Shadows of Forgotten Ancestors

Nucleic acids compete, individual organisms compete, social groups compete, perhaps species compete. But there is also competition on a very different level: Sperm cells compete. In a single human ejaculation there are some 200 million sperm cells, the fittest among them with tails lashing, racing against each other, speeding along at an average clip of five inches per hour, each striving—or so it seems—to be first to reach the egg. A surprising number, though, from normal, fertile males have deformed heads, multiple heads or tails, kinked tails, or are just motionless, dead in the water. Some swim straight, others in convoluted paths that may turn back on themselves. The egg may actually choose among sperm cells. Chemically, it cries out to them, egging them on. Sperm cells are equipped with a sophisticated array of odor receptors, some oddly similar to those in the human nose. When the sperms obediently arrive in the vicinity of the calling egg, they don’t seem to have sense enough to stop swimming and thrashing, and molecules on the eggs surface may cast out a kind of fishing line, hook the sperm, and reel it in. The fertilized egg then promptly establishes a barrier that turns away all future sperm cells who may come blundering in. These modern findings are rather different from the conventional view of the passive egg waiting to be claimed by the champion sperm.¹³

But there is, in an ordinary impregnation, something like one success and 200 million failures. So conception, while controlled to a significant degree by the egg, is still in part the result of a competition among sperm cells for speed, range, trajectory, and target recognition, at least.*

Odds anywhere approaching 200-million-to-1 in every conception, continued once a generation through geological ages, imply an extremely strong selection of sperm. Leaner, more streamlined sperm cells with more swiftly lashing flagellas that can swim straight and that have superior chemical sensors will probably arrive first; but that has very little to do with the characteristics, once grown up, of the individual so conceived. Getting to the egg first with genes for boorishness, say, or stupidity, seems a dubious evolutionary benefit. A great deal of effort would appear to be squandered in natural selection among the sperm cells.¹⁴

But then it seems odd that so many sperm cells are dysfunctional. We do not understand why this should be.

Many other factors affect which sperm succeeds: Who’s conceived must depend on the progress of the egg into the fallopian tubes, the precise moment of ejaculation, the position of the parents, their rhythm of motion, subtle distractions or encouragements, cyclical hormonal and metabolic variables, and so on. At the heart of reproduction and evolution, again we find a surprisingly strong random component.

The monkeys and apes are preeminent among animals where many males mate, one after the other, with the same female. They can hardly contain themselves, jumping up and down with excitement, awaiting their turn. In chimpanzees, as we’ve noted, there may be dozens of copulations in quick succession with an ovulating female. So the act itself cannot be prolonged or rich in nuance. Several pelvic thrusts, roughly one a second, and it’s over. For an average male there’s a copulation maybe once an hour, all the livelong day. For females in estrus it’s much more than that.

In ten or twenty minutes many males may have copulated with the same female. So consider the sperm cells of these various male chimps, racing against one another. Essentially, they set out from the same starting line. The probability of insemination by a given male is proportional to the number of sperm cells delivered, other things being equal; and thus the chimps with the largest number of sperm cells per ejaculation, the chimps able to copulate the most times in succession before exhaustion sets in, have an advantage. Having more sperm cells requires larger testicles. The very large testicles of male chimps amount to about a third of a percent of their entire body weight—twenty or more times the endowment, relatively speaking, of primates who are monogamous or who live in breeding units of one male and several females. In general it is found that males have considerably larger testicles for their body size in species where many males mate with each female. Not only is there selection for testicular volume, but also for an interest in copulation. This may be one of the routes—there are many mutually reinforcing trajectories, as we’ve described—to the highly sexual social proclivities of our primate order. Because men, compared to male chimps, have such relatively small testicles, we might guess that promiscuous societies were uncommon in the immediate human past. But a few million years ago, say, our ancestors may have been substantially more indiscriminate sexually and substantially better endowed.

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