Parasites have a bad reputation—most people find the idea of a creature that secretly mooches off another to be pretty creepy. Lately much has been made of the weird zombie fungus that hijacks the bodies of ants, but parasites influence host behavior in a variety of ways, some more disturbing than others
Many parasites have an elaborate life cycle involving more than one host organism. Given that many parasites cannot survive outside of a host, moving from one host to another can be difficult. Some parasites can be spread passively, e.g. through feces. But many parasites rely on their hosts themselves to facilitate transmission. One classic technique is a real bummer for the host—the parasite makes the host easier for predators to eat. Writing in the Journal of Parasitology, biologist Nadia Carreon and coauthors describe flatworms of the genus Polypocephelus, which infect white shrimp. Once inside a shrimp, the parasite inhabits the nervous system, where it may influence the shrimp’s behavior. To complete its life cycle, the parasite is angling to get inside a ray, which feed on the sea bottom. Under the worm’s influence, a shrimp slows down, spending less time swimming and more time walking on the sea floor where a ray is more likely to eat it. The parasite, therefore, gets what it wants by manipulating the shrimp.
Stephania Fucini at al., writing in Behavioral Ecology and Sociology, describe a different tactic taken by the cuckoo wasp. This wasp, like its namesake bird, lays its eggs in the nests of rival wasps. Then it splits, leaving the other wasp to raise its young. Adults raising the parasitic larvae have to spend more time foraging and less time resting than adults raising their own ordinary larvae. The parasite larvae get more resources at the expense of the adults. Plus, all the time foraging means less opportunity for reproduction. Limiting reproduction limits additional larvae that require adult attention. Unlike many host manipulators, these social parasites, as they are known, do not at any point reside inside a host. Rather than mind control, the usurper larvae apparently use behavioral interactions to manipulate the host adults into running themselves ragged. (Perhaps some human parents can relate.)
While it sounds like science fiction, parasites are extremely widespread. According to ecologists Chelsea L. Wood and Pieter T. J. Johnson, parasites may substantially impact entire ecosystems. By manipulating host behavior, parasites influence how infected individuals interact with each other and may impact how easily an organism invades a new environment. Most significantly, by making predation easier, energy is more easily transmitted from lower to higher trophic levels. In fact, the amount of energy transfer facilitated by parasites may make up such a high percentage of available energy in an ecosystem that without parasites there might actually be fewer predators. Maybe a little mind control isn’t so bad.