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 transmitted parasites to force their hosts to have sex more. But biologists have found very few examples of this in nature. A new study may explain why.

To figure out why there isn’t more “sexual mind control” in nature, theoretical ecologist Ludek Berec, of the Biology Centre of the Academy of Sciences of the Czech Republic, 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 did not make its hosts have sex more, 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.

If the mutant strain replaced its ancestor, the researchers introduced a new mutant that had even more power over its host’s sex life. They then watched the two strains compete again, introduced yet another, stronger mutant when the old one outcompeted its predecessor, and so on. In this way, the species as a whole could “evolve” to exert more or less sexual mind control over its host.

In most simulations, the mutants did not evolve toward making their hosts have more sex, the team reports in the 7 February issue of the Journal of Theoretical Biology. Instead, they either lost the ability to influence their host’s mating behavior or they died out. The researchers speculate that sexual mind control either costs too much energy for the parasite, causes too much harm to the host, or both. For example, the host might become so focused on sex that it doesn't spend enough time searching for food and water. Furthermore, for a parasite to alter its host's sex drive, it might have to pour a lot of its energy into pumping out powerful hormones, which could weaken it in the long term.

The findings will help epidemiologists better understand how sexually transmitted diseases spread—not only parasites visible to the naked eye, but also bacteria, viruses, and other microscopic invaders, says evolutionary biologist Patrick Abbot, of Vanderbilt University in Nashville, who was not involved with the work. Describing the characteristics of mating-enhancing parasites and their host populations may also help field biologists find more of such parasites in the real world, he notes.