Ecologists uncover links between fever and living fast, dying young

Fever is an effective defence against disease, but new research suggests that not all animals use it when exposed to infection.

The study, published online in the British Ecological Society's journal Functional Ecology, found large differences in fever responses among closely related species of mice and suggests that an animal's reproductive strategy could explain some of this intriguing variation.

Although often treated in the medical world as part of disease, fever is an essential way for many organisms to prevent or eradicate infections. But even though fever is an effective strategy for animals to protect themselves against pathogens, fever varies a great deal within and between species, and scientists are unsure why immune defences vary so much when evolution should strongly favour animals with the strongest immune systems.

Part of the reason for this variation seems to lie in the fact that fever is expensive and therefore involves trade-offs with other physiological processes. According to the lead author Professor Lynn Martin of the University of South Florida: “Fever is very costly energetically. A 1oC rise in body temperature in a warm-blooded animal requires around a 10% increase in metabolic rate.”

Martin's hypothesis is that an animal's reproductive strategy could hold important clues as to why some species favour fever more than others. In particular, he wondered whether animals that “live fast, die young” might invest fewer resources in mounting fevers to combat infections because their life history involves putting their resources into breeding quickly rather than living to a ripe old age.

He tested this hypothesis by examining fever in five species of mice of the genus Peromyscus with different reproductive strategies – some being fast living and others slow. According to Martin: “We found that fever was used by fast-living but not slow-living mice, which was surprising because we expected that fast-living mice would be unable to invest heavily in fever given their already large investments in reproductive effort. The fact that fever is robust in some species but absent in their close relatives is probably because of a combination of factors. Fever, like a lot of other immune defences, is broadly effective but not as targeted as other immune defences; this characteristic can often cause damage to host tissues, not just the parasites. Thus, fever may be a viable defence mechanism for only those organisms that can afford its high costs in terms of calories and self-destruction.”

As well as shedding new light on the interplay between animal physiology and life history, the study also may provide some perspective on human and animal health research. Rodents are the model of choice for understanding the vertebrate immune system, but this study reveals that even closely related mice use fever very differently. How might the use of fast-living rodents in biomedical research affect understanding of slow-living animals' immune systems" Professor Martin's next steps are to identify the molecular and hormonal pathways that mediate fever differences among species and determine the consequences of the various defence strategies for fitness.