The Female Male

Word may not have filtered down to local chapters of Women's Lib, but a renegade group has been found among the ranks of Australian females. Both sexes of Labroides dimidiatus, an unprepossessing little fish of the wrasse family that lives in the waters of the Great Barrier Reef, are rampant male chauvinists. Not only do the males rule the reef, but the females like the idea so much that they turn into males themselves at the first opportunity.

After careful observation of many groups of wrasse, D.R. Robertson, a zoologist at the University of Queensland in Brisbane, found that the death or disappearance of a male has a startling effect on one of the several female fish in his harem: she becomes a male. Within a few hours of her liberation from the influence of a male, the head female in the harem begins to display male mannerisms. She reconnoiters the borders of the absent male's territory and pays aggressive calls on the other females. Within four days her courtship and spawning behavior cannot be distinguished from a male's. From 14 to 18 days after her accession to power she actually becomes a functioning male, physiologically capable of releasing sperm.

Over a period of 25 months, Robertson studied 19 different harems of L. dimidiatus and recorded 48 instances of sex transformation. "Probably all females are capable of changing sex," Robertson writes in Science, "and most (possibly all) have testicular elements within perfectly functional ovaries." What prevents wholesale sex transformation by the females in a harem is a chain of authority extending downward from the lone male. A former female (like most other males of this species), he bullies the strongest female, who in turn is overbearing toward the female under her, and so on down the social scale. As long as a wrasse remains under the stress of domination, its body apparently does not produce the hormones necessary to effect a sex change. Once that stress is removed, however, hormone production is no longer inhibited, and a rapid transformation occurs.

Protogyny (the development of a male from a functional female) is not uncommon among other kinds of tropical fish. But the rigid hierarchy of the wrasse harems represents a much more stringent control over the production of males than is found in the comparatively random sex reversals of "schooling" fish. Robertson believes that the inbreeding produced by this aquatic pecking order is genetically advantageous to the species. Because "the social organization is a framework within which the selective process works," he concludes, the genes passed on by each male wrasse are those best adapted to the environment of the harem.

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