Biological size as a predictor of physiological performance and evolution: Evidence from lungless salamanders

Gross physiological performance in multicellular organisms coordinates
processes operating from the cellular to whole-organism level, and thus
may be influenced by physical constraints imposed by both cell size and
body size. A useful measure of joint cell and body size variation is
biological size, defined here as the ratio of body volume to mean cell
volume. We surveyed literature data across four vertebrate clades and
documented wide variation in two proxies for biological size, even between
species of equivalent body size. We evaluated these proxies in a sample of
lungless salamanders (Urodela: Plethodontidae) and used linear and
evolutionary regressions to compare their power to predict variation in
metabolic and water loss physiology to that of body size alone. One proxy
(the ratio of body length to the square root of genome size) consistently
outperformed mass in predicting water loss rates, and residual variation
from evolutionary trends in the other proxy (the ratio of mass to genome
size) consistently explained the most variance across all physiological
responses. These results suggest that for some traits in some taxa,
biological size may be a more functionally relevant predictor of
performance than body size alone. Physiological ecology is replete with
studies characterizing the effects of body size. We suggest that future
studies of size effects additionally consider the role of cell size
variation (and genome size variation as one of its determinants) to
improve our understanding of when and how biological size influences
organismal form and function.