Postprint version. Published in Integrative and Comparative Biology, Volume 50, Issue 1, July 1, 2010, pages 6-16.
Copyright © 2010 Oxford University Press. This is a pre-copy-editing, author-produced PDF of an article accepted for publication in Integrative and Comparative Biology following peer review.
The definitive version is available at http://dx.doi.org/10.1093/icb/icq015.