Climate Change Can Cause Animals to Switch Sex

Climate has such a powerful effect on animals that it can cause some males to develop as females, according to a new study on wild populations of Australian central bearded dragons.

The discovery, reported in the latest issue of the journal Nature, documents the first known case of sex reversal in the wild for a reptile.

“Sex reversal occurs when the genetic sex of an individual, usually represented by sex chromosomes, is reversed to the other sex,” senior author Arthur Georges explained to Discovery News. “An example would be in humans, where an XY individual, normally a boy, develops as a girl because of some mutation that renders the key male determining gene on the Y chromosome inoperable.”

Georges is a professor and chief scientist at the University of Canberra’s Institute for Applied Ecology. He and his colleagues combined field data from 131 wild-caught adult lizards with controlled breeding experiments.

Among the caught lizards, the researchers identified 11 sex-reversed individuals. For this species, the lizards have a ZZ/ZW system of chromosomal sex determination. A ZZ is male and a ZW is female. The system is similar to that of birds.

For the 11 sex-reversed animals, however, individuals had male ZZ chromosomes yet were anatomically female. What’s more, they could reproduce and often became supermoms within their population. As Georges said, “dads make better mums,” at least in this case.

“We showed that sex-reversed individuals, ZZ females, are larger more robust individuals and lay twice as many eggs as normal ZW females,” he added. “Dads reversed to become mums make better mums than ordinary mums.”

The 11 sex-reversed lizards were caught near the border of the Australian central bearded dragon’s range, close to the border of Queensland and New South Wales. This is a semi-arid region that tends to get hotter than the rest of the lizard’s range.

When the sex-reversed females were mated with normal males, none of the offspring had sex chromosomes, and their sex was entirely determined by egg incubation temperature. Previously it was thought that chromosomes solely determined the sex of a lizard in the wild. Now it is known that the temperature of egg incubation can affect a wild-born individual’s sex as well.

When the offspring themselves later mated with others, their young were more likely to be sex reversed, presumably because of an inherited propensity.

Sex reversals have been reported in other animals too. The phenomenon is common among fish, happening because of aging, environmental temperature, and other factors. It has also been reported in amphibians, but only in a lab setting so far.

In terms of people, Georges said, “Sex reversal occurs occasionally in humans, and presumably in other mammals, but usually comes to attention because the individuals present with clinical symptoms.”

“In some cases, though, it can come as a complete shock to a man to discover that he has XX chromosomes, or for a woman to discover that she has XY chromosomes,” he added.

There is little chance, though, for climate to influence sex determination in humans. Georges said that “the developing embryo is buffered from temperature variation,” given that it is within its mother’s body.

Read more at Discovery News