Hot chicks, cool dudes: How temperature affects turtle eggs

Blog Post By: Karoline, Sea Turtle Intern
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        One thing I love about public engagement is that oftentimes you get a question from someone that really makes you think. This week at our first “practice” night walk, we had a question about the temperature-dependent sex determination of sea turtle eggs. Fellow intern Ashlyn and I knew the answer to the question, but I realized I only know the answer to the question, not the reasoning behind it. Public engagement teaches you that there’s always more to learn!  
        To start off, temperature-dependent sex determination or TSD is when the sex of the offspring depends on the temperatures the offspring experiences during development in the egg. TSD is a type of environmental sex determination, and only happens in reptiles and some fish (most vertebrates use chromosomal sex determination systems). The sex of the egg is determined during a specific time during incubation known as the thermosensitive period or TSP. In sea turtles, we use the saying “Hot chicks, cool dudes”. This means that the eggs will be male if they incubate at a temperature below 81.86 Fahrenheit, and they will be female if they incubate at a temperature above 87.8 Fahrenheit. Eggs in a fluctuating temperature zone between these two will be a mix of male and female.  
        The original question asked was: Because the temperature determines the sex of the egg, would an entire nest be all one sex, or can there be a mix of males and females in one nest. Our answer was: it depends! For example, if the entire nest is at a temperature above 87.8, the nest would be all females. But, if the bottom of a nest is cooler than the top, the bottom eggs could be males and the top eggs could be females! This question was a great starting point for me to dive in deeper about TSD.  
        After researching the facts of TSD, I started to wonder about the effects of climate change on the sex ratio of sea turtles. With temperatures rising, will we have more females, leading to less genetic diversity? After some research, I found that I was not the only one with this concern. It is possible that sea turtles won’t be able to adapt to increasing temperatures fast enough, and we could end up with only female sea turtles, leading to extinction. However, some animal species have already started to change their behavior by nesting earlier in the year in order to preserve the sex ratio! It is believed that increasing temperatures might also start to affect the nest choice of the turtles, meaning that they will choose a nest site that stays cooler. So, maybe the turtles can overcome hot temperatures! Another more positive outlook on temperature effects is that maybe within a certain range, maybe having more females will help sea turtles increase their population size :) .  
     On a less positive note, the interaction between TSD and climate change is fascinating, but the current problem with increasing temperatures has nothing to do with the sex ratio of sea turtles. Right now, a large percentage of eggs/hatchlings are dying due to heat stress. The immediate temperature threat we are seeing is nests getting cooked, not the sex ratio. In some places with high temperatures, turtle researchers (and turtle patrols!) are finding new ways to keep the nests cool to avoid heat stress, such as adding shade. I think that’s pretty cool! 


Sources: 
https://oceanservice.noaa.gov/facts/temperature-dependent.html#:~:text=Most%20turtles%20are%20subject%20to%20temperature%2Ddependent%20sex%20determination.&text=Research%20shows%20that%20if%20a,the%20hatchlings%20will%20be%20female.  
https://oceana.org/blog/rising-temperatures-are-cooking-baby-sea-turtles-their-nests-8 
https://onlinelibrary.wiley.com/doi/10.1002/bies.202000146 
https://en.wikipedia.org/wiki/Temperature-dependent_sex_determination