Do sexual differences in life strategies make male lizards more susceptible to parasite infection?

1. Female and male hosts may maximise their fitness by differential evolution of strategies that compensate for the costs of parasite infections. The resulting sexual dimorphism might be apparent in differential relationships between parasite load and body condition, potentially reflecting differences in energy allocation to anti-parasitic defences. For example, male lacertids with high body condition may produce many offspring while being intensely parasitised. In contrast, female lacertids may express a different outcome of the trade-offs between body condition and immunity, aiming to better protect themselves from the harm of parasites. 2. We predicted that females would have fewer parasites than males and a lower body condition across parasitaemia levels, because they would invest resources into parasite defence to mitigate the costs of infection. In contrast, the male strategy to maximise access to females would imply some level of parasite tolerance and, thus, higher parasitaemia. 3. We analysed the relationship between body condition of lizards and parasitaemias of Karyolysus and Schellackia, two genera of blood parasites with different phylogenetic origin, in 565 females and 899 males belonging to 10 species of the Lacertidae (Squamata) that were sampled during a period of 12 years across 34 sampling sites in southwestern Europe. 4. The results concerning the Karyolysus infections were consistent with the predictions, with males having similar body condition across parasitaemia levels even though they had higher infection intensities than females. On the other hand, females with higher levels of Karyolysus parasitaemia had lower body condition. This is consistent with the prediction that different life strategies of male and female lacertids can explain the infection patterns of Karyolysus. In contrast, the parasitaemia of Schellackia was consistently low in both male and female hosts, with no significant effect on body condition of lizards, suggesting that lizards of both sexes maintain this parasite bellow a pathogenic threshold.

1. 雌雄宿主可通过演化差异化策略以补偿寄生虫感染所带来的代价，进而最大化自身适合度。由此产生的雌雄二态性（sexual dimorphism）可能体现在寄生虫负荷与体况之间的关联差异上，这或许反映了二者在抗寄生虫防御的能量分配上存在区别。例如，体况优异的雄性蜥蜴科蜥蜴即便承受高强度寄生，仍可产生大量后代；与之相反，雌性蜥蜴科蜥蜴则会在体况与免疫之间呈现出不同的权衡结果，以更好地抵御寄生虫的危害。
2. 我们提出如下预测：在所有寄生虫血症（parasitaemia）水平下，雌性宿主的寄生虫数量均少于雄性，且体况更差，这是因为雌性会将资源投入到寄生虫防御中以减轻感染代价。与之相反，雄性为最大化交配机会所采取的策略，意味着它们会在一定程度上耐受寄生虫感染，进而呈现出更高的寄生虫血症水平。
3. 我们分析了565只雌性与899只雄性蜥蜴科（Lacertidae，有鳞目Squamata）10个物种的体况与两种血寄生虫属——卡氏孢子虫属（Karyolysus）和谢拉虫属（Schellackia）——的寄生虫血症水平之间的关联。这些样本采集自欧洲西南部34个采样点，历时12年完成。
4. 有关卡氏孢子虫属（Karyolysus）感染的研究结果与预测一致：尽管雄性的感染强度高于雌性，但它们在不同寄生虫血症水平下的体况并无显著差异。与之相对，寄生虫血症水平更高的雌性个体体况更差。这一结果印证了“雌雄蜥蜴科蜥蜴不同的生活策略可解释卡氏孢子虫属的感染模式”这一预测。而谢拉虫属（Schellackia）的寄生虫血症水平在雌雄宿主中均始终处于较低水平，且未对蜥蜴体况产生显著影响，这表明雌雄蜥蜴均能将该寄生虫维持在致病阈值以下。