Matching pairs of colonies grown under the same conditions were then exposed either to consistent optimal environmental conditions, or to environmental stress. The stressed colonies faced either a two-hour temperature shock in frigid water, ten periods of ten minutes exposure to air, starvation, cramped conditions, or physical damage with a needle.

"The results were always the same…there was a wildly significant increase in the number of male modules [in stressed colonies]," said Hughes.

"In a stressful environment animals have a dramatically reduced survivorship," commented the University of Basle's Baur. "If they have no future, it benefits to quickly invest in the gender that is least costly," he said. Baur has found some similar results in hermaphroditic land snails deprived of food.

"In C.hyalina, eggs full of yolk take up a lot of energy, and once fertilized require a three-week gestation period," said Hughes, "If the colony won't last this long, there's no point in producing more eggs," he said. Sperm can be liberated immediately, and stand a chance of reaching non-stressed mates elsewhere.

Gender Bending

"The relative benefit of producing males or females varies with the environment," commented Stuart West, an evolutionary biologist at the University of Edinburgh in Scotland.

"Hermaphrodites can change the relative allocation of resources to male and female [organs]," to follow environmental conditions, he said. But non-hermaphroditic animals also have some influence on the ratio of male to female offspring to benefit most from environmental conditions, said West.

Lower ranking and less healthy red deer does, for example, are more likely to give birth to females, said West, as only the strongest and healthiest young stags are likely to father offspring later in life. European blue tits are more likely to lay eggs containing male offspring when they've mated with the best quality males—low-quality females are more likely to reproduce than low-quality males, said West.

Even humans are able to exercise some control over offspring sex ratio. Data shows that the number of boys born in Europe increased after both the First and Second World Wars, though it's not possible to say whether this was due to stress or some other factor, added West.

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SOURCES AND RELATED WEB SITES Proceedings of the National Academy of Sciences Stuart West's Research Group University of Wales, Bangor