Male–male interactions select for conspicuous male coloration in damselflies

Male ornamentation, such as conspicuous male coloration, can evolve through female mate choice. Alternatively, in species without overt female mate preference, conspicuous male coloration can evolve as a badge of status via intrasexual selection to avoid costly fights or prevent mating attempts with other males. Here, we investigated the drivers of conspicuous male coloration in an ontogenetic colour-changing damselfly, <i>Xanthagrion erythroneurum, </i>where the juvenile males are yellow and change colour to red upon sexual maturity. We first showed that red males were chromatically and achromatically more conspicuous than the yellow males. We then quantified the condition of the males: red males were larger and in better condition than yellow males. We tested female preference in a choice experiment where we artificially manipulated male colour and found that females did not choose mates based on male coloration. We further tested whether the male coloration affected male–male interactions. We presented red and yellow males at the breeding site and measured behavioural responses from conspecific males and heterospecific (<i>Ischnura heterosticta</i>)<i></i>males. Red males received less aggression from conspecifics and heterospecifics than yellow males. Our study experimentally showed that male conspicuousness is not a target of female mate choice in damselflies. Instead, intra- and interspecific male–male interactions appear to be the driver of its evolution.

雄性装饰特征（如醒目的雄性体色）可通过雌性择偶选择演化形成。而在缺乏明显雌性择偶偏好的物种中，醒目的雄性体色可作为地位徽章（badge of status），通过同性选择（intrasexual selection）演化而来，以避免代价高昂的争斗，或阻止其他雄性的交配尝试。本研究针对一种个体发育变色蟌（Xanthagrion erythroneurum）展开探究，该物种的幼年雄性体色呈黄色，性成熟后会转变为红色。我们首先证实，红色雄性相较于黄色雄性，在色彩与非色彩维度上均更为醒目。随后我们量化了雄性的身体状态：红色雄性体型更大、身体状态优于黄色雄性。我们通过人工操控雄性体色的选择实验测试雌性择偶偏好，结果发现雌性并不会依据雄性体色选择配偶。我们进一步测试了雄性体色对雄性间互动的影响：在繁殖位点放置红色与黄色雄性个体，记录同种雄性与异种雄性（异痣蟌 Ischnura heterosticta）的行为反应。结果显示，红色雄性相较于黄色雄性，受到的同种与异种攻击更少。本研究通过实验证实，在蟌类中，雄性醒目体色并非雌性择偶选择的作用靶点。相反，种内与种间的雄性间竞争互动似乎是该特征演化的驱动因素。