Data from: When do trade-offs occur? The roles of energy constraints and trait flexibility in a bushcricket

In many animal species, the expression of sexually-selected traits is negatively correlated with survival traits such as immune function, a relationship termed a ‘trade-off’. But an alternative in which sexually-selected traits are positively correlated with survival traits is also widespread. The nature of inter-trait relationships may be largely determined by overall energy expenditure, energy availability, and trait flexibility, with trade-offs expected when individuals are subject to energy constraints. We tested this hypothesis in Ephippiger diurnus, a European bushcricket in which males are distinguished by two prominent sexually-selected traits, acoustic calls and a large spermatophore transferred to the female at mating, and where immune function may be critical in survival. E. diurnus are distributed as small, isolated populations that are differentiated genetically and behaviorally. We analyzed songs, spermatophores, and the immune function in male individuals from eight populations spanning a range of song types. As predicted, we only found trade-offs in those populations that expended more energy on song and were less flexible in their ability to adjust that expenditure. Ultimately, energy constraints and resulting trade-offs may limit the evolution of song exaggeration in E. diurnus populations broadcasting long calls comprised of multiple ‘syllables’.

在诸多动物类群中，性选择性状的表达与免疫功能等生存相关性状呈负相关，这种关联被称为‘权衡（trade-off）’。但性选择性状与生存相关性状呈正相关的另一种模式也广泛存在。性状间关联的本质在很大程度上由整体能量消耗、能量可获得性以及性状可塑性所决定，当个体面临能量限制时，便会出现权衡现象。我们以欧洲灌木蟋蟀Ephippiger diurnus为对象验证了这一假说：该物种的雄性具有两项显著的性选择性状——鸣声以及交配时传递给雌性的大型精包（spermatophore），而免疫功能对其生存至关重要。E. diurnus以小型隔离种群的形式分布，各种群在遗传和行为层面均存在分化。我们对覆盖多种鸣声类型的8个种群的雄性个体的鸣声、精包以及免疫功能进行了分析。正如假说所预测的那样，我们仅在那些鸣声能量消耗更高、且调整该消耗的可塑性更低的种群中观察到了权衡现象。综上，能量限制及由此产生的权衡现象，可能会限制那些发出由多个‘音节（syllable）’组成的长鸣声的Ephippiger diurnus种群的鸣声夸张化进化。