Estimating energy expenditure of sperm whales living in social units

Assessing species' vulnerability to stressors (e.g. changes in prey availability, noise) can be done with bioenergetics models, often within frameworks such as the Population Consequences of Disturbance. However, to successfully quantify the cumulative effects of stressors on individuals it is crucial to understand the link between behavioural change and metabolic costs. Measurements of energy expenditure (e.g. field metabolic rates, FMR) are difficult to obtain for large cetaceans because traditional methods are impractical due to whales' size. Consequently, energy expenditure must be estimated indirectly, using proxies such as respiration rates and overall dynamic body acceleration (ODBA). Here, we estimated daily FMR of sperm whales (Physeter macrocephalus) from social units by combining in situ data with these two methods. The estimated mean daily FMR, including estimated basal metabolic rates (BMR), was 412 MJ/day (95% CI: 262.20-616) using the respiration-based method and 620.5 MJ/day (95% CI: 402-839.3) using ODBA. This study provides the first estimates of daily FMR for sperm whales and revealed that averaged-sized individuals from social units have FMRs between 1.59 and 2.39 times the predicted BMR of similar-sized terrestrial mammals, based on respiration rates and ODBA estimates, respectively. Our findings, combined with data on energy acquisition, can contribute to improving predictions of how environmental stressors impact energy balance, health and the long-term population viability of deep-diving marine mammals.