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Life

Watch a baby sea turtle being hypnotised so we can weigh it

By Brian Owens

24 June 2016

Close-up of a baby green turtle

You’re feeling very sleepy…

Joel Satore/National Geographic Ark

Baby sea turtles are an energetic bunch. As soon as they emerge from their sandy nests they scramble down the beach and swim out to sea. This frantic burst of activity helps the turtles evade predators, but it can be a real headache for researchers who want to gather measurements from these tiny, squirming subjects.

“We often heard about novice researchers dropping hatchlings,” says Mohd Uzair Rusli, a biologist at the University of Malaysia Terengganu in Kuala Terengganu. A drop in the lab from table height can be deadly, damaging their fragile internal organs.

Mohd Uzair’s team studies how hatchlings use their energy reserves while they escape their nest. This information is important for improving conservation practices, but to get this they need to be able to measure young turtles.

Many animals, including rabbits and sharks, will temporarily freeze as if hypnotised when flipped onto their back – a condition called tonic immobility. But this does not work for baby turtles – when you flip one over, it will frantically try to right itself.

To take simple measurements like weight, most researchers put the hatchlings in a small cup, but they won’t stay in it for long enough to get the highly accurate measurements – to four decimal places – that Mohd Uzair needs for his research. They also won’t stay still for photos or X-rays.

Turtles all the way down

The team has now found a trick for putting the baby turtles into a trance to safely study them. Mohd Uzair had noticed that hatchlings will stop moving if placed in a dark container – probably because they rely on visual clues when moving away from their nest.

He also observed that they would stop moving if stacked on top of one another – reminiscent of the Dr Seuss story Yertle The Turtle. This might be because placing turtles on top of each other replicates the pressure they would feel on their bodies from other hatchlings while still in their nest.

To imitate these conditions, Mohd Uzair tried flipping turtles onto their backs, closing their eyes and gently pressing on their chests with one finger. And it works – the technique makes baby turtles freeze for an average of 25 seconds, long enough to take precise measurements without harming them.

Mohd Uzair developed this method for his work with green turtles, but he thinks it should work on other species, such as leatherbacks. “All of them show the same behaviours, and rely on the same cues during the nest escape process,” he says.

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