Who’s chasing whom?

Originally published 6 May 1991

Recent­ly, there has been a spate of let­ters to Ann Lan­ders from moms con­cerned about sex­u­al­ly aggres­sive girls pur­su­ing their teenaged sons — lit­tle temptress­es hang­ing out on the front stoop, or call­ing in the mid­dle of the night, that sort of thing. Don’t these girls know, the moth­ers ask, that it’s a boy’s role to do the chasing?

Ah yes, the boy is the chas­er, the girl the chasee.

Back when I was a teen there was a song that went “A guy chas­es a girl until she catch­es him.” Even then the female of the species was not entire­ly pas­sive. And now comes a bit of research on sperms and eggs that sug­gests the song’s theme might also apply on the cel­lu­lar level.

A team of biol­o­gists from the Unit­ed States and Israel have dis­cov­ered that the human egg does not wait pas­sive­ly in the fol­li­cle of the female repro­duc­tive sys­tem for a fer­til­iz­ing sperm to arrive. Rather, the egg sends out a chem­i­cal attrac­tor that draws the sperm.

And there is more. But first…

Among the crea­tures of Earth, nature has con­trived an aston­ish­ing vari­ety of ways to bring sperm and egg togeth­er, and any­one inter­est­ed in the sub­ject could do no bet­ter than read Adri­an Forsyth’s delight­ful A Nat­ur­al His­to­ry of Sex.

Millions of years of evolution

But it’s humans we are inter­est­ed in here. The basic plumb­ing of human sex we have inher­it­ed from mil­lions of years of mam­malian and pri­mate evo­lu­tion. And biol­o­gists tell us that all of that machin­ery has but one pur­pose: Get­ting the best and strongest sperm to the egg.

Why, for exam­ple, do human females opt for inter­nal fer­til­iza­tion? Why don’t they just eject their eggs into the open, like frogs? Biol­o­gists have an answer: Inter­nal fer­til­iza­tion gives the female more con­trol over who does the fer­til­iz­ing of the egg. Not just any male, but the male with the fittest genes.

Before we go any fur­ther, please under­stand that the pre­vi­ous para­graph is full of short­hand. Ani­mals (includ­ing humans) don’t “opt” for a repro­duc­tive strat­e­gy. And con­scious “con­trol” nev­er enters into it. That’s just the way biol­o­gists talk.

Accord­ing to the cur­rent Dar­win­ian doc­trine, what’s real­ly hap­pen­ing is this: When­ev­er life repro­duces, the genet­ic mate­r­i­al (DNA) is copied. Occa­sion­al errors occur in the copy­ing process (muta­tions). Errors that pro­duce fit­ter off­spring (bet­ter able to sur­vive and repro­duce) are pre­served among the genes.

And that’s why female humans have evolved those long dark pas­sages to be nav­i­gat­ed by sperms: Only the most vig­or­ous sperms will thrash their way to the wait­ing egg.

But as the long dark pas­sages evolved, pre­sum­ably so did penis­es. A penis cuts down the dis­tance a sperm has to swim. The male who puts his sperm clos­est to the egg will more like­ly have his genes repro­duced, a sub­tle evo­lu­tion­ary pres­sure toward longer penises.

A penis, how­ev­er, can also be a dis­ad­van­tage, par­tic­u­lar­ly before our ances­tors devised loin­cloths and Jock­ey shorts. As Adri­an Forsyth points out, a dan­gling appendage is at risk from thorns, snags and net­tles: “The risk of dam­age and trans­port prob­lems are undoubt­ed­ly why most species that have penis­es have evolved erec­tions that enlarge the penis only when it is need­ed for insem­i­na­tion or display.”

The com­bined length of the vagi­na, uterus, and the low­er por­tion of the fal­lop­i­an tube, where most fer­til­iza­tion occurs, is about twelve inch­es. Com­pared to the size of the sperm, that’s a human jour­ney of about six miles, a long way for a sperm to swim. So the male ejac­u­late is loaded with chem­i­cals to help sperms along — hor­mones, for exam­ple, that induce mus­cu­lar con­trac­tions in the uterus that move sperms for­ward. A pres­sur­ized ejec­tion sys­tem gives the sperms anoth­er edge.

In a typ­i­cal ejac­u­late, as many as 300,000 sperms start out on the jour­ney toward the egg, but only a few hun­dred make it. The first sperm to arrive dis­solves its way into the egg, and instant­ly the egg changes its out­er lay­er to bar entry to the run­ner-ups. To the win­ner of the race goes the genet­ic future.

Not so perfect

All of this incred­i­ble machin­ery can be account­ed for by nat­ur­al selec­tion, but don’t get the idea that it was inevitable, or opti­mal. Noth­ing in nature is per­fect. Stephen Jay Gould has called life “a quirky mass of imper­fec­tions, work­ing well enough (often admirably); a jury-rigged set of adap­ta­tions built of curi­ous parts made avail­able by past his­to­ries in dif­fer­ent contexts.”

The very quirk­i­ness of life is tak­en by some biol­o­gists as proof of evo­lu­tion (with its gen­er­ous ele­ment of chance). Human sex is about as quirky as any­thing else. And, with the recent­ly announced dis­cov­ery of a sperm attrac­tor, it’s look­ing quirki­er all the time.

The egg, it now appears, is no pas­sive par­tic­i­pant in the fer­til­iza­tion process. It beck­ons the sperm for­ward, lay­ing down a chem­i­cal path that leads the sperm upstream. There’s anoth­er spin to the sto­ry: Appar­ent­ly, only the most active sperms are attract­ed. The egg’s chem­i­cal attrac­tor is like a beck­on­ing call that only the sharpest ears are able to hear. This may be one more way the egg selects the best and strongest sperm for fertilization.

Like the old song says, the sperm chas­es the egg until she catch­es him.