Fever appears to protect ectotherms against infectious disease perhaps because it increases their aerobic metabolic capacity, which is temperature-dependent. Mammals, however, have a high aerobic capacity and normally regulate a high body temperature. Thus, the further increase in temperature induced by interleukin-1 may be dangerous, and the resulting increase in aerobic capacity may not be necessary for an effective defense. In fact, recent evidence suggests that although the neuroendocrine cold defense responses that are stimulated in fever enhance the defenses of the host, the increase in temperature harms these defenses. Data, however, are scarce and equivocal, and the function of fever in mammals is still uncertain.