After death, male guppies can keep on siring offspring because females store sperm for so long. As a result, a living male in a stream in Trinidad can end up competing with long-gone fish from his grandfather’s generation.
At its most posthumously successful, stored ghost sperm sired about one in four of the offspring among wild guppies released into a stream, evolutionary biologist Andrés López-Sepulcre of École Normale Supérieure in Paris and his colleagues report June 5 in Proceedings of the Royal Society B.
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Biologists have long known that female Poecilia reticulata guppies store sperm. The cells clump in little pockets in a female’s ovarian cavity and feed on sugars released by ovarian tissue. Storage in itself isn’t unusual, López-Sepulcre says. Some crabs, turtles, lizards, bats and other creatures preserve sperm for later use.
Posthumous reproduction by stored sperm also isn’t unheard of. “The fun part of our study,” López-Sepulcre says, “is that you have males who are alive and males who are dead competing with each other.”
Researchers deployed guppies in several streams as part of a study on evolutionary change. Every month researchers catch, check and release as many fish as possible to track deaths and births. They also genetically analyze parenthood of the fish. Female guppies give live birth to broods of two to about 10 youngsters, with not all sired by the same male. Females live about 15 months; males about three.
Genetic testing revealed that guppies fathered offspring up to two generations after they died. Their share of offspring increased to about a quarter for months eight through 10, the final months of data available so far. The researchers continue to track the population.
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It’s unclear why female guppies in this stream rely so much on old sperm, says Tim Birkhead of the University of Sheffield in England. “Sperm of most species — the social insects are an exception — clearly deteriorate with time,” he says. Since the stream still has living males, females could presumably remate.
Females’ reliance on old sperm might turn out to be a stage in the process of settling into a new home, speculates evolutionary biologist Kelly Zamudio of Cornell University, who has studied posthumous reproduction in lizards. Using a wide variety of sperm, even from dead males, might give offspring of a small group of settlers a better chance of a genetic mix that suits their new home. Zamudio is curious about whether guppies will still use so much old sperm when they’re no longer new in the neighborhood.