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Can Sea Turtles Cope with Climate Change?

Adult male turtles may become less abundant as the climate warms, but the population sizes may actually increase because they breed more often than females. Credit: Kostas Papafitsoros

Adult male turtles may become less abundant as the climate warms, but the population sizes may actually increase because they breed more often than females. Credit: Kostas Papafitsoros

By Jacques-Olivier Laloë & Graeme Hays

Rising temperatures due to climate change are skewing gender ratios among turtles and increasing the mortality rate of hatchlings. Can they adapt?

It is now widely accepted that warming temperatures are having profound impacts on a range of ecosystems and animal populations. Reptiles are particularly at risk because much of their physiology is intimately linked to temperature. Most reptiles are cold-blooded, with their body temperature driven by the external temperature.

Many reptiles also exhibit temperature-dependent sex determination (TSD), whereby the sex of an individual is determined not by genetics but by the incubation temperature of the developing eggs. In these species, when an egg is laid the developing embryo has no gender – it can develop into either a male or a female depending on the incubation temperature.

Sea turtles undergo TSD, with low incubation temperatures leading to the production of males and high temperatures resulting in females. Therefore, in the context of climate change, there are concerns that warming global temperatures might produce single-sex sea turtle populations and lead to the extinction of turtle species.

Since many sea turtle populations are already endangered, this topic has attracted major scientific and international attention, and in recent years there have been many breakthroughs in this field of research.

Female-Biased Populations

The discovery of TSD in sea turtles generated huge scientific interest in the 1980s. Soon, scientists started recording primary sex ratios – the number of male and female hatchlings inside the nest– at key nesting sites across the world. When all these data were put together, a pattern emerged: primary sex ratios were skewed in favour of females.

In 2014 our group co-authored a meta-analysis of all published data in Frontiers in Marine Science (http://tinyurl.com/nxb975o). We reported that female-biased hatchling ratios dominated in sea turtle populations globally. In more than half of the records, females outnumbered males three to one. The dominance of female-biased primary sex ratios reiterates previous concerns for sea turtles, especially in the context of warming temperatures.

The study made another key finding. Satellite transmitters that tracked the movement of 15 male and eight female turtles from the breeding ground revealed that the breeding periodicity between males and females differs: whereas many males returned to the breeding ground the following year, no females did.

The study provided empirical evidence that male turtles breed more frequently than females. This was an important result: if males breed more often than females, this leads to more balanced adult sex ratios at the breeding sites. On top of that, a male turtle can fertilise the eggs of several females, which means that a healthy population does not need equal numbers of adult males and females.

Hence, concerns of increasingly skewed primary sex ratios are less acute than previously thought. In fact, a study we published earlier this year in Proceedings of the Royal Society B reported that extremely biased primary sex ratios do not compromise the overall population size at 75 nesting sites across the world (http://tinyurl.com/m7e8uge). This paper showed that some of the biggest sea turtle populations in the world have extremely female-biased primary sex ratios. Warmer temperatures, it seems, may actually benefit sea turtles.

Taking into consideration that male sea turtles breed more often than females, it is possible to model future population growth. Another study we published in Nature Climate Change (http://tinyurl.com/knhfjyg) projected how nest numbers at a globally important sea turtle rookery would change under different warming scenarios issued by the Intergovernmental Panel on Climate Change. This study showed that, all else being equal, climate warming will increase population size: more females are produced at the nesting grounds, and they in turn will contribute to laying more egg clutches in the future.

Projecting this pattern into the future, we predicted that by the middle of the 22nd century the number of females recruited to the adult population would increase by approximately 60%.

Temperature-Linked Hatchling Mortality

Where there has been a lot of attention given to the feminisation of sea turtle populations, there has been less focus on another effect of rising temperature: extremely high incubation temperatures are lethal to developing clutches. A turtle embryo only develops successfully at temperatures between approximately 25–35°C. The concern here is that if incubation temperatures increase as part of climate warming, more sea turtle nests will fail in the future.

In our paper in Proceedings of the Royal Society B, the effect of incubation temperature on both primary sex ratios and hatchling mortality was considered. This revealed that the primary concern for sea turtles should be a potential increase of hatchling mortality rather than feminisation of populations. If incubation temperatures are too high in the future, entire clutches of eggs might fail.

Adapting to Climate Change

Sea turtles evolved hundred millions of years ago, and have survived major climatic changes in the past. Since they have experienced past temperatures that were both warmer and cooler than the present day, this suggests that they were able to adapt to the changing environment and may be able to do so again in the future.

One of the most cited ways in which future warming may be mitigated is if turtles shift their nesting season to times of the year when it is cooler. Such phenological shifts in breeding have been noted before in both sea turtles and other taxa, including birds. The key question here is whether the speed of phenological change is sufficient to ensure that incubation temperatures in the future do not rise to levels that cause massive mortality of sea turtle embryos.

Another way in which sea turtles might adapt to the changing environment is by shifting their nesting sites to cooler geographic regions. However, it is difficult to assess whether such a pole-ward shift is feasible for an animal that returns faithfully to breed at the same site where it had hatched.

Mitigation Strategies

Sand temperatures at nesting beaches are currently being recorded all around the world by sea turtle researchers. These important data help keep an eye on the current state of sea turtle populations. If temperatures are to increase beyond the viable range for embryonic development, it is possible to put in place management strategies.

One primary intervention strategy is the use of shade nets to lower the sand temperature. It is also possible to relocate entire nests to a shaded hatchery where they can be closely monitored.

A better solution might involve planting vegetation at the back of the nesting area to shade entire sections of the beach. This conservation measure has already proven successful at important nesting sites in Florida, where trees have been used to screen nesting beaches from street lights that disrupt hatchlings. Since shading by trees may decrease incubation temperatures by several degrees, care needs to be taken when considering this management intervention to ensure there are no deleterious impacts.

Moving Forward

Sea turtles have recently been listed as one of the ten groups most at risk from climate change. While their life history traits and biology means that they are particularly at risk from rising temperatures, these characteristics also mean that sea turtles are an ideal group to study in order to understand the ecological risks and consequences of climate change. Furthermore, the widespread distribution and charismatic nature of turtles mean that global engagement in addressing the effects of climate change on turtles and their ecosystems can be realised. In this way, sea turtles are truly a great flagship species to rally under for the protection and management of our oceans as a whole.


Jacques-Olivier Laloë and Graeme Hays are based at the Centre for Integrative Ecology at Deakin University.