Lizard’s Choice: Mating test pits physique versus domain

When a female moves into her intended’s home, is she choosing the guy or his real estate? A novel experiment says that it’s the landscaping that counts, at least among side blotched lizards. Moreover, the ladies have their own ways to improve life with a puny gent.

Females of this common Western lizard, Uta stansburiana, usually prefer big dominant males, explains Ryan Calsbeek of the University of California, Los Angeles.

Those males typically control the best rocks for sun and shade throughout the day, so it’s hard to tell what drives a female’s decision.

Calsbeek and Barry Sinervo of the University of California, Santa Cruz took rocks from the lizard haves and gave them to the have-nots. Each male, nonetheless, remained loyal to his site. Most females deserted the large dominant males and moved to the better rock collections, Calsbeek and Sinervo report in an upcoming Proceedings of the National Academy of Sciences. This is the first experiment in the wild that has differentiated masculine allure from the power of real estate, says Calsbeek.

The researchers found yet another twist in the mating system. The females still visited their previous, studlier partners and, somehow, used their sperm to fertilize eggs that turned into sons. Daughters, however, had received sperm from Mom’s smaller rockmates.

“These females really can have their cake and eat it, too,” says Calsbeek.

Researchers had already established that what makes a territory desirable is its wide variety of rocks. During spring, Calsbeek monitored a group of males, all with blue blotches on their throats, as they staked out their domains. Then he spent a day playing Robin Hood, moving some 1,500 pounds of stones.

Afterward, lizards were in “complete chaos. Everybody was wandering around looking for rocks,” Calsbeek says. Out of 51 female lizards that started out in a big male’s empire, 37 eventually moved to territories with better rocks. As far as the researchers could tell, most of these new homes had had no female inhabitant because of their pitiful rock supply.

Later, it was easy to recognize and catch females just before they laid their eggs, Calsbeek says. “They looked like beanbags, waddling around on their little legs,” he recalls. The researchers brought them into the lab and checked their offsprings’ DNA to determine paternity.

Work in progress by Sinervo and Calsbeek suggests a genetic advantage for the lizards’ siring pattern. Large males pass on their advantageous size to their sons but handicap their daughters with a tendency to delay egg laying, says Calsbeek.

Research on other animals has turned up evidence that females somehow skew the sex ratio of offspring to fit conditions of greater or lesser food abundance, says evolutionary biologist John Alcock at Arizona State University in Tempe. He calls the newly described sperm allocation “astonishing.”

The question of whether rock abundance or male size influences females may pose an artificial dilemma. Mark Elgar of the University of Melbourne in Australia says, “Clearly, it’s both.”

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