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（Glucocorticoids, GC）与三碘甲状腺原氨酸（triiodothyronine, T3）是两类常用于量化资源限制的内分泌标志物，然而现有针对此类标志物与动物能量状态间关联的研究，多聚焦于圈养小型物种。但野生自由活动的动物会依据自身能量储备调节能量摄入，该行为被定义为状态依赖觅食。此外，生活史策略与代谢异速生长间的关联，使得小型动物的能量摄入与能量储备的耦合强度高于大型动物。鉴于GC与T3或可反映能量摄入或能量储备，状态依赖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生自由活动动物中的应用价值，我们以野生驼鹿（Alces alces）为研究对象，评估了状态依赖觅食、能量储备与能量摄入对其粪便糖皮质激素（fecal GC）与粪便三碘甲状腺原氨酸（fecal T3）浓度的影响。相较于能量储备充足的个体，能量储备匮乏的个体具有更高的能量摄入与粪便T3浓度，该结果印证了状态依赖觅食假说。尽管粪便GC水平未随能量储备出现显著变化，但粪便GC水平较高的个体，相较于粪便GC水平较低的个体，往往拥有更少的能量储备与显著更高的能量摄入。据此，能量摄入更高的个体同时呈现出更高的粪便T3与粪便GC浓度，这一模式与此前圈养动物研究的报道并不一致。我们推测，若GC与觅食活动增强及能量摄入增加存在关联，那么在表现出状态依赖觅食行为的动物中，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工作前，需针对不同体型与生活史策略的野生自由活动动物，开展更多关于GC-T3-能量关联的研究。