Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Parasites can induce various behavioral changes in their hosts.The types of these changes depend on the means of transmission of the particular species of parasite.The greatest amount of information on manipulation by a parasite has been obtained on the parasite-host system where the parasite is transmitted from the prey to a predator, i.e. through predation.This is understandable, because these induced changes are the most conspicuous.Under the influence of the parasite, the host is forced to act against its vital interests, to look for an opportunity to be killed and eaten by a predator.Cases are also known involving manipulation by a parasite transmitted by blood-sucking arthropods.In these cases, the both the behavior of the vector can be manipulated, leading to repeated and multiple blood sucking and also the behavior of the host can be altered, leading the host to stay at locations of greater occurrence of the vector, or to migrate over greater distances.

            It is far more difficult to obtain empirical documentation of manipulation on the part sexually transmitted parasites.While, in cases of parasite transmission by predation, simple comparison of the frequency of infected individuals caught by a predator with the frequency of infected individuals in the normal population can provide a clear evidence for the existence of manipulation; this procedure cannot be used for sexually transmitted parasites.A positive correlation of the occurrence of a parasite with sexual activity or promiscuity of the host individual can, of course, also be a consequence of manipulation by the parasite.However, it is highly probable that the more frequent occurrence of parasites amongst sexually more active individuals is just a secondary consequence of their greater sexual activity and therefore higher risk of infection.

A parasite can also frequently induce types of behavior in a host that do not occur at all in an uninfected individual.An example is the Lancet liver fluke Dicrocoelium dendriticum, which is capable of forcing a parasitized ant to spend many hours attached to a grass leaf by its mandibles, where it can then be eaten by a browsing ruminant, the definitive host of this fluke (Spindler, Zahler, & Loos-Frank 1986).Another example is the parasitic wasp Cotesia glomerata, whose larvae “preprogram” the parasitized caterpillar before leaving its body in such a way that the caterpillar weaves a spider-web cocoon around the pupa of the parasite and defends it, until it itself dies, against any enemies (Brodeur & Vet 1994).

 Another parasitic wasp, Ampulex compressa, employs a specific neurotoxin to prevent cockroaches from spontaneous motion; however, the cockroach is not paralyzed and can thus be lead the wasp to its nest, where the wasp lays eggs on it.Ants parasitized by horsehair wormsMermis(Nematomorpha) look for water after the parasite matures and drown themselves in it.About 10 minutes after they drown, the adult helminths leave their bodies and enter the next phase of their life cycle in an aquatic environment (Maeyama, Terayama, & Matsumoto 1994).