Animals first developed fever as a response to infections: the higher body temperatures primed their immune systems. At the time, 600 million years ago, virtually all animal species were cold-blooded. They had to spend long periods of time in warm areas of their habitat to achieve fever-range body temperatures. Michael Logan, a Tupper Fellow at the Smithsonian Tropical Research Institute in Panama (STRI), believes pathogens may be the reason why warm-blooded creatures first emerged.