650


JOHN D. ORCUTT AND SAMANTHA S. B. HOPKINS


FIGURE 3. Latitudinal body-mass gradients in modern and fossil canids. Land mammal age subdivisions are denoted by abbreviations (Ar, Arikareean; Ba, Barstovian; Cl, Clarendonian; Hh, Hemphillian). Spearman’s rho (Ρ) and p-value are indicated for each relationship.


should be positively correlated with latitude and negatively correlated with temperature. With the exception of only a small number of taxa, this is not the case in this study. In an earlier examination of Bergmann’srule, Mayr (1966) suggested that a biological rule should apply to 50% or more of taxa. The taxa examined here come nowhere close to this benchmark, with 14 out of 19 showing no evidence of directional latitudinal trends in body mass and with only three genera showing the expected positive relationship with latitude. Likewise, only one of the seven taxa compared directly to temperature shows a significant relationship between body mass and climate.


While the slope of such relationships might be expected to vary with time, their near absence within the genera analyzed in this study suggests they are the exception rather than the rule, falsifying the first prediction tested here. The second prediction derived from Bergmann’s rule—that the strength and fre- quency of latitudinal gradients should vary with climate—is likewise unsupported. Of the three genera with significant positive relation- ships between body size and latitude, two (late late Hemphillian Dinohippus and Recent Canis) do occur during cooler intervals, seemingly supporting Bergmann’s rule. However, other taxa from cool intervals show no such


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