Postprint version. Published in Integrative and Comparative Biology, Volume 50, Issue 1, July 1, 2010, pages 6-16.
The definitive version is available at https://doi.org/10.1093/icb/icq015.
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.