XIX.5.6 In some cases, the host actively helps to enhance the pathogenic manifestations of parasitization
In a great many cases, the pathogenic manifestations of parasitization tend to be caused by the defense mechanism of the host rather than the actual parasite activity.It is no exception for a host to die as a result of hyperactivity or autoreactivity of its immune system, while individuals with partial immunosuppression overcome the infection without difficulties.In most cases, it is apparently only a matter of failure of the relevant defense mechanisms, which are optimized for defense against certain species of parasites and that function disproportionately and counter-productively in defense against other parasites.However, it is also possible that, in at least some cases, this is an evolutionary adaptation on the part of the host, permitting elimination of infected individuals from the population and thus reducing the potential for spreading the parasite.
It is evident that a similar ability to “commit suicide” can emerge only through group orspecies or alsokin selection.If an infected individual were occasionally capable under normal conditions of recovering and reproducing, then the strength of individual selection acting against the emergence of suicidal behavior would be so strong that the probability of its evolutionary formation would seem negligible.However, in some situations, the conditions for the formation of similar behavior are far more favorable.For example, populations of butterflies bound to food coming from a rare plant survive at a single site for a long time, so that the individuals are very closely related.In this case, a caterpillar can increase its inclusive fitness if it commits suicide following attack by a parasite or parasitoid, e.g. in that it would let itself be caught by a bird (Trail 1980).This kind of behavior has actually been observed for caterpillars of the butterfly Harris' Checkerspot (Chlosyne harrisii).
In eusocial insects, the nonsexual casts do not participate at all in reproduction and express all their fitness in assisting sexual individuals.Here cases are also known that can be interpreted as voluntary suicide of parasitized individuals.Amongst bumble bees of the Bombus genus, individuals infected by parasitic flies of the Conopidae family stay out of the nest, both reducing the probability of transmission of the infection inside the nest and also increasing the probability that they will die (Poulin 1992; Muller & Schmidhempel 1992).However, according to some authors, the lower temperature outside the nest retards the development of the parasite and thus prolongs the survival time of the infected individual; for details, see (Poulin 1995a).