No quantification without calibration – estimating energy expenditure

Prof. Lewis Halsey, Environmental Physiologist in the Life Sciences Department at the University of Roehampton, discusses with us his recently accepted article, “Proxy problems: why a calibration is essential for interpreting quantified changes in energy expenditure from biologging data”, describes his research interests, and provides advice to fellow ecologists.

About the Paper

Every time an animal does anything, from moving around to reproducing to fighting off infections, it expends energy. Yet animals don’t have unlimited energy reserves – like spending money too quickly, it will run out. Thus, how much energy animals expend on all their different ‘costs of living’ is arguably crucial to their lifetime success – how long they survive and how many offspring they sire. Again, similarly to money, some animals will make their energy reserves go a long way while others are less energy efficient. Unfortunately, measuring an animal’s rate of energy expenditure, particularly in the wild, is next to impossible. What researchers do instead is measure a proxy on the animals they are studying – something that relates to the animal’s energy expenditure – to get an approximation. An intuitive example of a proxy for energy expenditure is heart rate, which as we all know from our own experiences tends to increase when the body is working harder and thus expending more energy. Another energy expenditure proxy that researchers are making use of is termed ‘overall body movement’. When the body works harder physically, such as during exercise, the amount of movement overall by the body increases, as does energy expenditure. These proxies of energy expenditure are measured in wild animals by instrumenting them with a small device that records heart rate or body acceleration.

Figure 1A from the publication. Examples of relationships for various animals between rate of energy expenditure (oxygen consumption) and a proxy (body motion, represented by the derivative ‘overall dynamic body acceleration’) during terrestrial locomotion. For every species, the relationship does not pass through the origin and thus is not proportional.
Figure 1A from the publication. Examples of relationships for various animals between rate of energy expenditure (oxygen consumption) and a proxy (body motion, represented by the derivative ‘overall dynamic body acceleration’) during terrestrial locomotion. For every species, the relationship does not pass through the origin and thus is not proportional.

However, at this point it is easy to make a mistaken assumption about how energy expenditure can be interpreted based on these proxies, and scientists often fall into this trap. That is, they assume that the relationship between these proxies and energy expenditure is proportional – for example, a doubling of heart rate represents a doubling in energy expenditure. Unfortunately, the relationships between energy expenditure and proxy, though often linear, are not so simple. This has led researchers to make mistakes about how energy expenditure in the species they are studying changes based on changes in heart rate or body movement. Dr Caleb Bryce and I wrote our paper to highlight this fallacy. First, we generated models based on published data to explore the lack of proportionality between actual energy expenditure and proxies of energy expenditure. We assessed the likely scale of the errors in published papers that had assumed proportionality. As predicted by our models, the size of the errors in reported in articles that assume proportionality is related to the size of the y-intercept of the relationship between energy expenditure and the proxy. That is, when the relationship between energy expenditure and proxy passes close to the origin then the error based on assuming proportionality tends to be low, but when the absolute y-intercept is large the error can be very considerable; unfortunately, the latter is most common. We hope that in the future, scientists will recognise that they can’t assume what the relationship is between their proxy and energy expenditure – they will need to have undertaken calibrations between these two measures, probably in the laboratory.

Undertaking calibrations is not easy – it requires simultaneous measurements of both energy expenditure and the proxy, and preferably while the animal is exhibiting a range of behaviours and states. After all, if energy expenditure was easy to measure, scientists would simply measure it instead of a proxy! The logistical difficulties of obtaining calibrations perhaps sometimes encourages scientists to collect ‘just’ measures of the proxy, and in turn to over-interpret them. We raise in our paper the possibility that when heart rate is the proxy of choice, energy expenditure can be predicted from measures of heart rate simply by knowing the animal’s heart mass or even body mass, thanks to all the previous heart rate-energy expenditure calibrations that have been undertaken on various bird species over the years. This doesn’t yet work for body movement as the proxy, but maybe it could become possible in the future as the data comes in for more species.

About the Author

What’s your current position?

Figure 1A from the publication. Examples of relationships for various animals between rate of energy expenditure (oxygen consumption) and a proxy (body motion, represented by the derivative ‘overall dynamic body acceleration’) during terrestrial locomotion. For every species, the relationship does not pass through the origin and thus is not proportional.
Author Lewis Halsey, with his 4-month old daughter, Charlotte. Photo by: Lewis Halsey

I’m a Professor of Environmental Physiology in the Life Sciences Department of the University of Roehampton, in London. I teach our undergraduate students basic physiology, and inferential statistics. My research centres around issues of energy expenditure in animals – how much energy do animals use and in what situations, what adaptations do they have to minimise energy expenditure, and what are the ‘breaking points’ when energy expenditure cannot be sustained by energy intake. I’ve been lucky enough to work on a menagerie of species, both in the field and in the lab, from lobsters to penguins to coypus, but I often turn to our own species – Homo sapiens – as a tractable model for investigating all sorts of fundamental energetics questions.

One piece of advice for someone in your field…   

Apply your skills broadly where you can. Don’t define yourself as a certain type of ecologist whose interests are in a specific sub-field and therefore that’s where all your research focus goes. Rather, look to collaborate with people from across ecology and potentially far beyond, bringing your skills to the research table. Indeed, the further away from your research niche you stretch, the more impressive your skills will look to your collaborators! There’s nothing wrong, and a lot right, with having a research ‘portfolio’ that includes specialisms in some areas, where you’re a leading expert, while also including broad collaborations in fields you know less well but bring something very new to that area of research.

Read the article in full here.

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