Languishing Leatherbacks

Leatherback turtles, Dermochelys coriacea, are the largest of all sea turtles, tipping the scales at up to 900 kg. Unlike other sea turtles, the leatherback lacks a carapace covered with scutes; instead its carapace is covered by thick leathery skin that is embedded with small bones forming seven ridges running along its back. This turtle has a wonderful set of anatomical and physiological adaptations, such as huge flippers and an efficient circulatory system, that make it a powerful swimmer and deep ocean diver. Males spend their entire lives at sea, while females usually return to their birthplace along sandy beaches to dig nests and lay eggs.


Leatherback female on the beach at Las Baulas National Park. Credit: Karla Hernández.

Unfortunately, from the perspective of conserving awesome animals in our world, some populations of leatherbacks are declining rapidly, and many are now listed as critically endangered by the IUCN Red List. Pilar Santidrian Tomillo wanted to know why leatherback populations in the Eastern Pacific Ocean have declined so much in recent years. Working at Las Baulas National Park in northwestern Costa Rica since 1993, Tomillo and her colleagues have tagged 1927 nesting females so they could measure survival and return rates to the nesting shoreline. They discovered an alarming trend of sharp decline as described by the graph below.

TomilloFig1Tomillo and her colleagues knew that many leatherbacks were killed every year as a consequence of bycatch – capture by fishing nets or lines cast by fishermen who are targeting other species. But leatherback bycatch is very difficult to monitor accurately, as few fishermen keep accurate records of dead turtles, and turtles may die after being entangled and subsequently freed. The researchers also suspected that climate variability could influence leatherback population size. El Niño Southern Oscillation (ENSO) is a large-scale atmospheric system that affects global climate. In leatherback foraging areas, El Niño years are associated with high atmospheric pressure and warm sea temperatures, while La Niña years are associated with low atmospheric pressure and cool sea temperatures. Importantly, cool sea temperatures stimulate upwelling of nutrient-rich water to the surface, increasing production of phytoplankton, thereby increasing the abundance of  jellyfish and other favored leatherback food items. So the researchers hypothesized that the leatherbacks might do better in La Niña years than in El Niño years.

But what do they mean by doing better? There are two important factors influencing population growth: survival and reproduction. Either one could be affected by climate. By recapturing marked individuals, Tomillo and her colleagues were able to measure both survival and one important aspect of reproduction, which is how often females return to lay eggs. Reproduction is a very energetically demanding process for leatherback females, as they must migrate long distances (often thousands of kilometers) from their feeding grounds, and their eggs are large and plentiful, so females require a huge investment in resources to reproduce. Consequently, at Tomillo’s field site, only 4.5% of females reproduced in consecutive years, while the average interval between reproductive events was 3.65 years.

Let’s consider leatherback survival. As you can see from the data below, annual survival probability is very variable from year to year, ranging from about 30% in 2012 to near 100% in several years. Disturbingly, the long-term trend is downward, and the overall mean adult survival rate of 0.78 is very low in comparison to viable populations of sea turtles. If survival rates do not increase, the future is very bleak for this population.

Tomillo Fig4

Annual survival probability of adult females tagged at Las Baulas National Park. Vertical bars indicate 95% confidence intervals.

How does climate variation influence survival and reproduction? The Multivariate ENSO Index (MEI) measures ENSO strength, with positive numbers (X-axis on graphs below) indicating El Niño years (with poor food availability), and negative numbers indicating La Niña years (with good food availability). The researchers found no climate effect on survival (top graph below), but a high reproductive rate associated with La Niña events (bottom graph below).


The question remains, why is survival so low? Climate does not appear to affect survival, so that brings us back to human impact. Tomillo and her colleagues recommend reducing bycatch levels and implementing beach conservation measures to eradicate egg poaching. They also warn us that increases in global temperatures reduce egg hatching success, and pose a severe stress to this and other critically endangered leatherback populations throughout the world.

note: the paper that describes this research is from the journal Ecology. The reference is Santidrian Tomillo, P., N. J. Robinson, A. SanzAguilar, J. R. Spotila, F. V. Paladino, and G. Tavecchia. 2017. High and variable mortality of leatherback turtles reveal possible anthropogenic impacts.  Ecology 98: 2170–2179. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Scientists MARCH against MADNESS (in April)

Back in my formative college years, my friends and I would indulge in many spontaneous gatherings, in which the question of “What is reality” occupied the stage front and center. As consummate dabblers in multiplistic world views, we considered all conceivable answers, and chose none. But time shuffled on, and so did we, to new adventures in which the reality problem no longer seemed so central, nor so puzzling. We recognized that reality is observable, but that your senses might betray you sometimes. That seemed like enough.

Going back a bit before my formative college years, Copernicus upset this sensory world view by publishing (almost on his deathbed) “De Revolutionibus”, which proposed, and gave some evidence for the hypothesis that Earth revolved around a fixed sun (heliocentrism). Originally this provocative alternative was mostly ignored, but many years later it raised some serious issues (particularly for Galileo) as it became more seriously considered. One major objection was that the reality imparted by our senses told us that the world was standing still – otherwise would we not be blown away by a world that was racing around a sun (and rotating at a frenzied pace to boot)? A second major objection was that the consensus of scientists at the time believed that that the world was standing still and occupied the central position. A third major objection was that a heliocentric world view, if taken literally, seemed at odds with some parts of the Bible, in particular when Joshua asked the sun to stand still so the Israelites had more time to deal with the Gibeonites.

geocentric universe

Figure of the heavenly bodies – An illustration of the Ptolemeic geocentric system by Portuguese cosmographer Bartolomeu Velho, 1568 (Bibliotheque Nationale, Paris)

The reality of our senses is a powerful argument. Copernicus, Galileo and scientists to follow would need to come up with a great deal of physical evidence to sway humanity from the commonsense observation that Earth is still. They did so with astronomical observations (aided by the invention of the telescope) and by developing theories of inertia, momentum and gravity, which propelled our understanding of universal laws for many different applications. As the scientific evidence became more compelling, scientific consensus shifted, and now most people accept the Sun as the center of the solar system, with Earth as one of eight (or nine) orbiting planets, even though these people (including many scientists) don’t understand the physics or the underlying mathematics. Lastly, even fundamentalist Christians and Jews can now argue that Joshua was simply using the language of his time when he issued his request to the Sun.

Advancing in time to 1896, Svante Arrhenius published a paper that proposed that atmospheric CO2 levels could influence atmospheric temperatures in ways that are now familiar to us. Like Copernicus before him, Arrhenius’ ideas were initially rejected by most scientists, until new technology was applied to measuring atmospheric CO2 levels, and to developing models of how the atmosphere and climate interacted. Ultimately, these new approaches led to the development of a scientific consensus that climate is changing, that Earth’s surface is warming, and that human behavior is responsible. Over the past 60 years we have measured the changes, we have developed a more coherent scientific understanding of atmospheric processes, we have made mathematical models that generate projections and we have validated these models empirically. Unlike Copernicus and Galileo, we can observe climate change using the reality imparted by our senses (either online, in books, or by journeying to shorelines or to polar regions that are losing their cool). Unlike Copernicus and Galileo, the consensus of the scientific community is in our camp. Unlike Copernicus and Galileo, the Bible does not make any claims about climate or CO2. This reality should be a no-brainer!

But it isn’t, and I lack the omniscience to understand why this reality is being denied. One hypothesis is that the geocentric hypothesis has been replaced by the corporate-centric hypothesis, which states that corporations and their shareholders are at the center of the universe, and that financial earnings are the currency of reality. The power of this system is that these earnings, if they are maximized and judiciously applied, can be used to purchase some people’s perceptions of reality, so that their reality denies the scientific consensus. This new corporate-centric hypothesis denies scientific facts, and downgrades them with alternative facts that claim to be equally valid.

On April 22 we march across the globe to celebrate and affirm the reality of our senses, the truth of our observations, and the beauty of our complicated world. We celebrate a universe with no center, and a world with millions of different species that interact with each other and their environment in meaningful and mysterious ways. We celebrate the pursuit of rational inquiry into the processes underlying these interactions, and the deepening of our understanding of who we are as humans, and how we can, as scientists, apply real knowledge to allow our Earth to flourish.

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.

img_6572  img_3367

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.

incubvsyr       hatchvsyr

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.