Researchers from Syracuse University found that in sperm competition in fruit flies (Drosophila melanogaster) slower and longer sperms outcompete their faster rivals.
The research team led by Stefan Lupold, a post-doctoral researcher in the Department of Biology in the College of Arts and Sciences made the discovery using fruit flies that were genetically altered so that the heads of their sperm glow fluorescent green or red under the microscope.
"Sperm competition is a fundamental biological process throughout the animal kingdom, yet we know very little about how ejaculate traits determine which males win contests," Lupold said in a statement.
"This is the first study that actually measures sperm quality under competitive conditions inside the female, allowing us to distinguish the traits that are important in each of the reproductive phases," he added.
The study was published in the journal Current Biology.
After identifying and isolating groups of males with similar ejaculate traits that remained constant across multiple generations, the scientists mated single females with pairs of males from the different groups.
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"This approach allowed us to simultaneously investigate multiple ejaculate traits and also observe how sperm from one male change behaviour depending upon that of rival sperm," Lupold says.
"The finding that longer sperm were more successful is consistent with earlier studies. However, the finding that slower sperm also have an advantage is counterintuitive," Lupold said.
"It could be that, when swimming back and forth in storage, slower sperm hit the exit less frequently and are therefore less likely to be pushed out," Lupold said.