From electrocardiography to energetics: methods, constraints, and practical considerations for heart rate biologging in small animals.

Physiological biologging is a flourishing field enabling researchers to answer fundamental questions about animal behaviour and physiology in their natural environment. As biologging devices are further miniaturised their use on small species (< 1.5 kg) is increasing-making this a pivotal time to review how this technology is being used, and what technological limitations and practices users must be aware of, to answer questions in small animals. Here we focus on the applications of devices that measure heart rate and their use as a proxy for energy expenditure in small terrestrial species. We outline essential considerations for programming (i.e. ECG sampling frequency) and placement of devices, and strongly advocate for the verification of heart rate measurements via ECG analysis where possible. As devices become smaller, cheaper, and more effective there is a growing capacity for integration of multiple sensors on a single device. This has broad implications for the study of energy budgets, stress and conservation management in small species. We encourage users of biologging technology to thoughtfully consider our recommendations to make the most of what this technology can offer.