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In-depth: Turtles

Turtles Take the Heat
by Robin Marks

What would the world be like if no men were born? Business for barbers would decline, Fatherís Day would be a moot point and finding a tenor to sing La BohŤme would be a challenge.

turtles on a stick
Below 32 degrees Celsius, or 90 degrees Fahrenheit, the turtle babies are male; above that temperature and the babies are female (Photo credit: Carrie Vonderhaar)
Click to enlarge

Most important, with only one gender the human species wouldnít be around much longer. In a less dramatic, but possibly no less drastic way, this is a plight facing many of the worldís turtle species, including river turtles like the yellow-spotted river turtle, Podocnemis unifilis, and the six-tubercled river turtle, Podocnemis sextuberculata, found in Brazilís Mamirauá Reserve. And climate change is the likely culprit.

Many turtle species already face a litany of threats: land and water pollution, loss of habitat, and consumption by humans. The impact of climate change on turtle populations is more subtle and less understood, but it is clearly having an effect -- rising temperatures lead to rising sea levels, which means more flooding and erosion at nesting sites along the shore. For some turtle species, this climatic turn could also result in fewer and fewer males.

tortoise
Although tortoises and turtles are still found throughout the Amazon, their survival is threatened for several reasons, including habitat loss and disturbance, egg collecting, hunting, and climate change.(Photo credit: Carrie Vonderhaar)
Click to enlarge

What does rising temperature have to do with a deficiency of males? The determination of sex in turtles is quite different than for mammals. In humans, for example, the gender of a child is determined by which chromosome, X or Y, it receives from its father. But the vast majority of turtle species donít have these sex chromosomes. Instead, the sex of the offspring depends on the temperature of the eggs during the first third of their incubation. This phenomenon is called temperature-dependent sex determination, or TSD.

There are two patterns of TSD in turtles. In the first, cooler incubation produces more males, warmer incubation produces more females, and a middling temperature results in a mixture of sexes. The second pattern produces mostly females at either end of the temperature spectrum and mostly males when temperatures are in the middle. The river turtles in the Mamirauá Reserve follow the first pattern. Some ecologists studying the area believe that temperatures are moving into the upper tier of incubation temperatures and therefore fewer and fewer males are being hatched.

Did you know?

The fact that temperature can determine the sex of offspring in some animals wasnít discovered until the 1970s. Incubation temperature determines not only the sex of turtles but also that of crocodiles and alligators. Some scientists believe that the gender of dinosaurs -- relatives of modern reptiles, including turtles -- might also have been dependent on temperature and that the rapid change in climate during the Cretaceous period, 100 million years ago, might have contributed to their extinction.

You might think that this sort of reproductive system could lead to imbalanced sex ratios even without a climate crisis, and youíd be right (at least in some cases). Under certain conditions, turtles like those in the Podocnemis family can have more females than males in a given population. But a favoring of females isnít necessarily a problem, as the gender distribution in the population may even out again over time.

Still, a population of turtles that consistently has too many females (or too few males, depending on how you look at it) isnít good for the overall future of a species. If the sex bias becomes extreme, a smaller group of males will be fathering the offspring, meaning there will be less genetic variability and therefore less resistance to disease, climate and other pressures. Ultimately, a less-varied population will be less healthy over generations.

diver with turtle
Turtles all around the world are vulnerable to climate change.
Click to enlarge
It doesnít take much to skew the sex ratios in some turtle populations: Some scientists predict that an increase of just a few degrees in the average temperature during nesting season could throw the gender balance off enough to threaten a species if it persists over a generation or two. What constitutes a generation for turtles varies, as different species have different life spans. Itís difficult for scientists to know exactly how many years of skewed gender ratios a population could endure. A species that lives an average of 40 years, for example, might not feel an impact for five or even ten years. Itís likely that the effects of a skewed sex ratio would accrue over time, rather than appearing suddenly, making it all the more important to document populations now and pay attention to subtle changes.

Some wildlife conservation groups have advocated taking advantage of TSD as a way of artificially producing more females in populations of endangered turtle species. Others have argued that this kind of intervention unnaturally restructures the population and so may not actually be a benefit over the long term.

baby turtle
This baby turtle was kept in a protective tank before it was released. Newly hatched turtles are vulnerable to predators that can smell egg on their shells.
Click to enlarge

It has been a challenge for researchers to get information on the long-term effects of temperature on turtle populations. Good data requires that scientists be able to constantly monitor the temperatures of nests in order to determine what kinds of fluctuations lead to what kinds of effects. Since this is hard to do in the field, most studies on TSD have been in labs. And although such studies yield useful information, they canít provide information on the larger picture of changes in a natural environment over time.

What scientists do know is that turtles are ancient creatures, with fossil remains dating back 200 million years. Theyíve proven resilient in the face of enormous natural challenges, surviving ice ages and the mass-extinction events that killed the dinosaurs. Perhaps they can survive the trials of a world dominated, and warmed, by humans.

Sources:

U.S. Geological Survey: "Turtles and Global Climate Change," by Geoffrey E. Lovich

Wikipedia.org: "Temperature-Dependent Sex Determination in Turtles"

Global Greenhouse Warming.com: Climate Change and Sea Turtles

Dr. Nicole Valenzuela, Iowa State University: Anders G.J. Rhodin, M.D., Chelonian Research Foundation