The Age of Sperm Can Have a Huge Impact on Reproduction, Say Scientists

Not all sperm are created equal.

High school sex ed textbooks all contain some variation on a classic photo: One big, beautiful egg, surrounded by a whole lot of tiny, wriggling sperm. Because all those sperm are simultaneously released in one big burst, we don’t often think about the unique characteristics of each individual swimmer, but as research published Thursday in Evolution Letters shows, some of those sperm are better off than the others.

There’s an advantage, the team of scientists from University of East Anglia and Uppsala University write, for sperm that live a certain length of time, at least for the zebrafish sperm they experimented on in their study. In particular, the sperm that ended up producing the most and healthiest offspring were those that were longer lived — or, put another way, those that are older.

“This is a surprising result, which suggests that it is important to understand how sperm selection may contribute to the fitness of the next generations,” said lead researcher Simone Immler, Ph.D., whose lab at the University of East Anglia’s School of Biological Sciences focuses on the biology of sexual reproduction.

The fertilisation of an egg
The classic "egg meets sperm" diagram suggests all sperm are the same. They're not!

You may be wondering: Aren’t all sperm the same age? It’s a good question, but as Immler and her team write, some sperm are naturally longer-lived than others — much like some people — making them “older” by the time they get to the egg. Sperm, after all, each have their own genetic material, and there’s a vast range of genetic difference between sperm in the same batch, as previous researchers have shown. The sperm that are naturally longer lived, the new study shows, are the sperm that go on to have healthier offspring.

In the experiments, the team took ejaculate from male zebrafish and divided it into two halves. One half went through a 25-second treatment that resulted in a “50% decline in the amount of motile sperm” before it was used to fertilize eggs. In other words, this batch selected for the longer-lived sperm (since all the others were weeded out in the 25 seconds).

The other half of the ejaculate went through a similar treatment but was then immediately used to fertilize the eggs, rather than wait 25 seconds. Since there was no chance for any short-lived sperm to die, “eggs could be fertilized by any sperm capable of fertilization,” as the team writes.

The eggs went on to produce fish babies, and for two years, Immler and her team monitored the health of these offspring. They eventually discovered that “when we select for the longer-lived sperm within the ejaculate of male zebrafish, the resulting offspring is much fitter than their full siblings sired by the shorter-lived sperm of the same male,” as Immler put it.

This is surprising, she says, because previously, scientists never thought it mattered which sperm fertilized the egg. All that mattered was that it could. But as they’ve found, the qualities of the individual sperm do matter — in particular, its lifespan — when it comes to the health of the kids.

For now, it’s not clear why some sperm live longer than others, but the explanation, more likely than not, has to do with their genes. Some sperm might simply be genetically better-equipped to make healthy offspring than others, and over the long course of evolution — many cycles of long-lived sperm leading to healthy kids — this process might have shaped the reproduction of these zebrafish, and possibly other species.

The team is already looking for the genes that lead to longer-lived sperm, paving the way for improved fertilization techniques. For now, we can turn to that classic textbook image with a fresh perspective: Though they may all look the same, each sperm is special — some more so than others.