Male box turtle. Credit: Constance McGuire
As global temperatures rise, male turtles may start to lose their cool.
Many reptiles, including most turtles, display temperature-dependent sex differentiation. In other words, the temperature that an egg incubates at determines the sex of hatchlings. In the case of turtles, warmer temperatures yield more female offspring, and cooler temperatures yield more males.
A new study by Stephanie Kamel and Nicholas Mrosovsky of the University of Toronto documents the peril of Caribbean hawksbill sea turtles as coastal forests disappear—and with them, cool, male-producing nesting sites.
“Long-term sex-ratio bias is a real concern for sea turtles,” Kamel said.
Worldwide, a population bias already exists against male sea turtles. Largely female sea turtle populations are common, whereas male-skewed populations are almost unknown.
In an effort to understand what nesting conditions produce a balanced sex ratio, Kamel and Mrosovsky logged temperature fluctuations at four types of sea turtle nesting sites in Antigua: an open beach, in low vegetation, in the forest, and along the forest edge.
Their temperature data suggest that only eggs laid in the forest itself are cool enough to produce male turtles—nests laid in all other environments produce exclusively female hatchlings. That means that as more Caribbean forests are cleared to make way for new resort development, fewer turtle nests will be able to produce male turtles.
“Deforestation is a proximal threat because it can happen rapidly,” said Kamel, “especially in the Caribbean, where so much development is going on.”
Climate change compounds the threat of localized habitat destruction for these sensitive reptiles. Global warming predictions range from a 1° to 6° Celsius mean temperature increase over the next century, and studies have shown that as little as a 1° increase in mean incubation temperature can dramatically reduce the ratio of male turtle hatchlings to female. A 4° increase effectively eliminates male turtles from a clutch.
Some turtles have responded to recent warming trends by nesting earlier in the season. Over the past ten years, studies by John Tucker of the University of Illinois and Frederick Janzen of Iowa State University have documented individual turtles nesting between 10 and 21 days earlier.
But behavioral change is limited to individual turtles, and there is no evidence that whole species, or even whole populations, will adapt in this way. According to Janzen, air temperature is only one of many indicators that signal the start of nesting season. Other factors less influenced by global warming, such as ground temperature and day length, ultimately influence how early turtles will start to nest.
“These turtles have a limited amount of behavioral plasticity,” Janzen said, “And then they have to evolve.”
Originally published September 25, 2006