Washington In the eternal war between the sexes, the female side-blotched lizard wins it all: she selects her many mates, decides where they'll live and even determines if they will have sons or daughters.
Virtually every element of the mating and reproducing cycle of the small American lizard is controlled by choices made by the female, said Ryan Calsbeek, a biologist at the Institute of Environment at UCLA.
"This is the ultimate example of a female having her cake and eating it too," said Calsbeek, the first author of a study appearing today in the Proceedings of the National Academy of Sciences. "It would be like a human female who marries a short, dumpy rich guy and then has an affair with a muscular 20-year-old to have a handsome son who grows up in a mansion and goes to the best schools."
The side-blotched lizard is the most common lizard in the American west. It lives among rocks west of the Rocky Mountains from Canada to the Baja Peninsula in Mexico. The animal is small with the male reaching about 2.3 inches and the female about half that but it has a complex mating and reproduction system, said Calsbeek.
Generally, the female picks the mate she'll live with. Calsbeek said he and his co-author observed that the female generally prefers a big male who lives on a big rock in the best location.
To see if the female was selecting the choice living site or the choice male, the researchers moved the rocks around, putting the big males on poor rocks and the little males on the best rocks in the finest neighborhoods.
The female lizards seemed to prize comfort over all else, choosing small males with the fancy rocks as the first mate and live-in partner.
But that's not the end of her choices.
Calsbeek said the female is "incredibly promiscuous," commonly mating with five or six males per reproductive cycle.
To see which male lizards are siring the female's young, the researchers did molecular tests for paternity and found the lady lizard was very clever, indeed.
The female collected the sperm from many partners in a special body cavity, called the spermatheca, before it was allowed to fertilize her eggs, Calsbeek said. Then, somehow, the female caused the sperm from big males to make sons, while the sperm from the small males was used to make daughters.
"We don't understand how the females do it," said Calsbeek, but the theory is that her body somehow selects the sperm's fate based on the sex chromosomes. Just as in humans, a single male sperm can carry either the X or the Y chromosome, while the female egg carries only the X chromosome. If the Y chromosome unites with the egg, it creates a male. If it's the X chromosome, the offspring is female.
"We found that the females were mating with the small male that she was living with, but would also mate with the big males," said Calsbeek. "Then she allows the big males to make sons, and the small males to make daughters.
"It appears to be ... an incredibly refined ability of the female to manipulate the investment of her partners," he said.