Halsey, Lewis; Bryce, Caleb
Animals expend energy for everything they do, from moving around to reproducing to fighting off infections. Yet they have limited energy reserves. Thus how much energy animals expend and under what circumstances is arguably crucial to their lifetime success – how long they survive and how many offspring they sire.
Measuring an animal’s rate of energy expenditure, particularly in the wild, is next to impossible. Instead, researchers measure a proxy – something that relates to energy expenditure – to get an approximation. As examples of such proxies, heart rate tends to increase when the body is working harder and thus expending more energy; so do levels of overall body movement. These proxies of energy expenditure can be measured in wild animals instrumented with a small device that records heart rate or body acceleration, respectively. Unfortunately, the relationship between these proxies and energy expenditure is not simply proportional – it is rarely the case that, for example, a doubling of heart rate represents a doubling in energy expenditure.
By generating models based on empirical data, our research explores this lack of proportionality between actual energy expenditure and proxies of energy expenditure, and assesses the likely scale of the resultant errors in published papers. As predicted by our models, the size of the error in the estimated change in energy expenditure reported in articles that interpret uncalibrated proxy measurements is related to the size of the y-intercept. That is, when the calibration between energy expenditure and proxy has a relationship that passes close to the origin then the error tends to be low, but when the absolute y-intercept is large the error can be very considerable.