Trimorphic males of Dorcus stag beetle exhibit three distinct fighting styles under female presence

Male trimorphism refers to the stable differences in external morphology such as body sizes and mandible shapes among male individuals, which is relatively rare in nature and usually related to alternative reproductive strategies. However, the fighting behavior of trimorphic stag beetle males with a female present has been less studied. Dorcus hopei Saunders, 1854 (Coleoptera; Lucanidae) exhibits male trimorphism, with alpha males having the largest mandibles, beta males having intermediate-sized mandibles, and gamma males having the smallest mandibles. In this study, we examined the fighting behavior of trimorphic D. hopei males in the presence of a female in the laboratory. We used the camera and BORIS software to observe and quantify males’ latency period, frequency of active attacks, duration of fighting, and the time to reach the female. Alpha males adopted a fighting strategy, exhibiting the shortest latency period, the highest probability of combat escalation, the highest frequency of active attacks, and the longest fight duration; beta males adopted a sneaking strategy; gamma males adopted a female-mimicry strategy, showing the lowest probability of combat escalation, the lowest frequency of active attacks, and the shortest fight duration. Our results indicate that the trimorphic D. hopei males exhibit three distinct fighting styles in the presence of a female. This study clarifies the dynamic relationship between the morphology and behavioral strategies of male D. hopei, providing an insight for understanding the coevolutionary mechanism of morphological polymorphism in lucanid species.