Immune Investment Is Explained by Sexual Selection and Pace-of-Life, but Not Longevity in Parrots (Psittaciformes)

Investment in current reproduction should come at the expense of traits promoting future reproduction, such as immunity and longevity. To date, comparative studies of pace-of-life traits have provided some support for this, with slower paced species having greater immune function. Another means of investment in current reproduction is through secondary sexual characters (SSC). Investment in SSC's is considered costly, both in terms of immunity and longevity, with greater costs being borne by species with more elaborate traits. Yet within species, females prefer more ornate males and those males are typically immunologically superior. Because of this, predictions about the relationship between immunity and SSC's across species are not clear. If traits are costly, brighter species should have reduced immune function, but the opposite is true if SSC's arise from selection for more immunocompetent individuals. My approach was to investigate immune investment in relation to SSC's, pace-of-life and longevity while considering potentially confounding ecological factors. To do so I assessed leukocyte counts from in a novel group, the Psittaciformes. Investment in SSC's best explained investment in immunity: species with brighter plumage had higher leukocyte counts and those with a greater degree of sexual dichromatism had fewer. Ecological variables and pace-of-life models tended to be poor predictors of immune investment. However, shorter incubation periods were associated with lower leukocyte counts supporting the notion that species with a fast pace-of-life invest less in immunity. These results suggest that investment in reproduction in terms of fast pace-of-life and sexual dichromatism results in reduced immunity; however, investment in plumage colour per se does not impose a cost on immunity across species.

当前繁殖投入理应以牺牲促进未来繁殖的性状为代价，例如免疫力与寿命。迄今为止，针对生活史节奏性状（pace-of-life traits）的比较研究已为该假说提供了部分支撑：生活节奏更缓慢的物种拥有更强的免疫功能。当前繁殖投入的另一途径是通过第二性征（secondary sexual characters，SSC）实现。第二性征的投入被认为具有代价，这种代价体现在免疫力与寿命两方面，且性状越复杂精致的物种承担的代价越高。然而在同一物种内，雌性更偏好羽色更艳丽的雄性，而这类雄性通常免疫能力更强。正因如此，跨物种层面上免疫力与第二性征之间的关系预测变得模糊不清：若第二性征的投入存在代价，则羽色更鲜亮的物种免疫功能应更弱；但若第二性征源自对免疫能力更强个体的选择，则结果恰恰相反。本研究旨在探究免疫投入与第二性征、生活史节奏及寿命之间的关联，同时考量可能存在干扰的生态因素。为此，我针对一个全新的类群——鹦鹉形目（Psittaciformes）——测定了其白细胞计数。研究结果显示，第二性征投入对免疫投入的解释力最强：羽色更鲜亮的物种白细胞计数更高，而两性色彩二态性更显著的物种白细胞计数更低。生态变量与生活史节奏模型往往难以有效预测免疫投入水平。然而，更短的孵化周期与更低的白细胞计数相关，这支撑了“生活节奏更快的物种免疫投入更少”这一观点。这些结果表明，以更快生活史节奏与两性色彩二态性为代表的繁殖投入会导致免疫功能下降；但从跨物种层面来看，羽色投入本身并不会对免疫功能造成代价。