The study, also found that sperm from promiscuous mice were more likely to form clumps of the optimum size, and that, when compared with sperm from Peromyscus polionotus a closely-related, but monogamous, species of mouse the trait is likely driven by sexual selection.
The new paper builds on a 2010 study conducted in Hoekstra's lab which found that sperm cells preferentially clump with those produced by the same male. Spurred by that earlier paper, Mahadevan approached Hoekstra with the idea of creating a mathematical model to understand whether and how sperm received an advantage by forming groups.
"I read the paper and thought we could make a quantitative theory of the observations they had made," he said. "But of course, the only way to know whether any model was capable of anything predictive was to make it testable.
"In this contextthe question was: Is it possible to make the aggregate do better than then individual?" he continued. "One way to do that is to get all the tails to synchronize, but that doesn't happen. The other way is to cancel out the random motion of the individuals in an aggregate because the sperm adhere to each other. Eventually, for large aggregates, the sperm point towards each other and thus cannot swim at all. This mechanism when quantified in a model that Luca and I developed and this led to testable predictions., When Heidi did the experiment, we found that this was essentially correct."
In addition to finding that sperm which group together to swim in a more linear fashion, researchers were able to identify at what clump size sperm reaped the largest reward for grouping together. Groups with too few cells, Hoekstra said, continued to swim along more meandering paths, while much larger groups often resulted with sperm swimming against each other.
"What we found is that both species have an optimum at around eight, which was wha
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