Onthophagus babirussa sexual size dimporphism and male sexual trait files and R codes

Sexual size dimorphism (SSD) arises when natural selection and sexual selection act differently on males and females. Male-biased SSD is rare in insects and usually indicates strong selection pressure on a population or species. Patterns of SSD can also vary between populations of species that are exposed to different environmental conditions, such as differing resource availability and diversity. Here, we investigate intra-specific variation in SSD as well as relative investment in precopulatory (horn length) and postcopulatory traits (sperm length and testes weight) in a tropical rainforest dung beetle Onthophagus babirussa across Singapore and Peninsular Malaysia. Overall, three out of four populations displayed significant male-biased SSD and SSD was greater in populations with smaller overall body size. Average male body size was similar across all populations while female body size was significantly smaller in Singapore, suggesting that the pronounced SSD may also be due to stronger sexual selection on male body size in Singapore populations. All populations showed significant investment in horns as a weapon used in precopulatory competition, while postcopulatory traits showed no clear scaling relationship with body size, suggesting a higher priority on precopulatory sexual traits in the mating system of this species.

性大小二态性（Sexual size dimorphism, SSD）指自然选择与性选择对雌雄两性的作用存在差异的现象。雄性偏向型性大小二态性在昆虫中较为罕见，通常预示着种群或物种承受了较强的选择压力。性大小二态性的模式还会因物种所处环境条件的差异而在种群间有所不同，例如资源可获得性与多样性存在分化的情况。本研究以分布于新加坡及马来半岛的热带雨林蜣螂Onthophagus babirussa为研究对象，探讨了该物种种内的性大小二态性变异，以及其在交配前（precopulatory）性状（角长）与交配后（postcopulatory）性状（精子长度、精巢重量）上的相对投入情况。总体而言，四个种群中有三个表现出显著的雄性偏向型性大小二态性，且整体体型越小的种群，其性大小二态性程度越高。各种群的雄性平均体型无显著差异，但新加坡种群的雌性平均体型显著更小，这表明显著的性大小二态性可能还源于新加坡种群中雄性体型承受了更强的性选择压力。所有种群均表现出对作为交配前竞争武器的角的显著投入，而交配后性状与体型未呈现明确的缩放关系，这暗示该物种种群的交配系统更优先重视交配前性性状。