Data from: Condition-dependent trade-offs between sexual traits, body condition and immunity: the effect of novel habitats

Background: The optimal allocation of resources to sexual signals and other life history traits is usually dependent on an individual's condition, while variation in the expression of sexual traits across environments depends on the combined effects of local adaptation, mean condition, and phenotypic responses to environment-specific cues that affect resource allocation. A clear contrast can often be drawn between natural habitats and novel habitats, such as forest plantations and urban areas. In some species, males seem to change their sexual signals in these novel environments, but why this occurs and how it affects signal reliability is still poorly understood.
Results: The relative size of sexual traits and level of immune responses were significantly lower for male palmate newts Lissotriton helveticus caught in pine and eucalyptus plantations compared to those caught in native forests, but there was no habitat-dependent difference in body condition (n= 18 sites, 382 males). The reliability with which sexual traits signalled body condition and immune responses was the same in all three habitats. Finally, we conducted a mesocosm experiment in which males were maintained in pine, eucalypt or oak infused water for 21 days. Males in plantation-like water (pine or eucalypt) showed significantly lower immune responses but no change in body condition. This matches the pattern seen for field-caught males. Unlike field-caught males, however, there was no relationship between water type and relative sexual trait size.
Conclusions: Pine and eucalyptus plantations are likely to be detrimental to male palmate newt because they are associated with reduced immune function and smaller sexual traits. This could be because ecological aspects of these novel habitats, such as high water turbidity or changes in male-male competition, drive selection for reduced investment into sexual traits. However, it is more probable that there are differences in the ease of acquisition, hence optimal allocation, of resources among habitats. Our mesocosm experiment also provides some evidence that water toxicity is a causal factor. Our findings offer insights into how plantations affect amphibian life histories, and how novel habitats might generate long-term selection for new resource allocation strategies in native species.

研究背景：将资源最优分配至性信号（sexual signals）与其他生活史性状（life history traits）的策略，通常取决于个体的生理状态；而不同环境中性状表达的差异，则由局部适应（local adaptation）、平均生理状态，以及影响资源分配的环境特异性信号所引发的表型响应（phenotypic responses）共同决定。自然生境与人工林、城市区域等新生境（novel habitats）之间往往存在显著差异。部分物种的雄性在这类新生境中似乎会改变其性信号，但该现象的成因及其对信号可靠性的影响机制仍尚不明确。
研究结果：与原生林（native forests）捕获的雄性掌状真螈（Lissotriton helveticus）相比，人工松林与桉树林中捕获的雄性个体的性性状（sexual traits）相对大小与免疫响应（immune responses）水平均显著更低，但不同生境下的个体生理状态并无显著差异（共18个采样点，382只雄性个体）。在三类生境中，性性状对个体生理状态与免疫响应的指示可靠性并无显著差异。本研究还开展了中型生态箱实验（mesocosm experiment）：将雄性掌状真螈分别饲养于松树、桉树或栎树浸提液配制的水环境中，为期21天。饲养于人工林来源水环境（松树或桉树浸提液）中的雄性个体，其免疫响应水平显著更低，但生理状态未发生明显变化，这与野外捕获个体的观测结果一致。但与野外捕获的雄性个体不同，水环境类型与性性状相对大小之间并无关联。
研究结论：人工松林与桉树林可能对雄性掌状真螈产生不利影响，因为这类生境与个体免疫功能下降、性性状偏小存在关联。该现象的潜在成因可能包括：这类新生境的生态特征（如水体浊度升高、雄性间竞争模式改变等）通过自然选择促使雄性减少对性性状的资源投入。但更有可能的解释是，不同生境中资源获取的难易程度存在差异，进而影响资源的最优分配策略；本研究的中型生态箱实验也为“水体毒性是致病成因之一”提供了相关证据。本研究结果揭示了人工林如何影响两栖动物的生活史，以及新生境可能如何对本土物种施加长期选择压力，推动其演化出新的资源分配策略。