Climate Change and Reptile Reproduction

As global temperatures continue to climb due to climate change, animals around the world will face new threats. Among the most animals most vulnerable to increasing temperatures are the turtle and other reptile species whose sex is determined by environmental conditions such as temperature. Unlike in mammals, whose sex is determined before development by their genes, many reptile species’ sex is determined by environmental conditions during development (Moyes & Schulte, 2005). Temperature, for example, determines sex in many turtle, lizard, and crocodile species. This mechanism of sex determination is called temperature-dependent sex determination or TSD (Bull, 1980). Climate change is expected to have drastic impacts on sex ratios in species with TSD and could affect the ability of these species to successfully reproduce and survive (Janzen, 1994a).

How does temperature determine sex in species with TSD?

There are many different patterns of TSD that may alter the effect climate change has on sex ratios. TSD species have a temperature at which an equal number of males and females develop, called their pivotal temperature. In some species, temperatures above the pivotal temperature result in eggs developing into females, as shown in the figure below. In other species, temperatures above the pivotal temperature cause eggs to develop into males. Some species have two pivotal temperatures. When temperatures during development are between the two pivotal temperatures, most eggs develop into males. Temperatures more extreme than the pivotal temperatures in either direction cause most eggs to develop into females (Ewert et al., 1994). In all of these cases, extremely warm temperatures could result in eggs developing into individuals of a single sex, skewing sex ratios and making reproduction impossible.

Higher temperatures result in more male offspring in some species. The star indicates the pivotal temperature.

Temperatures during development affect the embryo’s sex by changing the expression of sex hormones. Hormones are molecules that travel through the body to particular organs or tissues and cause that target organ or tissue to do something particular (Randall et al., 2008). Androgens, including testosterone, are an example of sex hormones that influence sexual maturity and stimulate the production of sperm. Estrogens, which are produced from androgens, are another sex hormone, and they play a vital role in egg development in females. Androgens and estrogens are present and play important roles in both male and female development, but the concentration of these hormones affects sex. Aromatase is the enzyme responsible for converting androgens to estrogens (Moyles & Schulte, 2005). In animals with TSD, the external temperature affects how much aromatase is produced. In red-eared slider turtles (Trachemys scripta), higher temperatures result in a higher expression of aromatase. High aromatase concentrations cause more androgens to be converted to estrogens, causing the egg to develop as female. At temperatures below the pivotal temperature, red-eared sliders have very low aromatase concentrations, which results in high androgen concentrations and male offspring (Ramsey et al., 2007).

Temperatures above the pivotal temperature cause physiological changes that result in development of female red-eared sliders.

What will happen to sex ratios if temperatures continue to rise?

      Based on current predictions for future climate change scenarios, models suggest that many turtle species will face problems within the next decade. Loggerhead sea turtles (Caretta caretta) in the southeastern United States are expected to produce almost exclusively female offspring if temperatures rise as little as 1º C (Hawkes et al., 2007). Painted turtles (Chrysemys picta) are expected to have highly skewed sex ratios with an increase of less than 2º C and offspring will be exclusively female if temperatures increase 4º C (Janzen, 1994a). The good news is that not all nesting areas are affected by temperature changes equally. The amount of vegetation shading a nest and the depth to which the nest is dug can significantly affect nest temperatures, and therefore sex ratios (Thompson, 1988; Janzen 1994b; Booth & Astill, 2001). Nests that are artificially shaded with screens have lower temperatures and more even sex ratios (Hawkes et al., 2007). Protecting nesting sites that have naturally lower temperatures, like heavily shaded sites, and artificially protecting nests from increasing temperatures may help mitigate the effects of increasing temperatures on turtle sex ratios in the short-term. If turtles and other TSD species are to survive long-term, however, we need broader solutions to climate change that will reduce the expected global temperature increases.

Works Cited

Booth DT, Astill K (2001) Temperature variation within and between nests of the green sea turtle, Chelonia mydas (Chelonia: Cheloniidae) on Heron Island, Great Barrier Reef. Australian Journal of Zoology 49: 71-84.

Bull JJ (1980) Sex determination in reptiles. Quarterly Review of Biology 55: 3-21.

Ewert MA, Jackson DR, Nelson CE (1994) Patterns of Temperature-Dependent Sex Determination in Turtles. The Journal of Experimental Zoology 270: 3-15.

Hawkes LA, Broderick AC, Godfrey MH, Godley BJ (2007) Investigating the potential impacts of climate change on a marine turtle population. Global Change Biology 13: 923-932.

Janzen FJ (1994a) Climate change and temperature-dependent sex determination in reptiles. Proceedings of the National Academy of Science 91: 7487-7490.

Janzen FJ (1994b) Vegetation cover predicts the sex ratio of hatchling turtles in natural nests. Ecology 75: 1593-1599.

Moyes CD, Shulte PM (2005) Principles of Animal Physiology. Pearson Education Inc., San Francisco.

Ramsey M, Shoemaker C, Crews D (2007) Gonadal expression of Sf1 and aromatase during sex determination in the red-eared slider turtle (Trachemys scripta), a reptile with temperature-dependent sex determination. Differentiation 75: 978-991.

Randall D, Burggren W, French K (2008) Eckert Animal Physiology: Mechanisms and Adaptations. W.H. Freeman and Company, New York.

Thompson MB (1988) Nest temperatures in the Pleurodiran Turtle, Emydura macquarii. Copeia 1988: 996-1000.

The Trouble with Turtles

If you’ve ever been to John F. Kennedy International Airport, you were probably worried about a lot of things: long security lines, delayed flights, grumpy New Yorkers. All are valid concerns. You probably didn’t spend much time worrying about turtles. Little did you know, turtles make their way onto JFK airport’s runways every year, sometimes delaying flights for hours (Reardon, 2011). The real trouble with terrapins, however, isn’t on JFK’s runways. It’s with their nests. Or really, the lack thereof. Researchers have been studying these terrapin populations for over a decade at Jamaica Bay Wildlife Refuge, a nature preserve just 5 km from JFK in New York. In that time, they’ve observed a 50% decrease in the number of diamondback terrapin nests laid each year (Rubenstein, 2014).

A female diamondback terrapin captured as part of a nesting behavior study at Jamaica Bay Wildlife Refuge.

What’s happening in Jamaica Bay to cause this decline? There are two parts to this problem. First, the whole terrapin population is declining (Reardon, 2011). Fewer females means fewer nests. The second problem is that females are laying fewer nests per year. Female terrapins usually next two to three times a year, but research suggests they may be nesting fewer times per year at Jamaica Bay Wildlife Refuge (Rubenstein, 2014). In both cases, it likely all comes down to one thing: food.

The health of Jamaica Bay, the body of water neighboring JFK airport and surrounding Jamaica Bay Wildlife Refuge, has long been declining. Four different New York City waste water treatments plants feed into Jamaica Bay, releasing tremendous quantities of nutrients such as nitrogen into the water (Benotti et al., 2007). Excess nutrients in the water have caused algae to proliferate, preventing other vegetation from getting the light and nutrients they need. Due to this nutrient pollution, as well as sea level rise, Jamaica Bay’s wetlands are declining at an alarming rate (Rubenstein, 2014). The amount of wetland vegetation in the bay declined by almost 40% from 1974 through 2002 (Hartig et al., 2002). This loss of wetland habitat has led to declines in terrapin’s main food sources: clams and mussels. So instead of eating protein-rich clams and mussels, terrapins are stuck eating algae (Rubenstein, 2014).

Jamaica Bay’s high nutrient levels cause algae to grow rapidly, covering beaches and depriving other vegetation of light and nutrients.

What’s so bad about eating algae? Algae doesn’t provide the same amount of proteins and other nutrients as clams and mussels. Proteins play a vital role in the production of energy and can have a big impact on an animal’s metabolic rate. Metabolic rate is the speed at which chemical reactions, such as the production of energy, occur in the body. All animals require a minimum amount of energy to keep their body functioning and survive. This minimum amount of energy needed, called the basal metabolic rate, is represented by the red line in the figure below. Metabolic rate depends on a number of factors, including temperature. For turtles and other ectotherms, which are animals whose body temperature depends on the external temperature, the minimum amount of energy an animal needs increases with temperature (Randall et al., 2008).

Adapted from:

An animal’s metabolic rate can also depend on the amount of food, and therefore energy, available. Any energy not used up by their basal metabolic functions can be used to fuel processes beyond just survival, such as growth and reproduction. The blue line in the figure above represents the maximum amount of energy an animal can use, called their maximum metabolic rate. The distance in between the basal and maximum metabolic rate represents the amount of energy that can be used for extra activities like growth and reproduction and is called the aerobic scope (Randall et al., 2008).

Since Jamaica Bay terrapins are stuck eating more algae and less protein-rich food, their ability to grow and reproduce may be compromised. All or most of the energy they get from their food may need to be used to maintain their basal metabolic rate just to survive. As a result, terrapins may not have the energy to reproduce as often, causing them to reproduce fewer times each year than in the past. If algae doesn’t provide enough energy for terrapins to meet their basal metabolic rate, individuals may not even be able to survive, which would explain the declines in population size that are also being observed.

If we truly want to understand why New York’s terrapins are reproducing less and less each year, further research about their metabolic needs, food availability, and population size is needed. But based on current evidence, large-scale efforts are needed to minimize nutrient pollution and to  preserve the remaining wetlands in Jamaica Bay if we hope to protect these terrapin populations.

By Becca Czaja 

Works Cited

Benotti MJ, Abbene M, Terracciano SA. 2007. Nitrogen Loading in Jamaica Bay, Long Island, New York: Predevelopment to 2005. USGS.

Hartig EK, Gornitz V, Kolker A, Mushacke F, Fallon D. 2002. Anthropogenic and Climate-Change Impacts on Salt Marshes of Jamaica Bay, New York City. Wetlands 22 (1): 71-89.

Randall D, Burggren W, French K (2008) Eckert Animal Physiology: Mechanisms and Adaptations. W.H. Freeman and Company, New York.

Reardon, Sara. “Why JFK’s Runway Has Turtles All the Way Down.” Science. 30 June 2011. Accessed 15 January 2019.

Rubenstein, D. “A turtle mystery in Jamaica Bay.” Politico. 30 October 2014. Accessed 15 January 2019.