I still remember the first time I saw a real heart. It was in a second year vertebrate zoology course, laying within the chest of a chicken whose breast bone had just been broken open by my lecturer and first mentor. The look of excitement and suspense on his face was echoed in mine and I knew I wanted to study cardiac physiology.
For decades, biologists have been trying to get a glimpse into the secret lives of animals. From their circadian behavioural patterns to their seasonal energy budgets, it’s clear that information regarding the physiological ecology of an individual is best obtained from animals in their natural environment. Hence the burgeoning field of biologging.
Biologging is enabling researchers to gain detailed knowledge about energy expenditure of animals in the field. While accurate methods of measuring field metabolic rate have existed for some time (like doubly labelled water) they only give a broad integration of a snapshot in time. Now we have the ability to measure an animal’s heartbeat, movement and/or body temperature remotely giving us a much more comprehensive understanding of their patterns of energy expenditure.
The principles behind heart rate biologging begin with Adolf Eugen Fick, and his formula that equates metabolic rate to cardiac output. Essentially, this formula explains that any change in heart rate will equate to a change in metabolic rate. However, this relationship is likely to differ under varied physiological states and behaviours, meaning we need to create robust models for each species that we investigate under the range of activities that they perform in nature. The problem then becomes measuring heart rate and metabolic rate simultaneously, which in and of itself is an often difficult feat for many species, let alone under natural behaviours and activities.
During my PhD I investigated the relationship between heart rate and metabolism in tiny, 10 g bats. I had to develop a new method for recording ECGs non-invasively in these small animals so that we could measure their heart rate while they were in torpor, a physiological state these animals must enter almost daily in order to conserve energy and survive. As the traditional methods of measuring metabolism require restraining animals in a respirometry chamber, it makes sense that measures of heart rate in the field often rely on extrapolations from resting data. Yet, even for different sedentary states like torpor and rest, there is a substantial difference in the relationship between heart rate and metabolism, and extrapolations often result in over or under estimates of energy expenditure.
The technology required to measure heart rate on board an animal has been developing for many years. Back in 1972 Scott and Johnson elegantly described the engineering specifications behind their heart rate telemetry device and its application on freely moving chickens. Since then numerous companies and individuals have tried their hand at producing devices to measure electrocardiograms (ECG) and heart rate from animals- with the latest devices weighing less than 1 g (O’Mara et al 2017, Duda et al 2020). Alongside this, advances in our ability to measure metabolic rate in unrestrained individuals means that we can now create more robust models of heart rate and metabolic rate, vastly improving our estimates in the field.
While the field of heart rate biologging is quickly expanding, there remain a few pitfalls and side points that are important to consider, beyond the need for robust models of heart rate and metabolism. First and foremost is the issue of invasiveness of the procedure. For high quality recordings, ECG electrodes must be close to the heart and often under the skin, which can cause unwanted discomfort to the animals. In addition, this requires a basic level of cardiac electrophysiology knowledge; the correct placement of the electrodes is ‘make or break’, and distinguishing ECGs from muscle artifact or poor quality data can be tricky without training. But what good is high quality data if you can’t get your hands on it? Then the second important consideration is the ability to retrieve your data, which often dictates the species that one can work with and incites the debate between telemetry and logging.
With all of these considerations in mind, heart rate biologging has vast applications from ecophysiology to behavioural ecology. Even more exciting is that with the miniaturisation of processors, internal storage and batteries, mean that new wave loggers are capable of combining multiple technologies into one. I, myself have worked with a number of these technologies that combine biopotential signals (ECG or EEG) with triaxial accelerometers, temperature sensors and even GPS, proximity sensor and microphones, all within a single device. Data can be collected for a year or more, and we can know how much energy an animal is expending, where they are doing it, for how long and even with whom.
Now, we are not only able to glimpse momentarily into the lives of animals, but we are offered a ‘bird’s eye view’ over extended life history phases, in a way that would have been impossible 50 years ago
To find out more about biologging, visit the Journal of Animal Ecology’s Special Feature: Biologging. This Special Feature includes novel analyses and insights, syntheses and how-tos, and covers a broad range of biologging technologies used to address a variety of fundamental questions in animal ecology. All papers are free to access.
N. Duda, S. Ripperger, F. Mayer, R. Weigel and A. Koelpin, “Low-Weight Noninvasive Heart Beat Detector for Small Airborne Vertebrates,” in IEEE Sensors Letters, vol. 4, no. 2, pp. 1-4, Feb. 2020, Art no. 6000104.
O’Mara M. Teague, Rikker Sebastian, Wikelski Martin, Ter Maat Andries, Pollock Henry S. and Dechmann Dina K. N. Heart rate reveals torpor at high body temperatures in lowland tropical free-tailed bats R. Soc. open sci. http://doi.org/10.1098/rsos.171359
Scott, N. R. & Johnson, A. T. Telemetry system and heart rate counter for determination of heart rate in small animals. T. ASAE 15, 14-18 (1972).