Photo of a Red Eared Slider turtle by Nightryder84.

Photo of a red-eared slider turtle by Nightryder84.

Dr. Thane Wibbels (University of Alabama at Birmingham) is interested in studying how temperature affects the sex of red-eared slider turtle embryos. For humans, the answer is simple: sex chromosomes. You know, the combination of XX means girl and XY means boy.

Turtles are not that simple. Temperature is a factor in determining whether the embryo will be male or female. If the eggs are incubated at 78.8 degrees F, the hatchlings will all be male. If they are incubated at 87.8 degrees, they will all be female. As I’m sure you’ve guessed, temperatures in between these points will result in a mixture of female and male hatchlings. This temperature-based sex determination is a reptile trait that is speculated to date as far back as 220 million years ago.

A new study published by Drs. Wibbels and Kayla Bieser (Northland College, Ashland, Wisconsin) in the journal Sexual Development tried to understand this mystery. They examined the expression of 5 genes in turtle eggs exposed to either 78.8 or 87.8 degrees. They discovered that a gene called dmrt1 is the earliest gene to be expressed in males and according to Dr. Wibbels, this gene in particular seems to be involved in male sex determination for all vertebrates.  The plan now is to find the “male producing” switch that dmrt1 turns on.


University of Alabama press release


  1. #1 Dimakatso
    April 1, 2015

    This kind of sex determination fascinates me so much. I was under the impression that it was unique to crocodiles and alligators but after having read the blog I realise that it is quite a norm in the reptilian family.
    Does this kind of sex determination result in there being more of a single sex in one area when compared to another?

  2. #2 Chanté Kritzinger
    South Africa
    April 2, 2015

    I have read that red-eared slider turtles can be found in New Mexico, north to Ohio, Illinois, Indiana, West Virginia, Kentucky, Tennessee, Georgia and northern Mexico. If in colder areas more males are born and in warmer temperatures there are more females, do these turtles migrate in any way during mating season so that the different sexes are dispersed evenly?

  3. #3 George
    April 2, 2015

    This information is very interesting. I also did not realize that this type of sex determination extended to turtles. I will definitely go do some more research on this.
    I was just wondering if Dimakatso is correct in saying that this type of sex determination is a norm in the reptilian family?

  4. #4 Sam (15036198)
    Pretoria, South Africa
    April 2, 2015

    Dimakatso, I was wondering the exactly the same thing about the ratio between males and females in certain areas. I know very little about this topic, so I went and did some research. I picked up one line of thought that made sense to me. The outcome of this type of sex determination is obviously directly linked to how it evolved. Currently the evolution of temperature-dependent sex determination is not understood. One theory is that it evolved due to one sex being more suited to specific areas. An example could be that in a warmer area nesting is more successful, so more females are produced to increase the amount that nest next season. This theory is supported by the fact that one male can mate and impregnate many females. So technically the species does need fewer males than females anyway.

  5. #5 Abby
    April 3, 2015

    It is common occurrence among animals to regulate the amount of babies and the gender of the babies according to environmental constraints in order to ensure the survival of the species.

  6. #6 Peter
    April 3, 2015

    This can also be seen in humans. Male sperm cells are more susceptible to heat and those carrying the female gene are not.

  7. #7 Carly
    April 3, 2015

    This is very interesting. Its amazing to see how animals adapt instinctively to ensure survival.

  8. #8 Vormaurer, K.F. (u15002782)
    University of Pretoria
    April 3, 2015

    I have read that rising temperatures are resulting in a larger female population and a smaller male population. It is better to have more female turtles because male turtles can mate each year and females cannot. However, the complete absence of males could be tragic to the species.

  9. #9 Batista
    April 7, 2015

    Does temperature close to 78.8 degrees F or 87.8 degrees F result in more male or females hatch-lings? Does this mean that if the average temperature of a area is closest to either one of the mentioned temperatures there will be more of a gender which the average temperature is closest to?


  10. #10 Vormaurer (15002782)
    April 8, 2015

    I have read that rising temperatures are resulting in a larger female population and a smaller male population. It is better to have more female turtles because male turtles can mate each year and females cannot. However, the complete absence of males could be tragic to the species. 15002782

  11. #11 Bennett M
    April 8, 2015

    I would have to disagree with the theory mentioned by Sam(15036198). If that theory were to be true then one would expect to see more of one sex than the other. With the passage of time this would result in a decrease in the specie’s population and population growth, as it would become increasingly difficult for 2 different sexes to find each other and mate. There would also be increased competition for mating partners which could result in conflict.

  12. #12 Chong H
    April 15, 2015

    I would have to disagree with Bennett above with the last part of his statement as for females, reproduction is a very energy intensive and risky venture where for males it is much less so. So females would not have to compete for males, but they would have to accept advances from lower quality males, leading to a degradation of the overall quality of the gene pool, and thus a higher mortality rate. However this will also be countered by increased resources like food and territorial spaces, so general population and productivity increases. So in all, it would largely depend on the rate of one versus the other, as factors like these are not easily identifiable for direct consequences on a population.

  13. #13 Verster, J
    April 15, 2015

    I partly agree with Chong, H. As the population increases the competition for food will also increase. Over time this will lead to a different gene pool according to natural selection. This might even change this mechanism of sex determination according to temperature (during the course of evolution).