Gliding through the warm south-western Indian Ocean, green turtles live a peaceful life feasting on seagrass meadows before returning to their deep coral reef homes. But their idyllic lifestyle is under threat from human activity; so understanding how these enigmatic creatures stay in tune with their environment is becoming increasingly important. ‘If they don't balance their energy budget there is going to be a problem for the individual and later on at a population level,’ explains Manfred Enstipp from the DEPE/CNRS Laboratory in Strasbourg, France. However, conventional methods for measuring energy expenditure may be unreliable in turtles, so Enstipp and Jean-Yves Georges decided to try another approach: measuring the animals' movements with accelerometers to estimate their energy use. First, the team had to discover whether there was a measurable relationship between the turtle's acceleration and their oxygen consumption in order to extrapolate the animals' energy use from their activity ().
However, when Enstipp and his colleagues embarked on the study they weren't even sure that they could measure the turtle's oxygen consumption rate. ‘They had to find this hole that is 40×40 cm to breathe so that we could measure the oxygen consumption rate,’ explains Enstipp, but many of his colleagues thought that the freely swimming animals could not be trained to surface repeatedly at a single point in long channel.
Travelling to Stephane Ciccione's Kelonia Turtle Aquarium on Reunion Island, Enstipp isolated a 13 m-long swim channel in the turtles' enclosure and then, over a period of months, he and Benoit Gineste slowly covered the channel's surface with sections of metal fence until the turtles were content to surface and breathe at the respirometry dome, which covered the channel's only remaining open surface. Despite his colleagues' misgivings, Enstipp found that the turtles successfully learned to surface and breathe at the respirometry dome. Then, Enstipp velcroed a two-axis accelerometer onto a turtle's shell before recording her oxygen consumption and movements as she swam to and fro.
Returning to Reunion Island with Gineste, Myriam Milbergue and Virginie Plot in the summer and winter, Enstipp successfully measured the oxygen consumption rates of six adult turtles at temperatures ranging from 24.8 to 30.1°C while also measuring their acceleration patterns. But could he use the acceleration traces to accurately estimate the turtles' oxygen consumption?
Developing an equation to calculate the turtles' oxygen consumption based on their acceleration and the temperature of the surrounding water – which determines turtle metabolic rate – Enstipp calculated the turtles' oxygen consumption rates and compared them with the animals' measured oxygen consumption rates. They agreed well: the team could use the turtle's acceleration pattern to calculate their oxygen consumption, which they could use as a proxy for the animal's energy consumption.
But how does the captive turtle's activity level compare with that of free-ranging green turtles grazing their seagrass meadows? ‘We are trying to apply this to Katia Ballorain's data from turtles foraging around Mayotte Island,’ Enstipp reports. However, early evidence suggests that the captive turtles were no couch potatoes and were every bit as active as their free-ranging cousins, offering Enstipp and his colleagues hope that accelerometery could provide much needed measurements of green turtles' energy consumption in the wild.
