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60-Second Science

Sperm Supply Tied to Competition

Studies in multiple species show that males may adjust sperm output when faced with romantic rivals, and females may seek more partners if males skimp. Karen Hopkin reports

[The following is an exact transcript of this podcast.]

If you paid attention during high school biology, you probably remember that girls are born with all the eggs they’ll ever have, whereas guys are churning out sperm pretty much all the time. Now, several new studies, in animals from mice to worms, suggest that sperm supply is closely tied to demand. And that how much sperm an individual male makes may influence whether females seek multiple mates.

One study, published by the Royal Society, showed that mice make more sperm when they compete with romantic rivals. That finding is the first demonstration that a male mammal can adjust his sperm output depending on social circumstance, in this case, making more sperm to better the odds that he’ll come out on top in a reproductive show-down.

Worms take the whole supply-and-demand thing to the extreme. According to a study published in the journal Current Biology, when females aren’t around, one type of nematode worm doesn’t make any sperm at all.

That kind of abstinence may work for a nematode, but for other species being stingy with sperm may backfire. According to a paper published in Science, when male fruit flies skimp on the sperm count, females simply mate with more partners. Something male flies might keep in mind if they want to avoid heartbreak this holiday season.

—Karen Hopkin 

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