Successful incubation and production of male sea turtle hatchlings is threatened by increased global temperatures. In their latest research, Clarke and colleagues test the efficacy of two potential nest intervention approaches in reducing nest incubation temperatures in a nesting loggerhead turtle population in the Eastern Atlantic Ocean.
Sea Turtles Are Vulnerable to Climate Change
The Intergovernmental Panel on Climate Change (IPCC) predicts increases in global mean surface temperatures of 3.7 to 4.8 °C by 2100. While an overall loss of global biodiversity has been widely predicted, climate change is expected to have a more insidious effect on marine turtles, by skewing the proportion of female and male offspring produced.
Sea turtles lack sex chromosomes. Instead, they have ‘Temperature Dependent Sex Determination’, where females develop at high incubation temperatures and males at low incubation temperatures. Sex ratios of up to 90% female have been documented in many places and future projections predict almost complete feminisation in some populations with warmer temperatures, with serious implications for population viability.
High sand temperatures can also cause embryo mortality – ambient temperatures affect the date when the nesting season starts, the interval between nests, the duration over which eggs incubate, hatching success, and hatchling fitness. In addition, metabolic development of the embryos themselves can increase nest temperatures by around 1°C – termed “metabolic heating”.
There has been a lot of research recently on interventions to mitigate the effects of elevated nest temperatures on sea turtle reproduction. Researchers have investigated the artificial shading of nests, incubation of eggs in laboratories and artificial incubators, sprinkling of nests to cool sand temperatures and even changing the sand on the beach to a lighter colour to reduce the amount of heat absorbed. However, as turtle rookeries are often in remote locations in developing countries, effective management needs to be simple and cheap. It is therefore important to investigate potential mitigation tools for conservationists.
How Can We Help?
In our study, we tested two management tools to try to reduce the temperature of incubating turtle nests and hence mitigate the negative effects of climate change: clutch shading and a new approach – ‘clutch splitting’ – whereby we experimentally halved the number of eggs in a clutch to try and reduce the amount of metabolic heat produced.
We collected eggs from 60 female loggerhead Caretta caretta turtles during nesting on Boa Vista, Cape Verde, and reburied them in a protected hatchery on a beach that forms part of an ongoing turtle conservation programme. We shaded 20 nests with dark fabric material, “split” 20 nests in half before burying (we reburied the remaining eggs elsewhere in the hatchery) and reburied the remaining 20 nests whole as controls.
We also buried a temperature data logger in each nest to monitor incubation temperatures, along with six loggers throughout the hatchery to record sand temperatures. These were used to work out how much metabolic heat was being produced in the experimentally split clutches.
Did It Work?
Yes – average nest temperatures in the nests we shaded and split in half were 1.1 °C and 0.5 °C lower than control nests on average, respectively – which may not sound like a lot, but we estimated it was enough to reduce the sex ratio (the percentage of female hatchlings produced by 24% for split clutches and by 68% for shaded clutches.
Splitting the clutches in half also reduced the metabolic heat produced by the incubating clutch itself by 1.4 °C, but only in the last third of the incubation period. By this time sex determination has already occurred, but the reduction in incubation temperature then may help avoid lethal high temperatures being reached.
Importantly, we found no difference in the hatching success between our treatments, and hatchlings were the same size and mass, and could run and right themselves as quickly between treatments. These metrics are important for avoiding predators and survival after hatching.
Our results show that these simple tools can effectively change the thermal properties of incubating turtle nests at very little cost and investment of personnel and could be useful in the future to mitigate the effects of climate change without compromising hatch success or the fitness of hatchlings.
It is important to have a better understanding of sea turtle population dynamics before we intervene, as sex ratio is such a fundamental life-history parameter. However, as the effects of climate change accelerate, these tools may offer managers effective and easily implementable ways to support conservation of these endangered species.
Read the full, Open Access article Low‐cost tools mitigate climate change during reproduction in an endangered marine ectotherm in Journal of Applied Ecology