Holling suggested that discontinuities in the body size distributions among
species of animals are a universal feature of terrestrial biomes. We compa
red the magnitudes of body size gaps of mammal communities of North America
and Australia to those generated by a simple random null model. in most bi
omes, no gaps were significantly larger than random, so discontinuities in
body size distributions are the exception, not the rule. We also made intra
- and intercontinental comparisons of size distributions to test two altern
ative hypotheses: (1) Holling's Textural-Discontinuity Hypothesis, that bod
y size distributions reflect structural characteristics of the habitat; and
(2) the Core-Taxa Hypothesis, that body sizes reflect the distributions of
widespread taxa. We found that the gaps in body size were similar in struc
turally dissimilar but adjacent biomes that shared the same or closely rela
ted species. We conclude that body size distributions of biomes are not hig
hly discontinuous, and their structure reflects taxonomic constraints on bo
dy size.