The habitat and resource use of females critically affects their pattern of distribution and consequently their monoposibility by males and the mating system of a species. Shifts in habitat use are therefore likely to be associated with changes in mating system and sexual selection acting on males' phenotypes, consequently affecting patterns of sexual dimorphism. Although sexual dimorphism is often correlated with shifts in habitat use at the macroevolutionary scale, the underlying microevolutionary processes involved are typically unclear. Here, we used the New Guinean stick insect genus Eurycantha to investigate how changes in habitat use and mating system were associated with a change in sexual dimorphism seen specifically in the thorny devil stick insects (Eurycantha calcarata and Eurycantha horrida). Male thorny devils display sexually dimorphic and enlarged hindlegs endowed with a sharp spine. Sexual size dimorphism is also very reduced in these species relative to other phasmids. Using field observations, morphological measurements and radiotelemetry, we investigated changes in mating system associated with the reduction of sexual dimorphism and tested predictions from the hypothesis that sexual selection drove the evolution of this unusual male morphology. We found that thorny devils switched from solitary roosting in the canopy during the day to communal roosting inside cavities of a few host trees, shifting the distribution of females from scattered to clumped. Male thorny devils used their large hindlegs to fight with rivals for positions on the tree close to cavities containing females, and larger males were associated with cavities containing relatively more females. In contrast, the sister species, Eurycantha insularis, displays relatively small and unarmoured males (ancestral state). Adult female E. insularis were always scattered in the canopy, and this species displayed a scramble competition mating system typical of other phasmids, where mobility, rather than fighting ability, is probably critical to males' reproductive success. Overall, our study illustrates how a drastic change in sexual dimorphism can be associated with a switch from solitary to communal roosting and from a scramble competition to a defense-based polygyny mating system.

中文翻译：

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姊妹物种之间的性二态性差异与多刺魔鬼竹节虫的栖息地利用和交配系统的转换有关

雌性的栖息地和资源利用严重影响它们的分布模式，从而影响它们被雄性的垄断性和物种的交配系统。因此，栖息地使用的变化可能与交配系统的变化和作用于雄性表型的性选择的变化有关，从而影响性二态性的模式。尽管性别二态性通常与宏观进化尺度上栖息地使用的变化相关，但所涉及的潜在微观进化过程通常尚不清楚。在这里，我们使用新几内亚竹节虫属 Eurycantha 来研究栖息地利用和交配系统的变化如何与多刺魔鬼竹节虫（Eurycantha calcarata 和 Eurycantha horrida）中特有的两性异形变化相关联。雄性多刺恶魔表现出两性异形和增大的后腿，后腿具有锋利的脊椎。相对于其他 phasmids，这些物种的性别大小二态性也大大减少。通过实地观察、形态学测量和无线电遥测，我们调查了与性别二态性减少相关的交配系统的变化，并测试了假设性选择推动了这种不寻常的雄性形态进化的预测。我们发现多刺魔鬼从白天在树冠中单独栖息转变为在少数寄主树的空腔内共同栖息，从而将雌性的分布从分散变为丛生。雄性多刺魔鬼用它们的大后腿与对手争夺靠近雌性蛀洞的树上的位置，较大的雄性与含有相对较多雌性的腔有关。相比之下，姊妹物种 Eurycantha insularis 表现出相对较小且没有装甲的雄性（祖先状态）。成年雌性 E. insularis 总是分散在树冠中，该物种表现出其他 phasmids 典型的争夺竞争交配系统，其中流动性，而不是战斗能力，可能是雄性繁殖成功的关键。总体而言，我们的研究说明了性别二态性的急剧变化如何与从单独到公共栖息以及从争夺竞争到基于防御的一夫多妻交配系统的转变相关联。insularis 总是分散在树冠中，这个物种表现出其他 phasmids 典型的争夺竞争交配系统，其中流动性，而不是战斗能力，可能是雄性繁殖成功的关键。总体而言，我们的研究说明了性别二态性的急剧变化如何与从单独到公共栖息以及从争夺竞争到基于防御的一夫多妻交配系统的转变相关联。insularis 总是分散在树冠中，这个物种表现出其他 phasmids 典型的争夺竞争交配系统，其中流动性，而不是战斗能力，可能是雄性繁殖成功的关键。总体而言，我们的研究说明了性别二态性的急剧变化如何与从单独到公共栖息以及从争夺竞争到基于防御的一夫多妻交配系统的转变相关联。