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