Game of Cojones: The microscopic drama of sperm

One chilly autumn evening two years ago, I was sitting in a college dorm room with a friend who was stoned out of his gourd, and blew his mind by talking to him about sperm.  

One of my favorite perks of working in science (and in particular, evolutionary biology) is that just by staying on top of the literature, you amass a wealth of crazy stories about the unexpected workings of the world: evolutionary wars raging unseen, all around us, all the time — tales that would blow anyone’s mind. But, as I learned quickly in college, they work particularly well on the totally baked.

That autumn night, I was telling my friend about the battles waged, sacrifices made, and alliances forged by individual sperm cells as they race by the millions to reach what I assume, from a sperm’s perspective, is the Holy Grail of all existence: an unfertilized egg. This microscopic drama was uncovered by Heidi Fisher and Hopi Hoekstra at Harvard University, and published in the journal Nature in 2010.

Just like human beings, every sperm cell on Earth is unique. Sperm are produced through a process called “meiosis,” a specialized cell division that scrambles the DNA in the cell to produce combinations of genes that have never existed before in all of history. And like human beings, sperm cells have brothers: any two sperm produced by the same male share 50% of their genes, as do James and Dave Franco or any two siblings.

What Fisher and Hoekstra discovered was that ALSO like human beings, sperm cells know who their brothers are, and cooperate with their family in the face of competition. The researchers were studying a species of wild mouse called the deer mouse, which has a promiscuous lifestyle. (This is technical terminology. Few realize that Nelly Furtado’s music video was actually an interpretive biology master’s thesis.)


The adorable, highly promiscuous deer mouse (Peromyscus maniculatus).


Anyway — females of this promiscuous species often mate with several males in a short time period. This means if a sperm cell finds itself inside a deer mouse’s fallopian tube, it’s not unlikely that the place is abuzz with not just his own brothers (sperm cells from the same male) but also STRANGER sperm. And to add to the little guy’s anxiety, all of the millions of sperm cells around him are trying to beat him out to get to the only thing he’s ever wanted in his life, the egg. Only one sperm can win — and only the winning sperm’s genes will get passed down to the next generation, to make new sperm for years to come.

You might think that, under this acute pressure, each sperm would adopt an every-man-for-himself attitude. The best strategy is to put your head down, ignore your competitors, and swim as fast your little spermy tail lets you, to make sure YOU, and no one else, get there first. Right?

Wrong. Faced with this intense competition, the sperm cells cooperate. Deer mouse sperm each have a tiny hook on their head (if you ask me it looks a bit like an emo hair flip). Using this built-in tool, they find other sperm cells and hook onto each other, forming a little clump with all of their tails facing the same direction. Attaching to each other in this way helps all of them swim faster than they would alone. (If you’re interested in the physics of this, check out an equally fascinating paper that Fisher, Hoekstra, and others released a few years later.) By cooperating, they increase the chance that one of them will reach the egg in time.


Left, deer mouse sperm “cooperating” to swim faster.
Right, close-up of a sperm hook. No, that’s not Davey Havok from AFI.
(Source: James Weaver / Wyss Institute)


The idea that sperm can cooperate is cool in and of itself. But there’s more! Sperm are cliquey. Our little hero knows that, while most of the sperm around him are strangers, some are his brothers — sperm from the same male. Fisher and Hoekstra showed that individual sperm cells preferentially cooperate with their brothers.

That is astounding. To a human eye, all deer mouse sperm look identical. How does a sperm cell discern which ones are related to him? Your guess is as good as anyone’s. And it doesn’t stop at brothers: Fisher and Hoekstra discovered that individual sperm cells can even recognize more distant relatives — say, cousins — and preferentially cooperate with them, as well.

It’s a mystery how a sperm cell can possibly distinguish a sibling from a stranger, but to an evolutionary biologist, it’s no mystery why. From an evolutionary perspective, our hero benefits — selfishly! — from helping his brother. If a stranger wins the race to the egg, none of our hero’s genes are passed to the next generation. But brother sperm share 50% of their genes — so if our hero helps his brother win the race, 50% of his genes will successfully be passed down. (Or 25%, in the case of a cousin.) Not the Holy Grail, but much better than the alternative.

It’s a mystery how a sperm cell can possibly distinguish a sibling from a stranger, but to an evolutionary biologist, it’s no mystery why.

Evolutionary biologists had known that this would theoretically be the case — if a sperm could tell his sibling from a stranger. But until this study was published, that would have seemed crazy. Fisher and Hoekstra demonstrated experimentally that sperm can detect their relatives, and they act on it exactly as the math predicts! I love stories like this, because they remind me that when evolutionary pressure is applied, nothing is impossible.

To me — and to my stoned friend, whose universe I had successfully exploded at this point — this story is exciting because it’s just one example of the millions of microscopic biological wars roiling beneath our radar. Who knows how many others remain unseen, just waiting for the right experiments — the right scientists — to unearth them?

Or as my friend summed it up: “Dude, whoa. Evolution is awesome.”

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