Abstract

Climatic gradients impose clinal selection on animal ecological and physiological
performance, often promoting geographic body size clines. Bergmann's rule predicts
that body size increases with decreasing environmental temperatures given the need
to retain body-heat through adjustments of body-mass-to-surface-area ratio. This
prediction generally holds for endotherms, but remains controversial for ectotherms. An
alternative interpretation, the 'resource rule', suggests that food abundance, primary
productivity and precipitation (which, unlike temperature, do not necessarily correlate
with geography), drive body size clines. We investigate geographic variation in body
size within 65 species of lizards and snakes (squamates) based on an intercontinental
dataset (6,500+ specimens belonging to 56 Israeli species, and multiple populations of
nine Liolaemus species from Argentina and Chile). Bergmann's rule is only rarely
supported by our data (in four species, 6%), whereas six species (9%) follow its
converse (hence, it is unsupported in 94% of cases). Similarly, size increases with
resource abundance in only 12 species (18%). Therefore, although neither of the rules
is supported, factors suggested by the resource rule are better predictors of body size
than temperature. Surprisingly, we show that some measures of the extent of a
species' climatic envelope do not affect the likelihood of it showing a size-climate
relationship. We conclude that negative size-temperature associations are an
exception rather than a generality among squamates.