Fever in Invertebrates
Fever is not a phenomenon unique to mammals. Febrile responses have been observed across a wide range of phyla, from organisms as ancestral as insects and crustaceans. Although many invertebrates are ectotherms and must thermoregulate through behavior, many of the same mechanisms and responses of fever are conserved through more derived species. For example, the TLRs found in humans were first discovered in fruit flies (3), and injections of prostaglandin E, which is essential for mammalian febrile response, has been shown to result in crayfish preference for increased temperatures (14).
A trial examining the thermoregulatory response of Senegalese grasshoppers to exposure to fungal infection showed that grasshoppers are capable of regulating their internal temperatures through behavior (15). Grasshoppers treated with the fungal spores exhibited a greater frequency of higher body temperatures than untreated grasshoppers over the course of 6 days. This increase in temperature appears to have been regulated by behavioral mechanisms such as basking or aligning with the sun. The grasshoppers usually switched between high and low temperatures so as to not overheat, a characteristic of behavioral thermoregulation which appears to be common in ectotherms.
The results of the study did not, however, show an increase in survival to the host insects. The authors of the study proposed that this might be due to the fact that the behavioral fever might serve more to restrict pathogenesis within the organism due to the shorter exposure times (rather than the continuous raised temperature that would be found in endotherm fever). Though not beneficial to the individual, this could still potentially allow an extended period for possible reproduction, thereby increasing the individual’s fitness. This hypothesis was reinforced by a separate study on the effect of fungal pathogen infection of desert locusts, in which it was found that only infected locusts able to thermoregulate freely and induce a febrile response were able to reproduce, even though all infected locusts died regardless of their ability to raise their body temperatures (12).
Honeybees use a slightly different behavioral mechanism to induce fever, although the basic principle remains the same. Rather than basking in the sun or other warm areas, honeybees will, as a colony, increase the temperature of their honeycombs when exposed to certain pathogens infecting the bee larvae (16). One proposed mechanism for doing this is by increasing the activity of their flight muscles (in a similar manner to bumblebees preparing for flight in cold weather). One particularly striking aspect of honeybee thermoregulation, however, is that the response is participatory on the scale of the colony rather than that of an individual. This further supports the argument for fever as adaptively valuable, as it is not only found as a trait in individuals but is also expressed on a behavioral and social level, reinforcing claims to its beneficence to maintaining a population’s fitness.
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Phylogenetic Conservation of Febrile Response
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