedbut not rigorously testedbelief, body mass recession from summer to winter (the Dehnel Effect) did not correlate with latitude. We concluded that shrews followed the converse to Bergmann's rule, and hypothesize that part of their body size variation along the west-east axis may be explained by character displacement. We also hypothesize that scarcity of food, especially in winter, is a major factor selecting for small body size in shrews in northern areas, as smaller individuals should require less food.

Bergmann’s rule for Neomys fodiens in the middle of the distribution range

“…Shrews, as the smallest terrestrial mammals, are good examples of an exception to Bergmann's rule. Five out of six studied Sorex species, the common shrew (S. araneus), the pygmy shrew (S. minutus), the tundra shrew (S. tundrensis), the Laxmann's (masked) shrew (S. caecutiens) and the masked shrew (S. cinnereus) do exhibit size depression in higher latitudes, while the taiga shrew (S. isodon) does not [3,4]. Not conforming to Bergmann's rule, both N. fodiens and N. anomalus are larger in the southern parts of the range.…”

“…In this paper we do not pretend to analyze factors other than geographical position on the south-north axis, i.e., latitude. However, we concede that not all published data may be directly comparable, as some authors used direct measurements such as body weight or condylobasal length of the skull, while others used residuals or corrected measurements ("controlled" according to [4]) or principal components [3,5,10,[20][21][22][23]. Shrews, as the smallest terrestrial mammals, are good examples of an exception to Bergmann's rule.…”

“…To minimize dispersion related to collection month, we did not use braincase depth measurement as it varies depending on the season [3]. While body weight …”

“…This has been investigated in several species of g. Sorex [3,4]. For N. fodiens, no significant latitudinal size trend was found between 44° N and 53° N [5].…”

Trophic cascades, invasive species and body-size hierarchies interactively modulate climate change responses of ecotonal temperate–boreal forest

“…However, an exception to this trend occurs in some taxa of belowground fauna, which get larger in warmer climates; this includes annelids that dominate soil processes in large parts of the world, with small enchytraeid worms in the boreal forest and progressively larger earthworm species found from southern boreal forests through temperate and tropical forests [4,5]. This review and synthesis examines why body sizes of dominant above-and below-ground fauna responsible for important ecosystem processes in the temperateboreal forest ecotone are expected to change in opposite directions as the climate warms.…”