It’s Getting Hot in Here: Sand, Sex, and Sea Turtles

Three baby sea turtles finally reaching the ocean at Playa Mazunte, in the southern state of Oaxaca, Mexico (note the little one diving into the wave!)
Credit: Nicole Keeney

It’s never been a better time than 2018 to be a male sea turtle.

You could be the ugliest male turtle in the world, with the most wrinkly neck and unkempt shell, but female sea turtles today are looking at slim pickings for mates — like really slim. In one population in Australia, lady sea turtles outnumber their male counterparts 116 to 1. That’s over 99% female! Less extreme but still notable, another Australian study of 75 turtle populations showed a female to male ratio of 3 females for every 1 male, indicating that the past few decades has seen a drastic shift in favor of majority-female sea turtle populations. Even the ugliest of male turtles are getting lucky in the Anthropocene, and scientists are pointing fingers at the common culprit— climate change. How are warming temperatures literally creating more female turtles, and how can you use that same tactic this Valentine’s Day to create a pretty woman to take to dinner? Keep reading, and I’ll answer one of these questions.

As global warming heats things things up, scientists are observing changes in gender ratios for species that have temperature-dependent sex determination; that is, the gender of the animal is determined by the temperature of the environment its egg is laid in, as opposed to the familiar X and Y sex chromosomes that determines sex for humans and most other mammals. For sea turtles, there exists a specific temperature called the pivotal temperature (around 84° Fahrenheit for most species of sea turtles) at which an equal ratio of male and female turtles will hatch. If the sand is much cooler than 84°, all male turtles will be formed, and if it’s warmer than 84°, females will form. That’s pretty crazy if you really think about it. This process would be analogous to a human fetus’s sex being determined by how warm the mom’s stomach was— maybe you’d wear a wool sweater for 9 months if you wanted a baby girl, or wear a light t-shirt if you wanted a baby boy!

Photo: Volunteers measuring the length of a female yellow-spotted river turtle at a conservation center in the Ecuadorian Amazon. The only way to non-invasively determine the sex of these turtles is by measuring them— juvenile females are slightly larger than males. Although not a sea turtle, this species also has temperature-dependent sex determination, and is seeing similar shifts toward majority female populations due to warmer temperatures of sand on the banks of the rivers.
Credit: Nicole Keeney

The science behind temperature-dependent sex determination has to do with aromatase, a large molecule (a catalyst, for any of my readers familiar with chemistry) that increases the production of estrogen, a hormone responsible for female sex determination. High temperatures in the egg’s environment (sand, in a baby turtle’s case) activate aromatase, causing an increase in the production of estrogen and the formation of female sex characteristics instead of male. As global temperatures heat up, so does the temperature of the sand, which in many places surpasses the pivotal temperature for normal, mixed gender turtle hatches, and creating all-female generations of turtles. While ideal for filming a season of The Bachelor: Turtle Edition, the implications in nature are complicated and concerning. The consequence of majority-female sea turtle populations is that reproduction gets a lot more difficult, and turtles won’t be able to adapt to maintain their population numbers as fast as climate change requires for the survival of the species.

Photo: Female Pacific Ridley sea turtles nesting at Escobilla Beach in the beautiful state of Oaxaca, Mexico.
Credit: Wikipedia.org

So, why don’t sea turtles just go lay their eggs in cooler sand to get more balanced sex ratios of their offspring? The answer lies in an characteristic called natal philopatry, which describes the curious tendency of some species to return to their place of birth to lay their own eggs, even decades after first leaving the beach as a hatchling. This means that female turtles will continue to return to the same beach that they hatched at, regardless of the fact that the warmer sand will give them only female babies, or even kill the developing embryos in the eggs if it’s too hot. It also means that most populations are unlikely to breed with other populations, even if the population doesn’t have enough males, as male turtles tend to hang around and breed near their natal beaches as well.

Sea turtles, however, have been around since the Late Jurassic period, and have evolved to changes in the temperature of the Earth’s oceans and sands for 150 million years— why is now different? The changes in temperature over the past millions of years have occurred at a natural rate, slow enough for sea turtles to evolve, allowing for changes in nesting beaches and breeding patterns to develop as the Earth warmed and cooled over long periods of time. The temperatures of the sand today are warming too fast for our turtle friends to adapt, especially considering how long it takes for a sea turtle to reach sexual maturity, which can take over 25 years! Some scientists and conservation groups, however, are lending a helping hand to turtle populations to help restore gender ratios in new generations, so we may not be doomed to a turtle-less future.

Although the root of our turtle problem problem— climate change— is incredibly complex, the problem itself is really quite simple: the sand is too hot. Everyone knows what to do when it’s too hot at the beach— you find an umbrella or a nice tree and cool down in the shade (or jump in the water, but this isn’t really an option for a turtle egg). Direct intervention from humans is looking like our best immediate solution to our turtle conservation problem, both to restore gender ratios and protect the eggs from natural and human obstacles to survival, including beach erosion, poachers, and animal predators.

Photo: My bucket, full of turtles! Baby sea turtles, just hatched, being transported from their temperature-controlled incubation area to the ocean in Oaxaca, Mexico.
Credit: Nicole Keeney

Mama turtles are hands-off parents. After breeding, they make their way to the beach, spend an hour digging holes with their flippers, lay some eggs in the hole, and then cover it up and head back into the ocean, never to see their eggs again. This means that after the mama turtle leaves, their eggs are left unguarded, allowing for conservation groups to come in and intervene to move the eggs to a safer place and directly control the incubation temperature of the sand they are in. Planting shade trees on nesting beaches is also a possible way to help cool down the sand to ensure development of male hatchlings.

It’s important to know, however, that there really isn’t a catch-all solution. The obstacles that sea turtles face en route to adulthood limit the number of eggs that will survive, the number of baby turtles that will make it to the ocean, and the number of adult turtles that will live long enough to reproduce. It is estimated that only 2 out of every 1000 sea turtles will survive from egg to reproductive age two decades later, a remarkably low offspring survival rate that further complicates conservation and population management strategies.

That being said, helping just one extra sea turtle make it to the sea is a remarkable feat in itself that could really make a difference in sea turtle populations, especially if it’s a male turtle. So plant a tree, watch your step on the beach, and be mindful of the seemingly small changes from global warming that have huge impacts. It’s getting hot in here— lets keep it cool on the beach.

Photo: More sea turtles, en route to the ocean in Oaxaca, Mexico.
Credit: Nicole Keeney

Photo: Me with a turtle in Ecuador, after giving it a bath, measuring its length, determining its sex, and marking its shell for identification. Unsurprisingly, this turtle was female!
Credit: Nicole Keeney

Photo: Protected turtle pond for juvenile yellow-spotted river turtles in the Ecuadorian Amazon, near the city of Tena.
Credit: Nicole Keeney

Bibliography:

https://www.cell.com/action/showPdf?pii=S0960-9822%2817%2931539-7

https://news.nationalgeographic.com/2018/01/australia-green-sea-turtles-turning-female-climate-change-raine-island-sex-temperature/

https://en.wikipedia.org/wiki/Sea_turtle

Nicole Keeney

Nicole Keeney is a second year majoring in Environmental Science, with particular interests in climate change, linguistics, and international environmental issues. Nicole loves to drink lots of tea, read cool books, make stir fry, water her kale garden, study Spanish and Swahili, and go for long walks on the beach. A San Diego native, she’s happy to complain with you about how cold the Bay Area is, or show you her basic succulent collection that she justifies by saying it “reminds her of home.” Find Nicole studying in the sun outside Brown’s Cafe in CNR, cruising around the Berkeley fire trails in the late afternoon, or in the library working on awesome Leaflet blog articles! Nicole Keeney covers environmental science.

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