| Summary: | Global biotic and abiotic threats, particularly from pervasive human activities , are progressively pushing large, apex carnivorous mammals into the functional role of mesopredator. Hunters are now becoming the hunted. Despite marked impacts on these animals and the ecosystems in which they live, little is known about the physiological repercussions of this role downgrading from ultimate to penultimate predator. Here we examine how such ecological role reversals alter the physiological processes associated with energy expenditure, and ultimately the cost of survival during peak performance. Taxonomic group, preferred habitat, and domestication affected the capacity of the oxygen pathway to support high levels of aerobic performance by carnivorous mammals. Fear responses associated with anthropogenic threats also impacted aerobic capacity. Allometric trends for three energetic metrics (maximum oxygen consumption, field metabolic rates, and the cost per stride or step), showed distinct trends in aerobic capacity for different evolutionary lineages of mammalian predators. Cursorial canids that chase down prey demonstrated the highest relative maximum oxygen consumption rates (10-25 times resting levels) and field metabolic rates, while ambush predators (i.e., felids) and marine mammals had aerobic capacities that were similar to or lower than sedentary domestic mammals of comparable size. The maximum energetic cost of performance for apex predators depended on whether the animals were hunters or the hunted. Escape responses were exceptionally costly for marine (narwhal, Monodon monoceros ) and terrestrial (mountain lion, Puma concolor) locomotor specialists, as well as semi-aquatic (polar bear, Ursus maritimus ) species; all showed a nearly two-fold increase in peak energy expenditure when avoiding threats. As the duration and frequency of threats to wild species continue to grow, cumulative energetic costs are becoming more apparent. In view of this, attention to the energy demands of apex predators will provide ...
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