Data from: State-dependent behavior alters endocrine-energy relationship: implications for conservation and management

Glucocorticoids (GC) and triiodothyronine (T3) are two endocrine markers
commonly used to quantify resource limitation, yet the relationships
between these markers and the energetic state of animals has been studied
primarily in small-bodied species in captivity. Free-ranging animals,
however, adjust energy intake in accordance with their energy reserves, a
behavior known as state-dependent foraging. Further, links between
life-history strategies and metabolic allometries cause energy intake and
energy reserves to be more strongly coupled in small animals relative to
large animals. Because GC and T3 may reflect energy intake or energy
reserves, state-dependent foraging and body size may cause
endocrine-energy relationships to vary among taxa and environments. To
extend the utility of endocrine markers to large-bodied, free-ranging
animals, we evaluated how state-dependent foraging, energy reserves, and
energy intake influenced fecal GC and fecal T3 concentrations in
free-ranging moose (Alces alces). Compared with individuals possessing
abundant energy reserves, individuals with few energy reserves had higher
energy intake and high fecal T3 concentrations, thereby supporting
state-dependent foraging. Although fecal GC did not vary strongly with
energy reserves, individuals with higher fecal GC tended to have fewer
energy reserves and substantially greater energy intake than those with
low fecal GC. Consequently, individuals with greater energy intake had
both high fecal T3 and high fecal GC concentrations, a pattern
inconsistent with previous documentation from captive animal studies. We
posit that a positive relationship between GC and T3 may be expected in
animals exhibiting state-dependent foraging if GC is associated with
increased foraging and energy intake. Thus, we recommend that additional
investigations of GC- and T3-energy relationships be conducted in
free-ranging animals across a diversity of body size and life-history
strategies before these endocrine markers are applied broadly to wildlife
conservation and management.