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Data illustrating fever’s beneficial effects originate from several sources. Studies of the phylogeny of fever have shown the response to be widespread within the animal kingdom.^[165] With few exceptions, mammals, reptiles, amphibians, and fish, as well as several invertebrate species, have been shown to elevate the core temperature in response to a challenge with microorganisms or other known pyrogens ( Fig. 47–6 ). It has been assumed, although not established conclusively, that such elevations in temperature are the poikilothermic corollary of fever. The prevalence of such “febrile responses” has been offered as some of the strongest evidence that fever is an adaptive response, on the basis of the argument that the metabolically expensive increase in body temperature that accompanies the febrile response would not have evolved and been so faithfully preserved in the animal kingdom unless fever had some net benefit to the host.