One night of passion with a male from New York shortens your life expectancy, at least if you're a worm of the species Caenorhabditis remanei. Although if you are, what are you doing reading this article? Go eat soil bacteria.

University of Oregon graduate student Colin Peden was conducting a study on nematode worms when he noticed something unexpected: mating with a male collected from New York halved the life expectancy of females, compared to those who died without experiencing male company. Being paired with worms from Germany or Ohio reduced female lifespans by 20%.

It wasn't a matter of being exhausted by the hectic lifestyle or tiny apartments either. Just a day in the company of New York C remanei did most of the damage. Early deaths were not a consequence of great fecundity either; New Yorkers only fathered half as many offspring as their co-speciesists.

Head of Peden's lab, Professor Patrick Phillips pondered: How have New York worms survived if they produce fewer offspring? Further experiments revealed the New Yorkers have super sperm, capable of out-competing that of other worms when each mated with the same female. New York females had shorter life expectancies, irrespective of mates.

"Despite their small size, nematode sperm is actually much larger than human sperm, and it is thought that the sperm from different males literally battle it out inside the female for access to her eggs," says Phillips. "So a reasonable evolutionary explanation would be that these males make bad mates but highly successful fathers." The males also produce copulatory plugs that interfere with subsequent matings, although these are less likely to shorten the females' lifespans.

In combination with a team at Bowdoin College, Maine, Phillips tested the related species Caenorhabditis elegans, the model organism that became the first multicellular organism to have its genome sequenced, reporting the findings in BMC Evolutionary Biology. Unlike C. remanei, C. elegans are normally hermaphrodites, although exclusive males exist.

The Bowdoin team blocked the gene for sperm production in C. elegans to make some female, and mated these with the rare males for 60 generations. They found that when the C. elegans were mating competitively in an environment without hermaphrodites their sperm quickly evolved to be larger and more aggressive. Females fertilized with this sperm died earlier than those that bred with males who did not have generations of competitive mating behind them.

Phillips says the researchers still don't know how the males exert their detrimental effects. "It could be a change in the behavior of the males, or it could be something in the seminal fluid that they transfer during mating," Phillips says. "We are following up on this work to figure that out." It is also not known what makes the competition in New York fiercer than that in other locations, although no doubt many singles in the city would could confirm.

The observations are an extreme example of the evolutionary mechanism sexual conflict. In this case the females appear to be collateral damage in a war between males. In other cases harm is inflicted in order to change female behavior.