Parasites often have an intimate, highly developed relationship with their host that formed over evolutionary time. Both definitive and intermediate hosts display an array of defense mechanisms allowing probenefispective hosts to avoid, eliminate or tolerate parasites, while parasites display ingenious ways to find and exploit their host. Among the most intriguing aspects of parasitism is the modification by parasites of host behavior when the parasite has successfully infected the host. This is usually presented as an adaptation on the part of the parasite to enhance its fitness.
An adaptation is a genetically determined feature that becomes prevalent in a population because it confers a selective advantage to the organisms bearing the feature. In the case of symbiotic organisms, one species may benefit, but not the other. In the case of parasitic organisms, the changes in host behavior are believed to enhance parasite transmission from host to host. Arthropod behavior seems to be readily manipulated by diseases and parasites. For example, caterpillars that are infected with insect - pathogenic viruses often climb to the top of their host plant before they die, where they attach with their prolegs. After they die, their integument ruptures, allowing their liquefying body contents to drip down. Thus, foliage at lower levels of the plant is contaminated with virus, and other caterpillars ingest the virus as they feed. This assists in the spread of the virus from insect to insect. Similarly, flies and grasshoppers infected with fungal disease climb to the top of vegetation and attach, the fungus grows out of the dead insects and produces pores, and the elevated position of the cadaver facilitates the spread of fungal spores by wind or rain. Birds infected with arboviruses often display reduced activities, including reduced preening and other antimosquito behavior, allowing other bird - feeding mosquitoes better opportunity to feed on the infected host, and to acquire the virus for further spread. A classic example of modified host behavior involves the Lancet fluke, Dicrocoelium dendriticum , and Formica ants. When ants are infected, instead of returning to their nest during the evening and cool periods like uninfected ants, they attach to foliage where they can be consumed by grazing animals. Once ingested, the flukes can infect their vertebrate host, completing the parasite’s life cycle.
It has become commonplace to infer that modified behavior in parasitized hosts is an adaptation induced by parasites, but often the evidence is lacking. Rarely is an improvement in the functioning of the parasite actually demonstrated. Usually we can observe only the present state of a character, not its evolutionary history, so caution is needed before drawing conclusions about the adaptive value of a trait. For example, although the tendency of insects that are infected with viruses and fungi to perish in elevated locations is well documented, and logically this behavior will enhance dispersal of the parasites, there is another reason for insects to ascend vegetation. Infected insects will climb vegetation and bask in the sun, elevating their body temperatures to levels that are unsuitable for the parasites that infect them, and optimizing the temperature at which the insect ’ s immune system functions. This phenomenon is called behavioral fever , a form of self - medication that can result in elimination of the parasites due to exposure of ectotherms such as insects to high temperature. Thus, there are multiple explanations for a single behavior, and it pays to be cautious about concluding adaptive significance to behavior without adequate experimental evidence.
