His hypothesis—that the egg could woo sperm with specific genes and vice versa—is part of a growing realization in biology that the egg is not the submissive, docile cell that scientists long thought it was. Instead, researchers now see the egg as an equal and active player in reproduction, adding layers of evolutionary control and selection to one of the most important processes in life.

“Female reproductive anatomy is more cryptic and difficult to study, but there’s a growing recognition of the female role in fertilization,” said Mollie Manier, an evolutionary biologist at George Washington University.

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The idea of sexual selection is as old as Charles Darwin himself. In On the Origin of Species, he wrote of the peacock’s showy tail and the elk’s giant antlers as examples of traits that evolved to help males show off their appeal as mates to females. For the next century, biologists focused on all the aspects of sexual selection that operated in the events leading up to copulation. After mating, the female had made her choice, and the only competition was among the sperm swimming to the egg.

This male-oriented view of female reproductive biology as largely acquiescent was pervasive, argued Emily Martin, an anthropologist at New York University, in a 1991 paper. “The egg is seen as large and passive. It does not move or journey but passively ‘is transported’ ... along the fallopian tube. In utter contrast, sperm are small, ‘streamlined,’ and invariably active,” she wrote.

Beginning in the 1970s, however, the science began to undermine that stereotype. William Eberhard, now a behavioral ecologist at the Smithsonian Tropical Research Institute, documented all the ways that females can affect which males fertilize their eggs even after mating. It’s a long list, and scientists still can’t say for sure whether they’ve documented everything. The belatedness of these discoveries wasn’t all due to sexism. Two walruses dueling with their tusks is easy to observe; games of hide-and-seek with sperm inside the female reproductive tract are much less so.

“As soon as you have eggs and sperm, you have sexual selection. There are incredible things that eggs and seminal fluid can do,” explained Andrea Pilastro, an evolutionary biologist at the University of Padova in Italy.

In those species in which fertilization happens outside the body, the females often coat their eggs with a thick, protein-rich ovarian fluid. Experiments in 2013 by Matthew Gage of the University of East Anglia in England showed that this fluid contains chemical signals to help attract the correct species of sperm. When they exposed eggs from salmon and trout to mixtures of sperm from both species, the eggs’ own species successfully fertilized 70 percent of the time, significantly more than to be expected by chance.