That time is now.
In many species of animals, males and females have a conflict of evolutionary interests. Males compete with each other for the opportunity to fertilize the eggs of females. Males use all sorts of strategies in these competitions. They fight with each other for territory, they scare off intruding males, they put scrapers into females to dump out the sperm from previous males, and they inject “anti-aphrodiasiacs” to make females unreceptive to other males.
A number of experiments suggest that females have to pay a steep price for these male shenanigans. Anti-aphrodisiacs are toxic to the females, shortening their lifetime. Why would males harm the females that carry their offspring? In many species, males can mate with many females. The long-term health of any one female doesn’t matter–in an evolutionary sense–to the male.
As natural selection favors increasingly deadly male mating strategies, this onslaught opens up the opportunity, in turn, for the evolution of counterstrategies in females. In some species, females may evolve antidotes to male poisons. The males, in turn, may evolve counterattacks to overcome these new defenses. Theoretically, this coevolution can become a never-ending cycle of sexual conflict, capable of producing some of nature’s greatest extravagances (like absurdly kinky ducks).
Up till now, the best evidence for this kind of sexual conflict came from experiments. Scientists manipulated Drosophila flies so that the males were free to evolve while the females couldn’t. The result: the lifespan of the females got shorter and shorter over the course of generations. In a flipped version of the experiment, scientists prevented males from mating with lots of females, as they normally do. Instead, the male flies were forced into monogamy. Now there was no evolutionary reward for competing with other males. Over time, the male fly toxins got less toxic, and the females lost their defenses.
Now Nicolas Rode of the the Center for Functional and Evolutionary Ecology in Montpellier, France, and his colleagues have found a new way testing this hypothesis: by having males travel through time to mate with females.
The time-traveling males in this case are brine shrimp (a k a sea monkeys). Brine shrimp produce tough eggs that can survive through droughts for years and then hatch into healthy young when water returns. In the Great Salt Lake in Utah, the brine shrimp egg cysts form layers on the lake bed going back decades. Rode and his colleagues gathered cysts from layers that formed in 1985, 1996, and 2007. They brought the cysts back to their lab and reared the sea monkeys. And then they orchestrated some sea monkey sex. They had females mate with males from their own time, as well as from the other years. For example, females from 1996 could mate with males from 2007 and 1985.
If sexual conflict is an ongoing evolutionary process, you’d expect females to fare differently with males from different time periods. They’d be best-adapted to the males of their own time, and worse adapted to males from other times. Evolutionary theorists have developed two different models for how this time-traveling sex would play out. It’s possible that males and females escalate their adaptations over time in an evolutionary arms race. It’s also possible that evolution moves more like a merry-go-round. For a while, one male strategy may dominate, and one female counterstrategy dominates as well. But then a new male strategy pops up, for which the females have no defense at all. That male strategy then rises to dominance, and a corresponding female counterstrategy eventually evolves as well.
Rode and his colleagues tracked the females, noting how many eggs they had and how long they lived. And they discovered, as predicted, that having sex with males from another time is bad for a sea monkey’s health. The further away in time the sea monkeys were, the sooner the female sea monkey died. When the male traveled 22 years to mate with a female, her life was cut short on average by 12%.
There are lots of caveats to this study–which you’d expect for the first study of its kind. The results weren’t clear enough for the scientists to pick the arms race or merry-go-round model as the best explanation for the conflict between the sexes. And over the entire lifetime of the female sea monkeys, time-shifting didn’t have a measurable effect on their reproductive success. That’s because they females who were dying faster also produced eggs at a faster rate.
Another mystery is how the time-traveling males are harming the females. Rode and his colleagues note one unusual aspect of brine shrimp sex: males and females can stay clasped together for hours–even days. They’re not cuddling in some erotic afterglow. Studies on other species suggest that the males are holding on tight and the females would prefer to get on with their lives. Amplexus, as this embrace is known, may be yet another way for males to outcompete their rivals. By holding on tight to females, they can prevent their mates from finding other males.
Females pay a price for this guarding; it can make them easier targets for predators and prevent them from eating. Scientists have found that female water-striders have evolved lots of acrobatic moves to get clasping males off of them. It’s possible that sea monkeys engage in a similar sexual wrestling match, and that the male and female moves evolve over time.
Whatever the answer to these questions, one thing seems fairly clear. If you’re a female sea monkey, and you see a blinding flash of light, and a male sea monkey suddenly appears saying he’s got to protect you from an army of sea monkey robots from the future–take care. Your well-being is definitely at risk.
Reference: Nicolas O. Rode, Anne Charmantier, Thomas Lenormand. Male-female coevolution in the wild: evidence from a time series in Artemia franciscana” Evolution: in press. DOI: 10.1111/j.1558-5646.2011.01384.x
[Updated to clarify the nature of sea monkey time travel and correct Marseilles to Montpellier]