Making overall dynamic body acceleration work: on the theory of acceleration as a proxy for energy expenditure

1. The energetic costs of different behaviours are critical in modulating the behavioural ecology of free-living animals. Despite this, measurement of energy expenditure in the field has proved difficult. 2. A new method with broad application for field studies has been proposed for determining the rate at which animals expend energy, based on measurements of overall dynamic body acceleration (ODBA) through the attachment of miniature acceleration data-loggers. This technique is easy to implement and has the promise to be able to resolve energy expenditure with fine (sub-second) temporal resolution, making it the only method which promises to able to determine the cost of short-lived behaviours. Increasing evidence supports the validity of the approach although the rationale behind it is vague. 3. This study explores link between metabolic energy and acceleration by examining what is known about how muscular tissue converts metabolic energy to mechanical work via muscular contraction and how Newtonian physics facilitates a derivation of Power (the rate at which work is performed) from acceleration. The link between metabolic energy and acceleration appears to involve three discrete processes: (i) the ratio of mechanical work to metabolic work performed by a single muscle (mechano-chemical efficiency); (ii) the ratio of external and internal work performed (mechanical work of the limbs in relation to that of the centre of mass); and (iii) the ratio of inertial to de novo mechanical work. These processes may vary according to the animal's mass, the medium in which it travels and its gait or behaviour. 4. Assessment of movement has limited application in defining non-movement energy expenditure such as that involved in specific dynamic action or non-shivering thermogenesis. However, this non-movement energy expenditure may often be modelled with reasonable confidence. The utility and appropriateness of the ODBA-energy expenditure method depends on a set of conditions, which we define and suggest should be assessed a priori. 5. This study explores the framework behind the ODBA-energy expenditure method to enable informed decisions to be made regarding the suitability for specific research questions addressed, as well as highlighting calibration needs.