After they emerge from their nests, baby sea turtles crawl towards the ocean and basically disappear for the next year or so as they drift thousands of kilometers with ocean currents. Eventually they’ll return to breed, but so little is known about that time in between that scientists call it “the lost years.” Brick-sized satellite tags on older, juvenile turtles have helped scientists puzzle together some parts of their life histories, but until recently, most hatchling studies had to rely on direct observations.
Now, as tracking devices get more and more tiny, researchers are finally able to follow newborn sea turtles on their very first day at sea during what’s called the initial “swimming frenzy.” This hyperactive offshore swimming can last for several days as the hatchlings try to survive predator-rich coastal waters before they can reach the ocean currents that will ultimately transport them to their feeding grounds. The findings were published in Proceedings of the Royal Society B this week.
GEOMAR’s Rebecca Scott and colleagues developed acoustic transmitters that are just 12 millimeters long, five millimeters wide, and weigh just half a gram. They glued these streamlined nano-tags (pictured above) onto 11 newborn loggerheads (Caretta caretta) from the island of Boa Vista in Cape Verde off the west African coast.
In just their first eight hours at sea, some hatchlings traveled distances of up to 15 kilometers. Their average travel speeds more than doubled from the initial 19 to 28 meters a minute up to 40 to 60 meters per minute when they joined the surface current flows passing near their natal areas.
As hypothesized, by the third night, the Cape Verdean hatchlings were essentially inactive at night. Previous work have shown that after the first day or so of the frenzy period, hatchlings become increasingly diurnal. Swimming during the day probably helps minimize predation risks, since many oceanic predators are the most active during twilight and nocturnal periods. Since the glue naturally breaks down, the tags detach from the babies' bellies after a few days.
To follow-up on their observations of swimming frenzy, the team monitored hatchling swimming behavior for up to a week in custom-made swimming pools using activity data loggers and tiny harnesses. Pictured to the right, one of two identical indoor swimming arenas used to monitor the proportion of time the turtles spent swimming in a day. In the lab, the team observed several of the unique, population-specific swimming behaviors they noticed in the field, such as nocturnal inactivity. The researchers believe these innate swimming behaviors are inherited from the mothers and evolved to maximize their survival chances based on local oceanic conditions.
Baby sea turtles who survive the frenzy in the wild eventually get to soak up some sun. Previous work with solar-powered, smartphone-sized transmitters on 17-day-old loggerhead hatchlings from Florida revealed that a few months later -- when they finally make their way to a gyre in the Sargasso Sea -- they get to hang out in floating seaweed mats and dine on crustaceans while staying safe and warm.
Images: 2014 R. Scott et al., The Royal Society