Six of the seven sea turtle species are endangered because of myriad factors including climate change, poaching, and habitat destruction. Moving turtle eggs from beaches into hatcheries is a common practice to protect them from some of these threats—at least until they hatch. But a study published June 13 in Frontiers in Ecology and Evolution finds that this off-site or ex situ incubation may negatively affect the developing turtles.
A team of Mexico-based researchers examined 150 sea turtles to determine how ex situ incubation affects female hatchling development. Previous research from this team found that this strategy negatively affects male gonads, brains, body size, and fitness, but they wanted to test whether this applied to female hatchlings too.
The team selected ten random olive ridley (Lepidochelys olivacea) nests to study from a beach in Lázaro Cárdenas, Michoacán, México. “We selected the marine turtles of these species because they are in lesser concern than the other species,” study coauthor Esperanza Meléndez-Herrera, a zoologist at the Michoacán University of San Nicolás de Hidalgo, explains. “But we think that our results can be applied to all sea turtles.”
The team left five nests in place (in situ), and moved the eggs of the other five to a protected hatchery area farther up on the beach. As the turtles emerged more than a month later, researchers selected five females at random from each nest—a total of 50 of the 150 hatchlings—to weigh and measure. They found the in situ hatchlings were on average 1.66 grams heavier and 3.34 millimeters longer than ex situ hatchlings. The 50 selected hatchlings were then euthanized for internal examinations.
Previous research indicated that two parts of reptile brains called the dorsomedial and medial cortices are akin to sections of mammals’ brains that are critical for learning and memory. In mammals, poor environmental conditions during development can disrupt neurogenesis in this region, which can lead to learning and memory disabilities as well as conditions like anxiety and depression, the authors write in their study. So the researchers examined these regions in the hatchlings to see if egg movement had any effects.
They found that, on average, in situ hatchlings had roughly 148 more developed neurons in the dorsomedial cortex and almost 665 more in the medial cortex. In the areas that give rise to new cells, in situ hatchlings had more early-stage neurons than ex situ hatchlings.
The researchers then looked at cell development in the ovaries and again found a higher number of proliferating cells in the in situ hatchlings.
For their last test, the team evaluated motor skills by placing the remaining 100 hatchlings from the 10 nests upside down in a tray of sand and recording the time it took to turn themselves over again. In situ hatchlings averaged 5.49 seconds faster.
“In almost everything we measured, the turtles from artificial nests were much worse,” says coauthor Bryan Phillips-Farfán, a neuroscientist at the National Institute of Pediatrics in Mexico. “So maybe, yes, we’re sort of saving them from many threats, but at the same time, we’re sort of hindering their development.”
While it’s possible that moving the eggs factored into the observed differences, the authors suspect external factors like temperature, moisture, and silt levels played a larger role in the results recorded. Temperature can affect brain, gonadal, and motor system development, the authors write, and the natural nests in the study were cooler than the hatchery—although average temperatures in both environments were higher than the optimal temperature threshold of 35 degrees Celsius. In situ nests were also located higher on the beach in coarser sand with lower silt levels.
Rick Herren, a biologist and project manager at the Sea Turtle Conservancy not involved in the work, writes in an email to The Scientist that the environmental differences between the treatments may have influenced the differing growth patterns, and that it was likely the egg microenvironment rather than the act of moving that led to divergences in the tested variables.
Because governments usually restrict how many endangered animals can be used in research, sample size was a limiting factor of the study. Such restrictions often lead researchers to measure “as many variables as you can possibly imagine,” says Phillips-Farfán, especially if the animals must be sacrificed. “Then you sift through the results and say, ‘oh, okay, yes, this variable is important. This other variable might not be that important’,” he says.
In this case, while the study found different growth patterns for in situ versus ex situ hatchlings, it’s unclear whether those differences will actually have detrimental effects on the turtles as they age, says Jeanette Wyneken, a biologist at Florida Atlantic University who has been researching sea turtles for about 40 years but was not involved in the study.
Ideally, researchers would follow turtles in the wild from egg to death to see how different conditions affect their development. But sea turtles take more than 10 years to mature and live for decades, and there’s currently no way to keep tabs on a single turtle for that long.
Wynecken says the way in which the researchers measured cell birth in the brain was inventive. “It’s a very novel study. Anytime you take a new look at a common procedure and from a different perspective, I think that’s valuable,” she says. However, she adds that, overall, “it was not a particularly well-designed study,” pointing to the small sample size and other details she considers flaws in the approach. She says she’s skeptical of the self-righting test in general, for instance, as it’s a behavior rarely performed by wild sea turtles. And she argues that moving half of the eggs in a nest instead of comparing wholly moved or not-moved nests would have been a “much better and much more compelling design,” as it would have controlled for possible genetic differences among nests.
Herren says he considers the work “important in that it adds further evidence of the potential detrimental effects of putting nests in hatcheries and suggests future research on embryonic development and the nest micro-climate.” Such research could be used to improve hatcheries and reduce their effects on development.
Even with the current caveats, moving nests “can be better than the alternative,” Herren says. The Sea Turtle Conservancy uses hatcheries in Panama to protect nests from poachers, he points out. In the past, he directed nest research and monitoring projects, which involved moving nests on beaches in Florida and Georgia. Those eggs were deemed unlikely to survive if they were left because of erosion, beach traffic, nest predation or beach construction.
As climate change warms beaches across the world, killing eggs and skewing temperature-dependent sex ratios, Phillips-Farfán says “we might be thinking . . . maybe it’s time to actually move every single nest that we can to hatcheries.”