Election Section

After that long siege of rain, it’s been warm enough this week for the turtles to be out basking. You can see them at Jewel Lake in Tilden Regional Park: the larger, darker ones are western pond turtles, the Bay Area’s only native chelonians; the green stripy ones with red patches on their necks are red-eared sliders, the descendants of inconvenient pets who were released in the lake. (“Slider” here pertains to a group of freshwater turtles, not to a curving fastball or a small hamburger.) Their lives appear peaceful, apart from the occasional jostle over the best spot on the log. 

Overall, though, western pond turtles aren’t doing so well. Like most freshwater creatures in California, they’ve lost a lot of habitat to farmland and urban sprawl. The streams they favor have been drowned by reservoirs; their hatchlings have been eaten by exotic bullfrogs. In the 19th century, thousands were harvested for the tables of San Francisco, selling for $3 to $6 a dozen; you could order local turtle at the Palace Hotel. Our resident subspecies, Clemmys marmorata pallida, is a California Species of Special Concern, and both it and the northern C. m. marmorata have been proposed for federal endangered status.  

According to the Turtle Conservation Fund, two-thirds of the world’s 270-odd land and freshwater turtle species are at risk of extinction. The Asian food and traditional medicine market takes a heavy toll, with wild-caught turtles from Burma and Indonesia shipped by the ton to China. And there’s a brisk and often illegal international pet trade: like parrots and orchids, some turtle species have been prized to the point of extinction. 

But all that may just be prelude to the big hit. Turtles, it seems, are especially vulnerable to the effects of climate change. Global warming could throw the sex ratios of turtle populations out of whack, leading to a terminal demographic crisis. 

We tend to think of sex determination as being all in the chromosomes. In most mammals (and in some insects, oddly enough), the X’s and Y’s dictate an offspring’s sex. If you get an X chromosome from your father, you become female; if you get Dad’s Y, you’re male. I say “most” because of the duck-billed platypus, which was recently discovered to have a set of 10 sex-determining chromosomes, and the howler monkey, which has 4. Birds have an analogous “ZW” system, in which a ZZ combination creates a male while ZW makes for a female. 

But reptiles are different. In almost all turtles, as well as alligators and crocodiles, some lizards, and the lizardlike tuataras of New Zealand, temperature during the first trimester of incubation dictates sex. At temperatures below approximately 30 C (86 F), turtle embryos develop into males; above 30, into females. I haven’t found specific studies of western pond turtles, but the phenomenon has been documented in many of their relatives, including the European pond turtle, painted turtles, map turtles, and sliders. The exceptions to the rule are snapping turtles, which produce females at warm or cool incubation temperatures and males at intermediate temperatures, and softshell turtles, whose eggs seem unaffected by temperature. Crocodilians show a reverse pattern: females at lower temperatures, males at higher.  

And fish are something else again. The sex of a juvenile Australian coral goby becomes the opposite of that of its first adult partner. This might not seem unusual to a visitor from Ursula LeGuin’s planet Gethen, but it’s fairly strange by Terran standards. 

Back to the turtles, though. Two French biologists, C. Pieau and M. Dorrizi at the Institut Jacques Monod in Paris, have worked out the mechanism for temperature-dependent sex determination in European pond turtles and two sea turtle species. At higher temperatures, the enzyme aromatase floods the gonads of the turtle embryos with estrogens. Experimental treatment with aromatase inhibitors and anti-estrogens turns off the temperature effect, producing male hatchlings at higher temperatures. 

And real-world studies have reinforced the lab work. Frederic Janzen of the University of Chicago spent five years monitoring a population of painted turtles on an island on the Illinois side of the Mississippi River. He found a strong correlation between average air temperatures in July—the critical period for sex determination, when incubation begins—and the sex of the hatchlings. In one year, 1992, all the hatchlings Janzen censused were female; in 1988, all were male. 

You see where this is going. Janzen projects that with an increase of four degrees in July mean temperatures over the next century—well within the parameters of climate change models—the result will be “an eventual extinction of this painted turtle population because no males will be produced.” The only hope would be a shift to earlier nesting dates. And although some creatures, notably migratory birds have already shown signs of adaptation to a warming earth, turtles are not known for their behavioral flexibility.  

It’s been speculated that skewed sex ratios driven by climate change may have contributed to the decline of the dinosaurs before the meteor finished them off. Maybe so, if their sex-determination system was more like that of alligators than that of their probable surviving next of kin, the birds.  

Granted, the extinction of turtles is not the first consequence of global warming you’re going to worry about if you’re a tropical medicine specialist, or a resident of New Orleans or Bangladesh.  

But the danger appears to be real. And what an ironic way to lose creatures that have become such icons of durability. In a drawer in UC’s Museum of Paleontology, there’s the shell of a western pond turtle from the Blackhawk Ranch fossil quarry. It’s about 10 million years old, and it looks like it could have come from Jewel Lake last week. It would be a shame if a species that outlived the four-tusked gomphotheres and sabertoothed nimravids finally succumbed to an excess of carbon dioxide. e