New research suggests that green turtle hatchlings ‘swim’ to the surface of the sand, rather than ‘dig,’ in the period between hatching and emergence. The findings have important implications for conserving a declining turtle population globally.

In a study published in Proceedings of the Royal Society B: Biological Sciences, scientists from UNSW’s School of Biological, Earth and Environmental Sciences used a small device, known as an accelerometer, to uncover novel findings into the behaviors of hatchlings as they emerge from their nests.

Sea turtle eggs are buried in nests 30–80cm deep. Once hatched, the newborn turtles make their way to the surface of the sand over three to seven days. But because this all happens underground, we have very little understanding of the first few days of a hatchling‘s life.

The results provided through this novel method revealed that buried hatchlings maintained a head-up orientation and unexpectedly, moved vertically through the sand by rocking forwards and backwards rather than tipping side-to-side as expected with digging.

“When I visualize a hatchling that has just come out of its egg, it is completely in the dark in its surroundings. There’s no sign to point which way is up toward the surface—yet, they will orientate themselves and move upwards regardless,” says Mr. Davey Dor, who led the study as part of his Ph.D. “Our initial findings and ‘proof’ of this new methodology opens the door for so many new questions in sea turtle ecology.”

How can you study something underground?

The image of newly hatched baby turtles moving enthusiastically across the sand and into the ocean is somewhat familiar. But what happens before then?

Once they emerge from their eggs, hatchlings move through the sand column and eventually emerge on the surface.

“It was about 64 years ago that the period of turtles hatching from their eggs and coming up to the surface was first observed,” says Mr. Dor. “And since then, people have tried different techniques to observe this phase, such as using a glass viewing pane to watch the hatchlings, or using microphones to listen to their movement.”

Each of these previous techniques has come with limitations which means it has remained difficult to study the first few days of life for turtle hatchlings.

“You just don’t think about how much work it takes for these tiny hatchlings to swim through the sand in the dark, with almost no oxygen,” says Associate Professor Lisa Schwanz. “It happens right under everyone’s feet, but we haven’t had the technology to really understand what is happening during this time.”

So Mr. Dor, A/Prof. Lisa Schwanz and Dr. David Booth, from the University of Queensland, set out to explore new ways to observe and research this obscure, little-known process.

Miniature accelerometer backpacks

Accelerometers, which measure changes in speed or direction, have previously been used to study animal movement, behaviors and physiology.

“The simple principle of the type of accelerometer we used is that it measures acceleration from three different angles,” says Mr. Dor. “So it can measure a change in velocity in a forwards and backwards motion, an up and down motion and a side to side motion.”

But until now, an accelerometer hadn’t been used in this context.

This research took place on Heron Island, a long-term monitoring nesting site for green turtles in the southern Great Barrier Reef, where nesting season typically runs from December to March.

“After locating the nests, we waited for approximately 60 days for the eggs to develop,” says Mr. Dor. “Three days before they hatched, we put a device called a hatch detector next to 10 different nests. This unique instrument measures voltage at the nest site and lets us know when the hatchlings had hatched out of their eggs.”

As soon as the team became aware that the eggs had hatched, they carefully dug down into the nest, selected the hatchling closest to the surface and attached a light-weight, miniature accelerometer onto the baby turtle, before placing it back. “We then gently layered the sand back in the way it was found,” says Mr. Dor.

It was then a waiting game to see when the hatchlings emerged. “We checked the nest site every three hours and when they did finally emerge, we retrieved the accelerometer from the hatchling carrying it.”

The accelerometer provided new data on the direction, speed and time it took for the ten hatchlings to emerge. “We analyzed the data and found that hatchlings show amazingly consistent head-up orientation—despite being in the complete dark, surrounded by sand,” says Mr. Dor.

“We found that their movement and resting periods are generally quite short, that they move as if they were swimming rather than digging, and that as they approach the surface of the sand, they restrict their movement to nighttime,” says Mr. Dor.

Conservation and nest intervention

Sea turtle populations are in decline in many parts of the world, with several species listed as endangered. The nesting phase is a major vulnerability for turtle populations and as a result, conservation management often focuses on nest intervention, including relocation, shading and watering.

Nest relocation has been used widely around the world for many years and the practice is expected to continue as the effects of climate change and rising sea levels are affecting turtle nesting. However, factors such as moisture and temperatures in the nest, which can vary when a nest is moved, can impact important performance traits of hatchlings, including their speed and movement.

“Altering nest characteristics, such as substrate moisture and depth, could have consequences for hatchlings that we currently don’t understand,” says Mr. Dor.

“This means knowledge of hatchling behavior in the sand column—and its links to offspring success—is key to future conservation practices.”

While we know that in the scramble across the sand to the water, hatchlings are at great risk from predators, “it’s also true that some hatchlings don’t even make it to that point,” says A/Prof. Schwanz. “We have so little knowledge of what makes one hatchling successfully emerge while another doesn’t, so it’s really important that we figure out what might contribute to this.”

Opening the door to further research

The latest publication confirms that using accelerometers to monitor hatchlings provides many benefits, including data of movement and behaviors, and crucially, the ability to study turtles when our visibility of them is limited.

These findings have also provided new insights and changed previous assumptions about hatchlings’ earliest days in the sand.

“There are lots of factors that we don’t really understand because we haven’t been able to observe this stage of their lives, but we hope this will change as a result of this new method, particularly in answering questions about best conservation practices,” says Mr. Dor.

The following summer, Mr. Dor returned to Heron Island to put accelerometers on multiple hatchlings in a single nest.

“So, using the next year’s data, we’ll get a sense of how coordinated the nests are, because there is a theory about whether the turtles coordinate their movements, or if they have a division of labor,” says A/Prof. Schwanz.

Fans of Clementine, the cat who recently captivated TikTok with her rare eye color, should take note. The piercing golden gaze of cheetahs, the striking blue stare of snow leopards, and the luminous green glare of leopards are all traits that can be traced to one ancestor, an ocelot-like feline progenitor that roamed Earth more than 30 million years ago.

In a new study published in iScience, Harvard researchers say this ancestral population likely featured felines with both brown and gray eyes, the latter paving the way for the rapid and wide diversification of iris color seen in cat species today.

“When I started this study I asked, ‘What do we know about eye color?’ And the truth is, very little, as there are basically almost no phylogenetic evolutionary studies on eye color,” said lead author Julius Tabin, a Griffin Graduate School of Arts and Sciences student in the Department of Organismic and Evolutionary Biology.

Most studies focus on the distribution of eye colors in a species, or on the genes involved in making eye color in humans and domesticated animals. Studies on eye color in animal populations are rare due to the challenges of preservation and lack of diversity—most animals have brown eyes.

While eye color in humans is likely a result of sexual selection, and in domesticated animals a result of artificial selection, Tabin wondered what spurred the wide diversity in wild Felidae. Without fossil preservation to rely on, Tabin took a novel approach by analyzing digital images from sources such as iNaturalist to identify and categorize the varied eye colors in 52 felid taxa.

Tabin and co-author Katherine Chiasson, a Ph.D. candidate at Johns Hopkins University, created an algorithm to map iris colors onto a phylogenetic tree of Felidae.

“We found a lot of variability of color between species,” said Tabin, “but shockingly, we also found a lot of intraspecific variability. Most species have a singular eye color with no variation. So, it’s really surprising that once you get into the cats—lions, tigers, panthers, etc.— we see all these different eye colors. There are actually very few Felidae species that have only a singular eye color in their population.”

Equipped with the colors mapped onto the phylogenetic tree, the researchers set out to reconstruct the ancestral state. They found that early, pre-felid lineages (the ancestor of felids and their closest relatives, the linsangs) had brown eyes only. However, after the linsang species branched off, gray-eyed felines appeared alongside brown-eyed ones.

“It’s likely this happened due to a genetic mutation that drastically decreased the pigment in the eye,” Tabin said. Melanin can be either eumelanin, which is brown, or pheomelanin, which is yellow.

To go from a brown eye to a gray eye would require a decrease of eumelanin. That decrease would lead to an eye that is not fully brown and not fully gray, but a brownish gray color, which is what the researchers found.

Those gray-eyed felines opened the door to a burst of greens, yellows, and blues, providing an anchor between brown eyes and the new colors.

“Blue eyes require carefully balanced low levels of pigment and are likely recessive in felids. A wild population would probably not be able to maintain blue eyes in a population with only one blue-eyed individual among a sea of brown eyes.

“It’s probable that you would need something lighter than brown, but not as light as blue, to be the mediator. And that’s what you see: In every single cat species with blue eyes, they also have gray eyes,” said Tabin.

Tabin and Chiasson also observed that brown eyes and yellow eyes rarely coexist in a species. They said they were surprised to find a positive correlation between yellow eyes and round pupils, and a negative correlation between brown eyes and round pupils.

The researchers found no significant correlations for activity mode, zoogeographical region, habitat, and uniformity of eye color, which leaves the adaptive benefit of having varied eye colors an open question to pursue.

The researchers not only reconstructed the general eye color types present at each evolutionary node, but they were also able to predict the exact color of each ancestor’s eye.

“Being able to reconstruct color quantitatively is one of the paper’s greatest strengths, because it means we are the first animals to see the color of these eyes since these felids were alive millions of years ago,” Tabin said.

For Chiasson the study was special in part because all the resources they used are freely available online. “The fact that rigorous studies like ours can be done by anyone with an internet connection and some curiosity is indicative of a field-wide revolution that is increasing the accessibility of science around the world.”

The study opens an opportunity for more investigation of the evolutionary importance of gray eyes, as well as eye-color evolution in natural populations, Tabin said. “I’m still riding high on the excitement of knowing that the felid ancestor had both brown and gray eyes, because that’s something I didn’t go in expecting or even thinking about.”

This story is published courtesy of the Harvard Gazette, Harvard University’s official newspaper. For additional university news, visit Harvard.edu.