What makes time-shifting sex hazardous to health is something called antagonistic coevolution, a way that different species (parasites and hosts, for example) or members of the same species (males and females) adapt to each other to promote their own individual reproductive interests. In nature’s sex wars, males campaign for more offspring—the proverbial seed-spreading—while females play hard-to-get because they bear most of the burden of reproduction and parenthood.

Evolutionary biologists say these conflicts are common in nature, and could occur either as an arms race, with each side forever re-upping the other with novel adaptations, or as a series of periodic trends, where the two sides take turns dominating each other over time with the same suite of weapons.

If males and females coevolve their sex organs in tandem, mating with a partner from a different time could leave you unprepared—sort of like heading into modern war with 17th-century armor. The brine shrimp experiment shows just this.