I remember head south to Lancefield in Victoria 6 or so years ago and being
amazed at the size of magpies when compared to the ones I see around the
Blue Mountains. Seems to work for me. Maybe its not applicable to
everything, but it is a 'rule' no a 'law.
Cheers
Graham Turner
Bergmann's Rule
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In zoology, Bergmann's rule is an ecogeographic rule that correlates
latitude with body mass in animals.[1] Broadly it asserts that within a
species the body mass increases with latitude and colder climate, or that
within closely related species that differ only in relation to size that one
would expect the larger species to be found at the higher latitude. The rule
is named after a nineteenth-century German biologist, Christian Bergmann,
who first formulated the rule in 1847. The rule is often applied only to
mammals and birds (endotherms), but some researchers have also found
evidence for the rule in studies of ectothermic species.[2] Bergmann's rule
is controversial amongst researchers and its validity has often been called
into question,[3] and there is division amongst scientists about whether the
rule should be interpreted to within species variation or among species
variation.[1] Although several mechanisms have been suggested to explain the
rule there is no clear explanation for why the pattern exists.
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