Biologists had long assumed that Bergmann's Rule--named after the 19 th century German biologist Christian Bergmann--reflected the adaptation of warm-blooded animals to become larger when they move in colder environments. The reason: Bigger animals have smaller surface to volume ratios and can more effectively conserve heat in cold environments. Similarly, smaller animals with larger surface to volume ratios are better adapted to warmer environments where they can more effectively dissipate heat.
However, this simple relationship doesn't explain why ostracodes and other cold-blooded creatures that do not regulate their internal body temperatures, such as mollusks to turtles, also follow this rule.
"It is a bit of a puzzle why Bergmann's Rule holds in cold-blooded animals like ostracodes," said Hunt.
Hunt and Roy found that as ocean temperatures declined by some 10 degrees centigrade, from 40 million years ago to the present day, the overall size of the deep-sea ostracode Poseidonamicus dramatically increased.
"It's not just that the small species got replaced by a larger species," said Roy. "The same species, the same lineage got bigger over time."
In addition, the biologists discovered that the body size increases in nine species of ostracodes that evolved over that 40 million year span were commensurate with the change expected given how much the ocean temperatures decreased over this time and how body sizes of living ostracodes vary with temperature. On average, for every degree centigrade of climatic cooling, each of the species of Poseidonamicus increased in length by about 29 micrometers.
Hunt and Roy said biologists are uncertain what may be triggering this biological response to larger size from cool environments. Nevertheless, the UCSD study is important because it establishes a firm link between climatic change and the body size of organisms, paving the way for a bet
Source:University of California - San Diego