Stress Brings Out Man in Hermaphrodites, Study Says

John Pickrell in England
for National Geographic News
August 19, 2003
Biologists already know that plants sporting flowers of both sexes focus on their masculine attributes during times of stress. Now, a new study provides the first experimental evidence that some hermaphrodite animals also benefit from promoting maleness when faced with troublesome conditions.

Researchers show that one type of marine invertebrate-—commonly found to foul ships, kelp, and other surfaces—produces many more male reproductive parts than female, when it's wounded or is faced with other damage.

Hermaphrodites are species that have both male and female sexual organs.

Reproductive Bailout

It's a kind of "reproductive bailout strategy," said ecologist co-author Roger Hughes at the University of Wales, Bangor. Producing fruit or carrying the relatively heavier burden of eggs and offspring is more expensive than producing sperm, he said. If an organism's time is limited, it pays to quickly invest in cheap-to-produce sperm or pollen.

Hughes and colleagues reveal the find in the August 19 online edition of the journal Proceedings of the National Academy of Sciences.

Since the late 19th century, researchers have noticed the same strategy in flowering plants. Many of these have flowers that include both male and female parts, or have flowers of both sexes on the same plant. These plants are typically found to increase the ratio of pollen to fruit and seed production when faced with drought, disease, herbivore attack, and other stressful conditions.

"People hadn't predicted this effect would be seen in animals before," commented Bruno Baur, evolutionary biologist at the University of Basel in Switzerland. The next step will be to see if other animals put more resources into the male sex during times of stress, he said.

Researchers have rarely considered the strategy relevant to animals, agreed Hughes. However colonial invertebrates, which are anchored to the same spot for life, share many attributes with plants, he said.

The tiny marine bryozoan organism Celleporella Hyalina, for example, consists of many modular units which are either male, female (like flowers), or feeding units. These tiny animals grow in an immobile, flat, disc-shaped colony, and release sperm into the water current, much like trees and other plants release pollen into the wind.

Male Multiplication

To test the idea that these animals might use the same strategy as plants, Hughes and his coworkers divided up the modules of one individual and allowed them to grow into a number of experimental colonies, grown on small plastic discs. This was repeated for several different genetic individuals.

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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