Does pseudosex enhance virgin birth?

DOES PSEUDOSEX ENHANCE VIRGIN BIRTH?

In the American southwest and in northern Mexico, virgin births occur as a matter of course. There lives the desert grassland whiptail lizard (Cnemidophorus uniparens), an all-female species that reproduces by parthenogenesis -- without any male participation. ''The species is a single clone,'' says David Crews, a professor of zoology and psychology at the University of Texas. The chromosomes of each lizard duplicate just before the ovum divides, producing ge netically complete and genetically identical eggs.

Parthenogenesis has its advantages, the most obvious being higher potential population growth, since all of these lizards (rather than half) can produce eggs.

Yet Crews says these unisexual whiptail lizards go through the motions of sex, at least in captivity. When two of them are placed in a cage, one acts like the female she is, the other like a male. They engage in courtship displays, mounting behavior and what Crews calls ''the doughnut pos- ture, in which the male-like lizard curls over the other lizard and grasps her.'' And every 10 to 14 days they switch roles. The lizard that has just ovulated has a surge of progesterone, which causes her to act like a male, and then after a week or so her ovaries become larger and secrete estrogen, and she reassumes the female's part. The riddle: If this goes on in the wild as well as in the lab, what reproductive purpose does it serve?

Since C. uniparens is descended from the hybrid union of two sexual species, one of them the little striped whiptail lizard (C. inornatus), Crews at first thought the pseudosexual behavior might be merely a useless vestige of a former way of life. But now he doubts it. In a study published recently in the Proceedings of the National Academy of Sciences, he reports that just as the female little striped whiptail lizard is more likely to ovulate when she's courted by a male, so the desert grassland whiptail is more likely to ovulate when she's with a male-like female. Crews also found that isolated females take longer to lay eggs. ''By going through this pseudosexual behavior,'' he says, these lizards ''maximize the number of eggs they lay.''

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