This summer I learned a lot about sea turtle reproduction. It’s not as fun if you’re a female turtle. You somehow mate in the water (difficult I would imagine), but only every 2-4 years (standard?). Then the guy leaves you, and it’s up to you to find a quiet beach somewhere to lay your eggs. You try to come ashore at night to avoid gawking humans, dig a big hole in the sand dune, drop about 100 ping pong ball eggs in it, cover it up, and haul ass back to the ocean. Takes about an hour. Rinse and repeat about six times over a few months.
I witnessed my first such event this summer (the egg laying, not the mating) when a loggerhead came ashore on the beach in the late afternoon. TBH it was quite boring for the most part – just a big shell laying there in a sand dune, followed by an extensive session of sand tossing. While we watched, we were told by the local “turtle patrol” that the gender of the hatchlings is controlled by the temperature of the nest, something called “temperature dependent sex determination”, or TSD. Turns out that when things get hot, there’s less male hatchlings. Kind of like the woodwind sections of your high school band, where there is 1 male (me) for every 35 females (shout out to the 3rd clarinet section, SHS, 1990).
As I watched her disappear in the surf, the meteorologist in me began to wonder if loggerhead turtles are doomed in this era of a warming planet. Are we moving towards a turtle society with a 49/1 female/male ratio? Is this good for anyone (except for the turtle males, of course)?
More than you want to know about sand
Not being a marine biologist, I did a little research. When you’re an egg, you get deposited in a sand hole about 18″-24″ deep. So if the sand encasing you is less than 29 degrees Celsius (about 84 Fahrenheit), you will probably be a dude. Higher than 29 C, female. As most of you know, sand can get pretty hot during the daytime. This is because sand has a low specific heat, meaning that it doesn’t take much energy to raise its temperature. There is another property of sand, though, that limits the temperature change of the nest. Sand has a low thermal conductivity, which means that it doesn’t transfer that heat easily. Long story short, you the egg generally do not experience much change in temperature from day to night or week to week, so your gender will be determined by the average temperature you experience over your 60 days of incubation.
Climate Impact on Turtle Gender
It’s impossible to know how much air temperatures will change at individual turtle nesting sites. But we can choose a reasonable number that will allow us to get some results. A recent study (Gammon et al. 2024) examined over 400 flatback turtle nesting sites in NW Australia to predict how the survival and gender of the hatchlings may change in a 2 degree and 4 degree C warmer world. For a 2 degree scenario (kind of middle of the road), the temperature at nest depth will rise about 1 degree C. With this change, they predict a 12% increase in hatchling mortality and a 70% decrease in male hatchlings. For 4 degrees C (severe but more unlikely warming), they predict a 2.5 C increase in nest temperature, resulting in a 97% decrease in male hatchlings! That’s a lot of clarinet players.
Are Sea Turtles Doomed?
In a overwhelmingly female population, successful mating becomes difficult. I experienced this first hand in college, where the female to male ratio was about 2 to 1. Or maybe the ladies don’t dig meteorologists, I don’t know. But in all seriousness, it seems to me that this could be a difficult era to be a sea turtle. Is there enough “cool beach” nests with large male production to offset the climate warming? Is the runway still long enough so that we can make substantive differences in the forcing and/or mitigation of the outcome? Will meteorologists who play clarinet ever get a fair shake? Stay tuned.