Why “sexual mind control” is rare in nature

Parasites don’t just cause nasty infections; they can also take over the minds of their hosts. The Ophiocordyceps fungus, for example, forces ants to climb up the stems of plants, where they die and release the fungus’s spores into the air to infect more ants. Likewise, it would make sense for sexually controlled parasites to force their hosts to have sex more often. But biologists have found very few examples of this in nature. To figure out why there isn’t more “sexual mind control” in nature, theoretical ecologist Ludek Berec, of the Biology Centre of ASCR and biological mathematician Daniel Maxin, of Valparaiso University in Indiana, turned to mathematical modeling. They created two strains of a hypothetical parasite species: an “ancestor” that didn’t increase the hosts’ sex urge and a mutant that did. Then they turned the two strains loose in a hypothetical host population and watched the parasites compete until the mutant strain either died out or replaced its ancestor. The research indicated the species as a whole could “evolve” to exert more or less sexual mind control over its host.