Hunters versus Hunted: New perspectives on the physiological costs of survival at the top of the food chain

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 vital predictive power to anticipate mismatches
between a species functional design and human-induced pressures, and allow
for the development of conservation strategies based on how species are
built to survive.