The optimum body temperature for warding off fungal infections, without burning too much energy, is 36.7 degrees Celsius (close to the core temperature for mammals), according to researchers. The research supports an emerging theory that fungal organisms may have been a driving force in the evolution of mammals becoming "warm blooded". It also explains why the core temperature of mammals is not even hotter, and why reptiles did not retake the earth after the extinction of the dinosaurs. Fungal diseases plague (no pun) reptiles.
Researchers examined the response to temperature of 4,802 types of fungi. Starting at 30C, every degree increase killed off 6% more of the fungi. The postulated reason mammals did not develop an even higher core temperature is the trade-off between controlling fungi and the need for additional food inputs.
The proposed model does not answer all the questions related to mammalian core temperature, but it would appear to demonstrate that the threat from fungi were an important factor.
The study can be found at http://mbio.asm.org/content/1/5/e00212-10.
An interesting earlier paper on this topic can be found at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WFV-4F60WWV-4&_user=10&_coverDate=02%2F28%2F2005&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1569430678&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=160bdc98ea92c897f69cd7ccd4c14901&searchtype=a