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.糖皮质激素（GC）与三碘甲状腺原氨酸（T3）是两类常用于量化资源限制的内分泌标志物，然而目前针对这两类标志物与动物能量状态之间关联的研究，主要集中于圈养小型物种。但野外自由活动的动物会根据自身能量储备调整能量摄入，这一行为被称为状态依赖型觅食（state-dependent foraging）。此外，生活史策略与代谢异速生长的关联，使得小型动物的能量摄入与能量储备之间的耦合程度高于大型动物。由于GC与T3或可反映能量摄入或能量储备，状态依赖型觅食与体型大小或会导致内分泌-能量关联在不同类群与环境中存在差异。为拓展内分泌标志物在大型野外动物中的应用潜力，我们针对野外自由活动的驼鹿（Alces alces）开展研究，评估状态依赖型觅食、能量储备与能量摄入如何影响粪便GC与粪便T3浓度。与能量储备充足的个体相比，能量储备匮乏的个体具有更高的能量摄入与粪便T3浓度，这一结果支持了状态依赖型觅食假说。尽管粪便GC未随能量储备出现显著变化，但相较于粪便GC水平较低的个体，粪便GC水平较高的个体往往能量储备更少、能量摄入显著更高。由此可见，能量摄入更高的个体同时具有更高的粪便T3与粪便GC浓度，这一模式与此前圈养动物研究的报道结果不一致。我们提出，若GC与觅食行为增强及能量摄入增加相关联，则在表现出状态依赖型觅食的动物中，GC与T3之间或存在正相关关系。因此，我们建议在将此类内分泌标志物广泛应用于野生动物保护与管理之前，需针对不同体型与生活史策略的野外动物，开展更多关于GC-T3-能量关联的研究。