Female lizards have developed a way to control which genes are passed down to their offspring, according to a study by two Dartmouth biologists. Dartmouth post-doctoral researcher Robert Cox and biology professor Ryan Calsbeek found that mother lizards can increase or reduce the number of male offspring they produce in order to maximize genetic effectiveness.

The study, which focused on the reproductive patterns of brown anole lizards, was published in the March 4 edition of “Science Express,” the advance online edition of the journal “Science.”

The two researchers concluded that female lizards must have genetic and morphological mechanisms to ensure that certain genes that are advantageous to male offspring — such as size — are passed down to them from large fathers, according to the study. As a result, females are more likely to produce male offspring when mating with large male lizards, but are more likely to produce female offspring when mating with small male lizards, according to the study.

“[The species has] figured out how to get the right genes into the right gender,” Cox said, according to a College press release.

After tracking the survival of young anoles that were released into the wild, the researchers concluded that while the survival chances of male anoles increased if they had large fathers, the survival chances of female anoles were not influenced by the size of their fathers, according to the study.

“This suggests that the genetic benefits of large size are specific to sons,” Calsbeek said, according to the press release.

Beneficial genes for males are often different from beneficial genes for females, according to the study.

“In an evolutionary sense, what’s good for the goose is not always good for the gander,” Cox said.

By manipulating opportunities in the laboratory for female brown anoles to mate with males of different sizes, the researchers determined that female brown anoles are more attracted to larger males, according to the study. When researchers only exposed females to smaller mates, the females produced fewer sons, according to the study.

Female brown anoles’ exact mechanisms for controlling the gender of their offspring are still a mystery, Calsbeek said in an interview with The Dartmouth, but that he and Cox have a few hypotheses regarding the question.

Female anoles have “pockets” to store sperm, and females are typically able to hold sperm for up to four months, Calsbeek said. He hypothesized that female anoles may be able to control the sex of their offspring by using their immune system to destroy Y-chromosomes in sperm.

Calsbeek told The Dartmouth he and Cox plan to continue their study on brown anoles to learn more about parents’ effort to transfer their advantageous genetic material to their offspring.

Cox and Calsbeek began work on the study three years ago, when they first began raising and breeding brown anoles, Calsbeek said. Calsbeek has studied brown anole lizards for nine years, he said.

Calsbeek said that he and Cox traveled frequently to the Bahamas to carry out field experiments because brown anole lizards are native the islands. The researchers spent about seven weeks in the Bahamas each year, Calsbeek said.

“We go in the spring to set up the experiment, and we catch and mark all the lizards in spring and we let those experiments run,” Calsbeek said.

Later in the fall, he and Cox will return to the Bahamas to collect the statistical data.

Cox, the lead author on the paper, did not reply to requests for comment by press time.

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