Fever in Vertebrate Ectotherms

Febrile Conservation

Although it could be argued that even mammals such as humans exhibit signs of behavioral fever (application of blankets, huddling in a fetal position), reliance on behavior to thermoregulate is a trait highly characteristic of vertebrate ectotherms such as reptiles and amphibians. Vertebrate ectotherms also make highly practical model organisms to study fever, as their internal temperature must change with their environments, making their body temperature far easier to control experimentally than endotherms would be. They also clearly show the distinction between a regulated change in set point and hyperthermia, as fever will cause them to actively seek out higher temperatures rather than passively adjusting to their external environment (17).

 

Figure adapted from Kluger 1985

Why Study Ectotherms?

Some of the earliest experiments on the effects and mechanisms of fever as an adaptive trait were performed in ectotherms—specifically, desert iguanas. It was shown that given the chance to select between a gradient of different temperatures, iguanas infected with pathogen would choose temperatures that would induce a fever-like response (17). This reaction resulted in increased survival for iguanas with higher body temperatures, while iguanas unable to increase their body temperatures were more likely to die after acute infection.

 

Concerns in Ectotherm Experiments

One concern with studying vertebrate ectotherms for insight as to how fever might play a clinical role in human disease, however, is that there might be significant differences in febrile mechanisms that would render information known about reptiles and amphibians inapplicable to humans (18). There appears to be a fair amount of mechanistic conservation, however, as a study on behavioral fever in toads showed that inhibition of prostaglandin synthesis causes inability to induce a fever in response to LPS infection. As PG is essential to human febrile response, it is likely that many of the same pathways are at least similar in function, once again reinforcing the long phylogenetic history of fever.