Turtles as climate change story-tellers

Contributed by Nature Seychelles

In Lewis Carroll's famous fantasy tale, "Alice in Wonderland" the sorrowful Mock Turtle, while telling Alice his history, makes this well-known statement, "We called him Tortoise because he taught us". He was speaking about the Master at his school in the sea who was an old Turtle. Among the lessons that Mock Turtle received were "mystery, ancient and modern, with Seaography."

Alice listens to the Mock Turtle's sad tale, illustration by Ryan Durney.

Now, as in Lewis Carroll's tale, the old masters of the sea have a few things to teach us. And although Alice had a difficult time understanding the characters of Wonderland, we are able to make sense of what the turtles are telling us.

Scientists have long recognized sea turtles' sensitivity to weather changes at nesting sites. On beaches where turtles nest, rising sea levels and violent weather can affect nesting areas and impede success.

Significantly, however, is that a sea turtle's gender is determined by nest temperature during development. Turtles therefore are seen as good environmental indicators that can reveal the effects of climate change on the natural world. For this reason many turtle management programs are incorporating temperature measurements into their monitoring. This is the case with Nature Seychelles' monitoring program on Cousin Island, thought to be the longest running program of its kind for hawksbill turtles.

"During this nesting season for hawksbill turtles, we have added the use of data loggers in nests to take nest temperatures," Says David Derand Nature Seychelles' Science Coordinator. "This is an innovative monitoring exercise we are carrying out with technical assistance from Kelonia, Reunion's marine turtle observatory. We believe it will enrich the information we are currently collecting for hawksbill turtles on Cousin and help us improve the chances of this still endangered species. It will also give us information about climatic changes." The data loggers inserted into a nest during laying of eggs are retrieved at the end of the incubation period.

Tail end of a data logger and nesting turtle, photo Liz Mwambui

In humans and in most familiar animals, the gender of an offspring is determined by the genetic contribution of the father. Offspring receiving an X chromosome from the father develop into females, while those receiving a Y chromosome develop into males.

However, in Turtles gender is based on nest temperature. This is commonly referred to as temperature sex determination (TSD). Turtles have a pivotal temperature at which the ratio of males and females is equal. Temperatures above this number result in more female hatchlings and below it, in more males. Therefore, small rises in beach temperatures can result in all-female populations and lower temperatures, all male. A bias towards either gender obviously becomes a problem for breeding of the turtles.

Recently marine scientists have begun to predict that global climate change and rise in temperatures will result in the male numbers in turtles being severely impacted. Furthermore, if temperatures are extremely high, the implications may be more serious, leading to high mortality of hatchlings in affected nests, says the World Wide Fund for Nature (WWF). High egg loss could lead to a decrease in population size, increasing the vulnerability of these species to extinction.

Implementation of temperature monitoring projects helps to assess what the local impacts of global climate change will be on beaches and sea turtles populations. Monitoring information from Cousin will add to management needed to try to mitigate climate changes.