Energy is the currency of life. Whether you're embarking on a 10,000 km migration or deciding whether it's worth catching the next meal, it's a matter of making sure that the energetic balace tallies up. Over the years biologists have come up with various ingenious ways of measuring energy expenditure,including measuring heart rate. But some techniques can be tricky to use in the field, which led Rory Wilson and a team based in Birmingham to come up with an alternative method: accelerometry (Wilson et al., 2006, J. Animal Ecology 1081–1090). Having shown that it is possible to measure an active animal's energy consumption from its acceleration while moving, Lewis Halsey and Jonathan Green wondered whether the method could also produce reliable estimates of energy consumption when an animal is stationary, and old English bantam chickens proved to be the perfect animals to test their ideas on (p. 471).
`The chickens were very cooperative; they are happy sitting still,'explains Green, who had access to an entire flock of them while working in Peter Frappell's lab in La Trobe University, Australia. Halsey travelled from the UK to join Green with some of Rory Wilson's accelerometers, and the team were ready to compare the bird's predicted energy expenditures, based on their acceleration, with their predicted energy expenditures based on their heart rate.
First they set the birds a jogging test on a treadmill. Fitting the chickens with a heart rate monitor, the team measured the birds' oxygen consumption (a measure of the amount of energy expended) as they trotted along. Then they swapped the heart rate monitor for an accelerometer and repeated the experiments. Calculating the birds' overall dynamic body acceleration (ODBA) from the acceleration traces, Green and Halsey found that it agreed well with the animals' exertions predicted by their heart rate.
But what happened when the birds exerted themselves during more static activities? After giving the chickens a hearty meal and switching the lights off to settle them down, the team recorded the animals' heart rates, body accelerations and oxygen consumption rates as the birds ramped up their metabolic rates and got on with digestion. The team also tested whether accelerometery could tell them anything about the birds' energy expenditure when they cooled the chickens down and warmed them up.
At first glance it was clear from the acceleration traces what activities the birds were up to, but the energy calculations based on the acceleration patterns didn't seem too accurate. However, when the team looked at the margin of error of the calculated energy expenditure, Green and Halsey realised that the acceleration data could provide a realistic estimate of the animals'energy expenditure. Green explains that when the birds were inactive, the error margin on energy expenditures based on heart rate was 6%, while the error margin from estimates based on the animals' weight was 20%. However,when the team calculated the error margin based on the animals' acceleration data, it was only 12%, significantly better than `guestimation' from the literature, and not far off the errors from heart rate measurements.
So it is possible to use an animal's acceleration pattern to identify their activity pattern and estimate their energy expenditure, whether they are on the move or digesting dinner. And Green suspects that there are many more acceleration data sets out there that could tell us a lot about animal activity and energy expenditure patterns in the wild.