ENVIRONMENT

At the Pacific Ocean’s largest green sea turtle rookery, a crisis is unfolding—up to 99% of the turtles are female. (National Geographic)

Download and print your own coloring page of a sea turtle.

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit.

Discussion Ideas

• The groundbreaking new study examines the sex ratio of a large population of green sea turtles. What are green sea turtles?
  • Green sea turtles are an endangered species found in tropical and subtropical ocean basins. The largest and most important green sea turtle rookeries in the Pacific are the tiny, uninhabited coral islands of the northern Great Barrier Reef off the east coast of Australia.

• The great Nat Geo article begins “The turtle wranglers landed on Ingram Island thinking about sex and heat.” Why were they thinking about sex and heat?
  • Scientists were collecting data on the possible impact of climate change on the sex of green sea turtles in the northern Great Barrier Reef. To do this, they were counting and comparing the numbers of female and male turtles in the region.
    • “Since the sex of a sea turtle is determined by the heat of sand incubating their eggs, scientists had suspected they might see slightly more females. Climate change, after all, has driven air and sea temperatures higher, which, in these creatures, favors female offspring.”
    • Since turtles return to the vicinity of the beach on which they hatched to breed, tracking turtles on islands in the northern Great Barrier Reef allowed scientists to track a large, specific population over decades. (This is great for research.)

• How do scientists determine the sex of sea turtles? (No, looking at their underbellies won’t help.)
  • physical characteristics. As adults, males are generally bigger, and have longer tails and larger claws on their front flippers. This is not always a reliable indicator of sex, and hatchlings all look the same.
    • Determining the sex of hatchlings could be reliably determined by a microscopic analysis of the animal’s gonads, but, as the researchers note, “the sacrifice of live hatchlings carries ethical implications.”
  • DNA. Scientists used an innovative new technique to decipher the sex of marine species based on hormone levels found in the animal’s blood. So turtle wranglers “stood atop skiffs and raced toward swimming turtles and launched themselves like bull wrestlers onto the animals’ carapaces. After gently steering each turtle to shore, they took DNA and blood samples, and made tiny incisions to inspect turtle gonads.”

• What were the results?
  • Females outnumbered males by at least 116 to one. The disparity was more pronounced among younger generations.
    • 86.8% of adult-sized turtles
    • 99.8% of subadult turtles
    • 99.1% of juvenile turtles
  • One site, Raine Island, has been producing almost exclusively female turtles for at least 20 years.

• Were any other green sea turtle populations studied?
  • Yes. In addition to rookeries in the northern Great Barrier Reef, scientists studied “turtles hatching from the southern reef near Brisbane—where temperatures have not increased as significantly … There, female turtles today outnumber males by only 2 to 1.”
    • “This combined with some neat modeling shows that cooler beaches in the south are still producing males, but that in the more tropical north, it’s almost entirely females hatching,” says one sea turtle expert. “These findings clearly point to the fact that climate change is changing many aspects of wildlife biology.”

• What does the feminization of green sea turtles in the northern Great Barrier Reef mean for the future of the species?
  • Ultimately “it is unknown how many (or what minimum proportion of) males is sufficient to sustain sea turtle populations.” This sufficient number is called a stable operational sex ratio (OSR).
    • Green sea turtles are polygynous, meaning one male breeds with many females. “Recent studies have used drones to successfully distinguish between male and female turtles at mating areas and shown that the increased breeding frequency of male turtles can result in stable OSRs, even for highly female-biased populations.”
    • Turtles face other population pressures, including hunting, pollution, disease, habitat loss, and accidental bycatch in commercial fishing.
    • Continued global warming could threaten the population—“a rise of just a few degrees Celsius could in many places eventually produce entirely female offspring. That could wipe out whole populations. If temperatures climb too high, things actually get worse; eggs literally cook in their nests.”
    • Local landscape and weather could mitigate the impact of climate change. For instance, “In the Chagos Archipelago in the west Indian Ocean, heavy bouts of sand-cooling rains, shade from the leaves of coastal trees, and narrow beaches that force [turtles] to nest close to water help maintain a healthy ratio of male hatchlings.”
      • This was the chief takeaway offered by the researchers: “Our study highlights the need for immediate management strategies aimed at lowering incubation temperatures at key rookeries to boost the ability of local turtle populations to adapt to the changing environment and avoid a population collapse—or even extinction.”

TEACHERS TOOLKIT

Nat Geo: 99% of These Sea Turtles Are Turning Female—Here’s Why

Nat Geo: Sea Turtle coloring page

NOAA: What causes a sea turtle to be born male or female?

(extra credit, great read!) Current Biology: Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World