Choosing the gender of babies may seem totally beyond our control without scientific intervention, but some animals have a remarkable capacity to manipulate the genders of their offspring naturally in a way that is not yet understood. Hen eclectus parrots, for example, have been known to hatch as many as 20 male chicks in a row, before switching over to producing only females. Now, however, a team of evolutionary biologists has found that tight control over the gender ratios of the young can have disastrous consequences for sexual selection.
|A pair of Eclectus—the male in this case can be recognized by his bright green plumage. This species represents the most extreme example of plumage coloration between the sexes, out of more than 350 different species of parrot. Photo courtesy fivespots.
Led by Dr. Tim Fawcett, a research fellow at the University of Bristol in the west of England, the team used mathematical models and computer simulations to investigate how gender ratio control and sexual selection interact. With gender ratios fixed at 50:50, the scientists found the classical pattern of sexual selection, with males evolving elaborate courtship displays to attract females. But when they changed the models so that mothers could control the gender ratio of their offspring, they discovered a completely different pattern.
"The effect was dramatic," Dr. Fawcett said. "When mothers choose the sex of their offspring, sexual selection collapses and male courtship displays disappear. This is because females no longer find the displays attractive."
The finding adds an unexpected twist to Darwin's theory of sexual selection.
"Previously, we thought that females needed to find a showy male to produce the best offspring” Dr. Fawcett continued. "But when they can control the sex ratio, this no longer matters. Mothers with drab partners do just as well.”
The scientists now plan to put their theory to the test. If they are right, this may help us to understand the great diversity in male courtship displays seen across the animal kingdom.
"In many animals, including humans, sex is determined by the genes, while other animals have a much more flexible system," Dr Fawcett added. "Some turtles, for example, produce male hatchlings when their eggs are kept cool and females when the incubation temperature is just slightly higher. We expect to find strong differences in how sexual selection operates between these systems."
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