Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [49]
In insects, however, and maybe in many other animals as well, fertilization is far from the end of the mating story. Many insect females, from butterflies to beetles, mate with more than one male in succession before they lay their eggs. This fact had been well known among biologists, but it wasn't until 1970, when Geoff Parker at the University of Liverpool wrote a landmark paper about what he called "sperm competition," that the consequences of such multiple mating began to be fully considered. Parker pointed out that while male competition for females is more commonly associated with the more flamboyant battles between bull elk or elephant seals, it could still occur after copulation has occurred. The males just continue to vie for the prize of siring offspring via the one-celled messengers of themselves they leave as a consequence of mating: their sperm.
The process would, Parker recognized, lead to different kinds of selection on males. On the one hand, male attributes that allowed their sperm to win at fertilization by circumventing the efforts of other males' sperm would be favored by selection; on the other, males that could prevent a female from mating with another male in the first place would do well because they would avoid the whole problem from the start. Insects are ideal candidates in which to observe such postmating activity because the females of most species mate with more than one male, often in rapid succession, and because in many insects females have specialized organs that serve as holding tanks, keeping the sperm in reserve until it is used to fertilize the eggs hours, days, or even weeks later.
The idea of sperm competition appealed to biologists, most of whom, at least at the time of Parker's insight, were male. Numerous mathematical models about the conditions under which a given male's sperm might be favored were developed, and the details of sperm structure in various species—which turn out to vary enormously, as I will explain later—were examined. But other scientists, including Bill Eberhard, pointed out that this emphasis on male competition missed the other half of the equation: the female. After all, it was the female that did the multiple mating that allowed sperm from more than one male to be in the same place at more or less the same time, and it was the female's body in which all the action occurred. Not to mention that the female too has a stake in which male sires her offspring.
So Eberhard and others suggested that females could influence the likelihood that a given male actually fathered her offspring, even after he had done the deed. This biasing of paternity after copulation is called cryptic female choice, a term originated by Randy Thornhill at the University of New Mexico. It is cryptic because it takes place out of view, inside the female's reproductive tract. Eberhard went further and pointed out that among insects and spiders at least, we should see that females control much of what happens in reproduction, and that we should stop focusing so short sightedly on that moment when sperm meets egg. In true infomercial fashion, we should wait, because there is more. Much, much more. The musician Björk said, "Football is a fertility festival. Eleven sperm trying to get into the egg. I feel sorry for the goalkeeper." One could, of course, take this the other way and point out that in fertility, both the goalkeeper and the players, not to mention the playing field itself, have a great deal to do with the outcome of the game. It isn't enough to just throw the team onto the field and wait for a goal.
Chemical Genitalia and an Embarrassment of Riches
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