Matters of Size

Size does matter. At least that’s what the tagline of the movie Godzilla claims (one of the many renditions of the franchise). A quick glance at the great beasts of our past—dinosaurs, marine animals, snakes, why even disturbingly massive dragonflies, scorpions and camels—would make it appear as if increased body size conferred some sort of advantage then. Yet biologists concur that overall, the body sizes of most species on Earth have been subjected to rather stringent limitations by factors such as land area, climate change, predation, and adaptability to changing environments (Roy, 2008; Smith et al., 2010). Larger animals typically live longer than smaller ones. Elephants should and will almost always outlive mice. But when one looks closer within a species, a more complex picture emerges. In dogs, for example, smaller breeds such as chihuahuas outlive Great Danes and St. Bernards (Galis et al., 2007), with the sole determinant of size being a single gene—insulin-like growth factor-1 (IGF-1) (Sutter et al., 2007). In fact, there is much evidence to support to the link between reduced body size and organismal longevity. A moderate reduction in cellular signaling pathways such as IGF-1, PI3K, or mTOR signaling results in enhanced resistance against various stresses and extrinsic insults and significantly increases lifespan across a number of species (Narasimhan et al., 2009). Whether this regulation exclusively occurs at the cellular level or whether there are cell nonautonomous mechanisms at play is not entirely clear. The link between cell size and organ size—and by extension organism size—is not necessarily linear. Studies in C. elegans and Drosophila provide evidence for both, the convergence and uncoupling of the pathways controlling cell size and cell proliferation that may then impact organ and/or organism size. While body size scaling and increases in cell size as well as number are important during development, organ and or-