Westward: A Journey to Lay Eggs and Save the Boys
The rising temperature of the Mediterranean is feminizing the loggerhead turtle population, which seeks cooler areas to reproduce and survive.
Twelve grams. That's the weight of a baby loggerhead sea turtle at birth. These chelonian reptiles normally breed on the coasts of the eastern Mediterranean, where both the sea and the coasts where they nest are rising. And this species is highly sensitive to these changes, which result in fewer males being born, which could have a disastrous outcome for the species. To avoid this fate, the loggerhead sea turtle has begun to colonize the west, in search of cooler temperatures for reproduction.
"At incubation temperatures of 29.5°C and above in the sand, the sex ratio changes, and more females than males are born," explains Irene Álvarez de Quevedo, a postdoctoral researcher at the Beta Technology Center. "Currently, populations are becoming feminized because in traditional egg-laying areas, such as Turkey and Greece, which were previously very good for incubation, practically 100% of the eggs are emerging as females due to rising temperatures," he adds.
What most worries the scientific community is not that reptiles can't find a mate, but that "genetic diversity is compromised," says Carlos Carreras, a researcher at the University of Barcelona's Biodiversity Research Institute. "The fewer males, the less genetic material, and that, in the long run, could jeopardize the future of the species," he warns.
Against the heat, migration
As sea turtles seek cooler areas to breed—"A way to achieve a balance between the sexes," Carreras emphasizes—increasingly, more and more turtles from the Turkish and Greek coasts are coming to lay eggs on the Catalan and Valencian coasts. Ten years ago, there were also some on the Catalan coast, but they were few. "We've always had turtles from the Atlantic and the eastern Mediterranean, which came here as juveniles and returned to their beaches to breed when they matured," the researcher points out, adding that, however, migratory habits are now changing and they're coming to stay. "The difference is that now they breed here."
"We're starting to see re-migrating families, that is, they return to the same area for more than one season," says Álvarez de Quevedo. This is the case of the Mascletà, a turtle that came to lay its eggs in 2016, 2020, and now, for the last time, in 2024, on the same beach of La Pineda, in Vila-seca (Tarragona). And this situation is expected to worsen because global and Mediterranean temperatures continue to rise.
"A turtle's track on the beach is very similar to what a tractor tire would leave behind," describes Giralt, a CRAM technician. A perpendicular rut, but "leaving half-moons, which are its fins dragging." In the middle, a flat, compact area, which "represents the weight of this animal, which in the case of females ranges between 70 and 80 kilos."
The difference with tractor wheels is that you see a "single trail coming out of the water and another going in." The second trail can be harder to see. "It can return almost the way it came out or it can go very far, depending on whether there has been disturbance, the beach, and the type of sand."
Although turtles usually emerge at night, Giralt reports that they are also encountering turtles that "emerge at 10 a.m." Therefore, "any trace should not be ruled out, even during the day." The important thing, if one is detected, is to "try to ensure that no one enters the area and call 112."
With an urbanized coastline and a burning sea, the scientific community is closely monitoring these migrations. "What's happening is unprecedented; it's a live colonization. As human beings, we have a responsibility to help species trying to adapt to changes as much as possible, because otherwise, they will obviously end up extinct," warns Álvarez de Quevedo.
In fact, this summer the Mediterranean Sea has been almost flammable. In many places, thermal anomalies exceeded 5°C; that is, the sea temperature was well above the historical average for the same time of year. On June 30th, the Dragonera buoy in the Balearic Islands reached 30.55°C, an absolute record since records began: a temperature more typical of a swimming pool than the open sea.
This summer, 31 nests have been identified on the Mediterranean coast: eleven in Catalonia, fourteen in the Valencian Community, two in the Balearic Islands, two in Murcia, and another two in Andalusia. Nesting in years past was what was called "sporadic," meaning that if a year was warmer, there would be some nests, but this changed in 2014, when it became more frequent.
According to Álvarez de Quevedo, new colonization areas, such as the Catalan and Valencian coasts, "are important because they will produce males, as temperatures are not yet as high as those in Turkey." Last year, on Savinosa Beach in Tarragona, 9 out of 10 turtles that were born were male. This, this researcher points out, is a "species strategy" to ensure its continuity.
"In summer, each female can lay between 5 and 7 clutches, and they are biannual, meaning they lay one clutch every other year," explains Silvia Giralt, oceanographer and technician in the research and conservation area of the Marine Animal Recovery Center (CRAM). "They begin to sexually mature." Only one in every thousand will reach adulthood and reproduce.
Screws, bottle caps, pieces of netting, plastic filaments. All of this ended up in a turtle's stomach. Pepita, a female admitted with a broken shell more than a year ago, expelled more than three cans full of plastic. "More than 80% or 90% of the specimens that pass through the CRAM expel plastics in their feces," Giralt laments. Turtles have an opportunistic diet, meaning they eat whatever they find at any given time. Some plastics are ingested directly, others come from "prey that already contained plastic." And not all are expelled.A study recently published in the journalMarine Environmental Researchshowed significant concentrations of microplastics in the heart and reproductive organs.
The perfect cocktail
Temperature not only affects biological sex, but also incubation time. "The higher the temperature, the faster the incubation," warns Carreras, from the UB. "In Calabria, Italy, turtles are being born after just 45 days of incubation, when the average process is 55."
This population boom is increasing and may clash with highly urbanized and overcrowded coasts. "Tourism is a problem. Often, society doesn't know how to react; it's a relatively new situation and increasingly common," he adds. Therefore, he says, modifications are necessary to adapt to the new situation.
"Perhaps the lights on the seafront promenades should face inward and not toward the beach," he points out. Artificial lighting has a negative effect on animals. "If there's a lot of white light, the hatchlings, when they emerge, instead of following the moonlight and heading towards the water, they head towards the promenade, where there's more light," warns Álvarez de Quevedo.
At the moment, no city council is adapting its lighting, as it's a relatively new phenomenon. In addition to the light, there are nighttime parties and recreational boating. "There's a lot of work to be done, especially in raising public awareness," emphasizes this postdoctoral researcher.