Evaluation of mammalian endurance using accelerometry <i>in natura</i> : bridging exercise physiology and ecology

The ability to evaluate fatigability during locomotion is crucial in various fields, from wildlife biology to clinical medicine. In wildlife, resistance to fatigue, or endurance, can determine the success of certain predator-prey encounters and underpins the ability of animals to migrate or disperse over long distances. In clinical contexts, endurance provides a reliable marker of physiological function, which could help guide exercise prescriptions and aid clinical decision making. However, current methods do not allow for accurate, non-invasive assessment of physical capacities over extended periods in natural and clinical settings. We propose a method for modelling the intensity-duration relationship based on dynamic body acceleration (DBA) records, from which we derived critical intensity, a key metabolic threshold in exercise physiology that delimits heavy from severe intensity domains. We recorded accelerometer data from 19 free-ranging species (n=272) across a wide interspecific and intraspecific range: from rats (10-2 kg) to elephants (103 kg), including oncology patients to regular runners. The three-parameter hyperbolic DBA-duration model revealed an excellent fit on experimental DBA records (median r2=0.995). By retrieving laboratory estimates of metabolic threshold for 15 species (n=688) from the literature, we demonstrated that critical DBA is a reliable proxy of metabolic threshold assessed in the laboratory both at the interspecific (r2=0.88, P<0.001) and intraspecific (Homo sapiens) levels (r2=0.90, P=0.051). The proposed method opens up new avenues for deciphering interactions among animals and between animals and their environment, through the lens of movement and physiology, but also for individualising the assessment of physical capacity in a clinical context.