Metabolic rate is a critical indicator of animal health, with respiration and heart rate used as common proxies for energetic expenditure. Infrared thermography (IRT) was previously paired with Eulerian video magnification (EVM) to capture changes in temperature associated with exhalation and blood flow for accurate and non-invasive measurements of respiration and heart rate. These techniques have been tested in a range of taxa, but only in controlled settings at zoological institutions and with imaging conducted at short range (~ 1 m distance). This study aims to validate IRT-EVM in wild populations with minimal disturbance and under variable environmental conditions, and to determine the maximum distance that animals can be imaged for accurate vital sign measurements. Infrared videos were taken of seals in two dramatically different environments including northern elephant seals (Mirounga angustirostris, n = 12) in North America and Weddell seals (Leptonychotes weddellii, n = 23) in Antarctica. ‘True’ respiration and heart rates were collected simultaneously by visually monitoring movement of the ribcage/nostrils and stethoscope/ECG readings, respectively. Using IRT-EVM, respiration rate was extracted from 74.3% of individuals while heart rate was measured in 93.3% of individuals, both with high accuracy (mean absolute error, respiration rate: 1.3 brpm; heart rate: 7.4 bpm). Infrared-derived measurements were resilient to individual characteristics (species, body mass, molt status, blubber thickness, age class) but were impacted by wind speed, animal movement, and ambient temperature. Signal noise introduced by wind could be filtered out to still yield accurate vital signs. Camera resolution directly influenced the distance that respiration and heart rates could be measured. In this study, we demonstrate that IRT-EVM is a powerful, non-invasive method for assessing vital signs in free-living pinnipeds. These imaging techniques are likely to be applicable towards study of other mammalian species, with appropriate validations for use in new field environments to ensure accuracy of IRT-EVM derived vital sign measurements is maintained. This work makes strides towards monitoring wildlife metabolic indices to evaluate impacts of intra-annual and longer-term environmental change on population health.
Rzucidlo et al. (Sat,) studied this question.