Effect of climate change on temperature-dependent sex determination

2021-08-24

 |  Science & The Environment

What comes to mind when you think of climate change? You may think of the ozone layer struggling to protect the Earth from cancerous UV radiation; prolonged, drought-inducing heat waves and the deserted towns of the midwest; icebergs melting, polar bears drowning, and flatlands sinking under the sea; the imbalance of males and females in egg-laying populations—wait, what?

What is temperature-dependent sex determination?

In temperature-dependent sex determination (TSD), environmental cues, in the form of particular incubation temperature ranges, triggers the genes that decide the sex of an offspring. 

Oviparous species bury their eggs underground to keep them at a warm, consistent temperature, regardless of the weather conditions above. These eggs are laid with no gender. Depending on the temperature that surrounds them during the thermosensitive stage of incubation (near when they hatch), the sex of the embryo is irreversibly determined. However, within a nest, eggs are often deposited on top of one another and the temperature ranges can be diverse. Even a single degree fluctuation can change the sex ratios of a clutch. 

Patterns of temperature-dependent sex determination (TSD) by Peteruetz/Wikimedia Commons

TSD varies from species to species, but there are two main patterns. In Pattern I, females are produced at higher temperatures and males at lower ones, often observed in turtles. On the contrary, TSD in the American alligator follows Pattern II, where cool and warmer temperatures select for females, while intermediate temperatures select for males. If the nest’s temperature remains consistant throughout, the majority, if not all the offspring, would develop into the same sex. 

Where’s the proof?

The hidden impact of climate change on temperature-dependent sex determination (TSD) in reptiles, fish, amphibians, birds, and insects is widely researched. The resulting plethora of evidence suggests that even slight fluctuations in global temperature patterns can affect TSD. A study published in March 2019 simulated a temperature increase of 5°C on a nest of Painted Turtles in Iowa, a species that typically produces males due to the colder temperatures of the region. Researchers found that 100% of the embryos were feminized. In the natural world, global warming can cause male-producing Painted Turtle nests to produce female offspring. As a result there may not be enough males to fertilize the eggs produced by the females and the population of Painted Turtles will plummet.

Which species will be most affected? 

Although all species that rely on TSD will eventually be affected if global warming continues, reptiles have been an early victim, in particular the eastern-lined skink, whose nests have seen a 1.5°C increase in temperatures from 2009 to 2019 and subsequently produced more females. 

Eastern-lined skink. Courtesy of PxHere Photos.

Likewise, by 2030, the percentage of male green turtles in the population is predicted to drop to 2.4%. Temperatures above 35°C are projected to not only throw off sea turtle clutches, but affect the mortality of their offspring, leading to their imminent extinction. A university study on loggerhead sea turtles nesting in southwestern Florida found that most clutches were only fertilized by one male, decreasing genetic diversity and their offspring’s chances of survival.

Green sea turtle. Courtesy of PxHere Photos.

Along with affecting animals species, plants that determine the sex of their offspring based on surrounding temperatures are also being impacted by climate change. In the last 40 years, the tobacco root plant population from the alpine mountains of North America has become largely male dominant. Although this skewed sex ratio has yielded high seed turnovers, TSD plants will lack female partners to reproduce with, causing a detrimental decline of their populations.

Tobacco root plant photographed by Matt Lavin/Flickr.

However, most alarming is the correlation the United States Bureau of Economic Research observed between hot temperatures and low birth rates in humans. A single hot day with temperatures over 26°C in 2015 was found to be the direct cause of over 1,100 miscarriages eight to ten months later. Additionally, a 2016 study in Japan concluded that extreme temperatures, whether hot or cold, produced a female-biased sex ratio and miscarriages of male newborns. Both studies led researchers to the provisional conclusion that climate change can cause reduced fertility in humans as well.

What are the long-term and short-term impacts of imbalanced sex ratios?

The short-term impacts remain manageable in species that were quick to adapt, such as the Australian water dragon, which burrows deeper into the ground to maintain its nest’s cooler temperature range, and male green, hawksbill, and leatherback sea turtles, who frequently mate with multiple females, due to the decreased occurrence of males in their population. In tobacco root plants, the increase in males has boosted seed production for the time being. However, scientists predict that these benefits will be short-lived and the population will plunge rapidly in the long term from the lack of reproductive partners. Regardless, the long-term prognosis for all TSD species, including humans according to growing evidence, is a grand decline in their populations, and without dynamic behavioural and physiological adaptations in the long run, extinction.

How can we help to inhibit climate change?

    “To survive today, other animals must endure global warming, pollution, and fewer habitats. More tragically, they must endure the silence of human hearts.”

  • Anthony Douglas Williams

Species that rely on TSD are struggling due to our irresponsible actions in the name of a prosperous economy and a comfortable lifestyle. Our industries burn fossil fuels and release 50 billion tonnes of carbon dioxide into the air, a greenhouse gas that traps heat and warms our planet. Nevertheless, these animals and plants are dependent on humans to bring an end to their reign of terror and live with empathy and consideration for life. 

Though personal initiative will have an impact, it takes both personal actions and those of governments and corporations. So on top of taking showers in colder water, shutting down your computer at the end of the day, and choosing renewable energy sources over those derived from fossil fuels, reach out to government officials and organizations to voice your concerns, attend town halls, and actively vote. Break the “silence of your human heart” and contribute in the fight against climate change.

Instagram handle: @bodhana.s