Russell Bonduriansky suddenly found himself with some unusually large flies.
The University of New South Wales evolutionary biologist had mated average-size neriid fly females with either unusually large or small males, and he had expected to see offspring that were larger or smaller than usual. (Read more about flies' preference for semen.)
Except he didn’t—the baby fly sizes were all over the map.
After poring over the data, the only factor that could explain his findings was that the females had been mated before.
Something in the original males' seminal fluid—the fly’s semen minus the sperm—had to be somehow altering the size of the female's subsequent offspring.
Given the importance of sperm, it’s not surprising that scientists have focused much of their attention on these little swimmers. (See "Sperm Tracked in 3-D—A First.")
But Bonduriansky believes that semen is also a major factor in mating and reproduction in a variety of species, from mammals to insects.
In fact, he and his team believe that semen not only has subtle effects on the next generation, but may also play a role in how a female selects her partner.
“No one was really thinking about the role of seminal fluid,” says Bonduriansky, whose study appeared March 3 in Trends in Ecology and Evolution.
More than 95 percent of fly semen is actually seminal fluid, a complex chemical soup of RNA and protein that help sperm survive the female reproductive tract and swim toward the egg.
Given that males of most animal species only produce a relatively small amount of seminal fluid with each ejaculation (humans only produce 3-4 milliliters of semen at a time), most scientists didn’t think that the fluid itself could have a big impact on females. (Find out how a man produces 1,500 sperm a second.)
Nothing could be further from the truth, says Cornell University evolutionary biologist Mariana Wolfner.
“The components in seminal fluid affect egg production and ovulation, as well as sperm storage, feeding and sleeping behavior, and the likelihood that the female would mate with someone else,” says Wolfner, who wasn't involved in the new research.
So how's it work? Bonduriansky believes that the RNA and proteins found in seminal fluid helps to switch various genes on and off in the female, which can affect many of her behaviors, from how many eggs she produces to how much she sleeps.
Such changes may make it more likely that the sperm will fertilize the egg and transform it into healthy offspring.
By changing the physical environment of the female reproductive tract, the seminal fluid acts as a potent factor in determining the fate of subsequent offspring.
Take Bonduriansky’s neriid flies. In the lab, he gave two groups of male neriid flies an under- or over-nutritious diet, which made them get smaller or bigger, respectively. (Also see "Sperm Works Best in the Winter.")
When these males mated, their seminal fluid affected the size of their own offspring, as well as those flies produced by subsequent matings, demonstrating that this fluid was more than just a swimming pool for sperm.
Seeking Male With Good Semen
Since the composition of seminal fluid varies in males of the same species, it's possible that females could also potentially select mates based on the quality of their seminal fluid.
However, there are several unknowns, including what makes seminal fluid high quality, as well as how a female would judge these qualities, Bonduriansky cautions.
Wolfner calls this hypothesis both “interesting” and “important,” although she says without evidence, it's impossible to say that females can select males based on their seminal fluid.
Bonduriansky admits that no one has yet tested this, or whether the quality of a male’s seminal fluid is independent from the quality of his sperm.
His lab is also trying to identify which components of seminal fluid can affect females and future offspring.