Climate change at loggerheads

In the late 16th century, “logger” referred to a large block of wood, so a loggerhead would describe a large and presumably hard head. Whether loggerhead turtles got their names from this source is not clear, but it is true that they are endowed with unusually large heads and powerful jaws that are ideal for crushing shellfish such as crabs, whelks, clams and mussels. Though they are still relatively abundant, many populations worldwide are declining, and listed as near threatened, vulnerable or endangered.


credit: Jaymie Reneker

Conservation biologists classify species in accordance to their probability of going extinct. The table below shows the classification definitions used by the International Union for Conservation of Nature (IUCN).

Extinct No known individuals remaining
Extinct in the wild Only survivors in captivity or outside their naturalized range
Critically endangered Extremely high risk of extinction in the wild
Endangered High risk of extinction in the wild
Vulnerable High risk of endangerment in the wild
Near threatened Likely to become endangered in the near future
Least Concern Species is widespread and abundant

The future survival of loggerhead turtles demands a concerted effort by humans to preserve nesting habitat along coastlines, and to avoid catching them in gill nets and longlines that are intended for other species. Hatchlings, usually about 100 per nest, are particularly vulnerable, and will move very rapidly from their nest along the beach into the water, and swim away from the shoreline without resting for several days. Most don’t make it, becoming dinner for many different predators, including a variety of shorebirds. Those that survive may, later in their lives, migrate over 12,000 km between nesting beaches and feeding grounds.

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credit both photos: Jaymie Reneker

As an undergraduate at Iowa State University, Jaymie Reneker worked with Dr. Fred Janzen on painted turtles along the Mississippi River. Janzen was interested in how climate change might affect sexual determination in his turtles, so when Reneker moved to the University of North Carolina to do Master’s research on loggerhead turtles with Dr. Stephanie Kamel, she wanted to know how climate change might be affecting sexual determination  in loggerheads. Reneker and Kamel knew from several previous studies that sex in loggerheads is determined by the egg temperature during the middle third of the approximately 60-day incubation period. In their study area on Bald Head Island off the North Carolina coast, eggs incubated above 30 ° C were primarily female, while eggs incubated below 28 ° C were primarily male. Females bury their eggs in the sand, and leave them to incubate on their own, so egg temperature is strongly influenced by the temperature of the surrounding sand.


credit: Jaymie Reneker

Egg temperature also determines how rapidly the eggs develop, with the result that rapidly developing eggs are primarily female, and slowly developing eggs are primarily male. This relationship is very useful, because as a threatened species, the researchers were legally barred from any actions that might harm the hatchlings. So they used the interval between egg laying and hatching (incubation duration) as a proxy, or substitute, for actual measures of sex-ratio (sex of loggerhead hatchlings can only be determined by highly invasive techniques).

Reneker and Kamel reasoned that climate change over the past 25 years would be associated with increased air temperature, increased sand temperature, shorter incubation duration, and thus a more female-biased sex ratio. The Bald Head Island Conservancy has monitored loggerhead nests since 1991 by patrolling the beaches with trained teams of observers every night during the nesting season, and surrounding each nest with a metal cage to protect it from predators. Temperature data were collected from a nearby weather station. Lastly, Reneker and Kamel estimated sex ratios using data from several earlier studies that correlated incubation duration with actual sex determination based on gonadal dissection of individual hatchlings.

The researchers discovered that air temperature has increased almost 2 ° C between 1991 and 2015 at Bald Head Island.


During that same time period, mean incubation duration has decreased about 7 days, leading to a sharp increase in the estimated % female hatchlings from about 40% to almost 70%. While Reneker and Kamel expected a decrease in incubation duration, they were startled to observe such a dramatic decrease.

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The researchers point out that global temperatures are expected to increase substantially over the next century, which they fear may lead to a continued and more pronounced feminization of this species. Over time, there may simply not be enough males around to fertilize all of the eggs. If this happens, the loggerhead turtle populations may become more critically endangered, and on the road to extinction.

note: the paper that describes this research is from the journal Ecology. The reference is Reneker, J. L., and S. J. Kamel. “Climate change increases the production of female hatchlings at a northern sea turtle rookery.” Ecology 97.12 (2016): 3257-3264. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2016 by the Ecological Society of America. All rights reserved.