Dr. Tatiana's Sex Advice to All Creation (6 page)

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That's a tricky one. The question of whether eggsâor femalesâactively select one sperm over another is contentious. To be sure, females can reject sperm from particular males. Consider the Caribbean reef squid. Males place packets of sperm anywhere on the female's head or tentacles. The female either moves the sperm packet to her sperm storage organ, the seminal receptacle, or she picks it off and throws it away. Then there's the farmyard chicken. Females who copulate with a male low in the pecking order are likely to eject his sperm as he dismounts. But whether females store sperm from several males and then choose the winning sperm or whether each egg actively prefers particular sperm, that's another matter altogether. I know of only one case where something like this does seem to be going on.

Have you ever met a comb jelly, or ctenophore? No? There are about a hundred known species, although given their penchant for life in the vasty deep, many more probably await discovery. To the untutored eye, comb jellies resemble jellyfishâthe typical member of each group is translucent, lives in open water, and can give a nasty sting. The similarity, however, is superficial. For one thing, comb jellies have firmer bodies. But the chief distinction is the eponymous “comb”âeight ridges of cilia that run down the sides of each comb jelly and that wave in unison to propel the animal languidly along.
BeroÃ« ovata,
one of the biggest species of comb jelly, is shaped like a bell. For whom does it toll? Ask not. If
you're another comb jelly, I'm afraid it tolls for thee:
BeroÃ«
is a voracious predator, swimming along mouth first to engulf comb jellies of other species. (If a
BeroÃ«'
s not hungry, it zips its mouth shut to cut down on drag.) But to get to the point,
BeroÃ«
has some singular reproductive habits.

Like most other comb jellies,
BeroÃ« ovata
is a hermaphrodite and sends both eggs and sperm out into the sea. Self-fertilization is rare: sperm released by the same comb jelly that released the egg are not usually allowed through the egg's outer covering. Nothing odd so far. The going only gets peculiar after the egg has been fertilized. As long as just one sperm enters the egg, the baby comb jelly will begin its development as you'd expect. But if several sperm enter the egg, things become interesting.

If several sperm penetrate a human egg, it won't develop. But for many animalsâsharks, for exampleâpolyspermy is not a terminal condition but the norm. In our friend
BeroÃ«,
it seems to provide an arena for the ultimate in mate choice. The nucleus of the egg moves around and “visits” each of the sperm nuclei in turn, before eventually “deciding” which one to fuse with. The process can take hoursâand the egg nucleus won't necessarily fuse with the last sperm it inspected but will sometimes turn around and go back to one that took its fancy earlier on. How does it decide? This system has been studied so little that it is hard even to speculate.

Of course, it's possible that, unbeknownst to us, similar shenanigans go on in other species. Finding out will be difficult, however. You see,
BeroÃ«
eggs are fertilized outside the body and are easy to look at under a microscope. In species with internal fertilizationâsuch as those that copulateâit's hard to get a microscope to the scene of the action. That means we can only draw inferences about sperm selection. Just because one male succeeds in fertilizing more of a female's eggs than another doesn't mean
the sperm have been expressly chosen. The successful sperm may be more competitive or more compatible. Or the effect may be due to chance. In the mallard, for example, females who have been artificially inseminated with a mixture of sperm from several males tend to use the sperm of one male for a given clutch. Which male is the lucky one changes each time, however, even though the female receives the same mixture of spermâsuggesting that the effect is due to sperm clumping rather than to an active preference for the sperm of one male in particular.

As for yellow dung flies, claims have been made that the female's decision to use one male's sperm rather than another's depends on whether she lays her eggs on a cowpat in the shade or one in the sun. It is a fascinating idea but extraordinary claims require extraordinary evidence, which we do not currently have. In any event, if I were you I'd concentrate on eliminating the option of your mate's choosing sperm. If a male yellow dung fly copulates for long enough, he can displace the sperm of previous males (to achieve this effect, small males have to copulate for longer than big males, because small males transfer sperm more slowly). Having replaced the sperm of your predecessors with your own, you should then guard the female until she has laid her eggs. That way, you won't have to worry: your sperm will be the only ones available. Go for it!

So you see, there are lots of reasons females might play the field, although we don't necessarily know the reasons in any given instance. Just in case you meet a girl on the prowl and you want to understand her motives, here's a checklist of possibilities: