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64 NEBRASKAland • NOVEMBER 2016 Temperature Matters Determining turtle gender. By Dan Fogell F irst off, I want to say that this is not a recipe for poached turtle eggs. That said, we all know the difference between boys and girls. Boys have an X and a Y chromosome, and girls have two X's. At least that's how it works for mammals. It's a little different with birds and a lot of reptiles, as males have two similar chromosomes and females ha ve two different ones. But at least males and females are genetically different. For most of Nebraska's turtles, however, this is not the case. Temperature-dependent sex determination is pretty much the rule for all of Nebraska's turtles except for the softshells. This means that whether they are males or females is determined solely by the temperature at which they incubated while they were developing in eggs (a trait that turtles share with the crocodilians, by the way). All species of turtle lay eggs. Think about turtle embryonic development in terms of trimesters, much like we think of human pregnancies. During the first trimester the sex is undetermined. Every egg in a turtle's nest has the potential to hatch as either a male or a female. The second trimester, however, is referred to as the thermosensitive period. This is when the developing turtle gets its gender. During this period, an enzyme responsible for fluctuations in levels of sex hormones is triggered by temperature. The enzyme converts male sex hormones, or androgens, into female hormones, or estrogens. How active the enzyme is depends on the temperature. If the enzyme is not very active, androgens will not be converted into estrogens and the turtle will develop male sex characteristics. But if the enzyme is very active, it then converts much of the male sex hormone into estrogen, which causes the developing turtle to become a female. So, by the third trimester, the sex has been determined and cannot be reversed. Cooler temperatures do not activate the enzyme as much as warm temperatures do. Therefore, males will hatch out of eggs incubated at cooler temperatures (less than 80 degrees Fahrenheit) and females will hatch out of warmer eggs (greater than 86 degrees Fahrenheit). Eggs incubated at temperatures between 80-86 degrees will randomly develop as either male or female. Since all turtles dig nests (another trait shared with crocodilians), those eggs at the top (nearest to the surface) have a better chance of hatching as females, and those buried deeper and cooler are likely to become males. I know what you're thinking: Can a changing climate have any effect on this physiological marvel? As temperatures during turtle nesting season become warmer, there is strong potential for females to significantly outnumber males. Fewer males available for breeding means some females may not find mates. No mating means no babies, which means a reduced population. Also, since fewer males would be contributing genes to future generations, genetic diversity would suffer. Population decline coupled with reduced genetic diversity are never encouraging characteristics for a species' survival. Turtles have a lot of interesting traits: shells, a lack of teeth, really long lives … but to me, the fact that temperature determines whether they will be boys or girls tops the list. ■ Dan Fogell is an author, herpetologist and biology professor at Southeast Community College in Lincoln. Above: A hatchling painted turtle. Below: a nesting snapping turtle with eggs. ■