Why Parasites Don't Turn Us Into Sex Zombies

Why Parasites Don't Turn Us Into Sex Zombies
The ultimate human motivator? © MAISANT Ludovic/Hemis/Corbis

Parasites use a host of ingenious and bizarre methods to hijack a host organism, all in order to continue their life cycles. They transform ants into stumbling zombies, make rats crave cat urine and turn mammals into rabid biters.

Yet cases of parasite-induced nymphomania are exceedingly rare in nature. Why is that ? Why don't drooling sex zombies streak naked through the streets, throwing themselves at everyone in sight like something out of an early Cronenberg film?

Math, Ecology and Sex Parasites

In a new study published in the Journal of Theoretical Biology, a biological mathematician and a theoretical ecologist examine this dearth of sexual mind control. As related at Science Now, the duo used mathematical modeling to investigate, creating a simulated "ancestor" parasite that doesn't hijack the host's sex drive and a mutated species that does.

They ran several simulations of this. Sometimes the mutated parasite went extinct, but each time it prevailed, they introduced a new, more-powerful mutated species that wielded even more sexual power. In effect, the simulated parasite could "evolve" into an even more effective sex-booster.

Results of the Study

Their results? Most of the simulations didn't end in the evolved enhancement of sexual mind control. The researchers presented these speculations as to why this would be the case in nature:

  • Sexual hijacking weakens the host: It's possible that heightened focus on sex might prevent the host organism form taking care of more important needs, such as food and water.
  • Sexual hijacking costs too much: Such a sex-boosting parasite might have to expend too much energy into the release of powerful hormones, weakening it overtime.

Naturally, the answer might be a little bit of both. Evolution inevitably favors the more economically-feasible design, and in the end there are far better ways to use a host creature to your parasitic advantage.