NATASCIA TAMBURELLOFemale ocellated wrasse release eggs into the water to be fertilized, but not all male wrasse make good mates. Some males are dedicated nest-builders that protect offspring, but others fertilize the eggs in another male’s nest and then disappear (so-called sneaker males). Normally, external fertilization limits the female’s ability to choose her mate, but according to a study published this week (August 16) in Nature Communications, the ovarian fluid surrounding a fish’s eggs provides a selective advantage to sperm of nest-building males.

This type of cryptic female selection is well studied among animals with internal fertilization, but unexpected among those with external fertilization. “These new results open up a whole new world of possibilities,” study coauthor Suzanne Alonzo of the University of California, Santa Cruz, said in a statement. “When we think about why marine species look and act the way they do, part of what...

Sneaker males release about four times as many sperm as nesting males, though nesting males’ sperm generally swim faster. Ovarian fluid appears to boost the speed of both types of sperm, increasing the relative importance of speed compared to volume and erasing the numerical advantage that sneaker males have, the researchers reported, noting that the mechanism by which ovarian fluid exerts this effect remains unclear.

“Rather than the default of treating females as being passive participants, we have to remember that evolution is also acting on females,” study coauthor Kelly Stiver of Southern Connecticut State University told The Guardian.

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