Dr. Tatiana's Sex Advice to All Creation Read Online Free PDF Page A

fifty million—a threshold below which he’ll have a hard time impregnating anybody. And some snakes positively fade away. The adder, a poisonous European species, loses a lot of weight at the start of the breeding season even though he’s doing nothing but lying in the sun making sperm. That’s one way to burn calories.
    But the clinching proof that sperm are often limiting comes from hermaphrodites—organisms such as garden slugs and snails that are both male and female at the same time. Under the sperm-unlimited theory, hermaphrodites should run out of eggs before they run out of sperm and, given a choice, should prefer the male role over the female one. But in many species, this is not what happens.
    Consider Caenorhabditis elegans, a tiny transparent roundworm beloved of geneticists. C. elegans is different from most hermaphrodites because it comes in two sexes: hermaphrodites and males. Among conventional hermaphrodites, there are two ways to have sex. Copulation can either be bilateral—both partners inseminate each other at the same time. Or it can be unilateral—in any given bout of sex, one partner plays the male role and the other partner plays the female. In C. elegans, however, the hermaphrodites cannot mate with each other, but they do make both eggs and sperm and can fertilize themselves. (Males, obviously, make only sperm.) A hermaphroditic C. elegans who never encounters a male will run out of sperm after laying about three hundred eggs. We know that the sperm are finished up first because
for a while afterward the animal carries on laying unfertilized eggs—sometimes as many as a hundred.
    But perhaps C. elegans is a special case. The hermaphrodite form does not make eggs and sperm at the same time; rather, it does sperm first. Therefore, the more sperm it makes, the longer it must wait before making eggs and the older it is when it starts reproducing. Too much delay is bad: among these worms, you’re likely to have more offspring if you get off to an early start.
    C. elegans and its relations are not, however, the only hermaphrodites that run out of sperm. Sperm limitation has also been found among sea slugs, sea hares, freshwater snails, and freshwater flatworms. (Although these organisms appear similar, they are only distantly related. Their looks and lifestyles have been arrived at independently.) In the freshwater flatworm Dugesia gonocephala (a bilateral copulator), sperm packets take two days to prepare, so individuals are thrifty with sperm and don’t give away more than they receive: they stop giving sperm when their partner does. In the sea slug Navanax inermis (a unilateral copulator), individuals appear to prefer the female role over that of the male—the opposite of what you’d expect if sperm were unlimited.
    And lest anyone still doubt that the male role can be expensive, look at banana slugs—enormous yellow slugs of the Pacific Northwest. In these hermaphrodites copulation is unilateral—and in several banana slug species, individuals may get only one shot at being male, whatever their sperm count. Banana slug penises are gigantic and complex. During sex, the penis often gets stuck. At the end of sex, therefore, the slug or its partner gnaws off the offending phallus. It never grows back: from that point on, the slug plays only the female role.
    That said, let’s take a closer look at your situation. Drosophila melanogaster males apparently suffer from two types of matinginduced
sterility. The first is temporary: after mating once, a male ought to rest for a day before mating again in order to replenish supplies. The second type appears to be permanent. Unfortunately, because of the design of the experiments done so far, we don’t know how quickly permanent sterility arises. All we know is that males presented with two females every two days are completely and irreversibly sterile by day 34—just under halfway through their