長短牙型鬼豔鍬形蟲咬合力與打鬥行為差異

Abstract

在動物的雄性競爭中，擁有不同武器型態的個體，可能採取特定的打鬥策略及打鬥行為使武器發揮最大功能(打鬥情境分化假說divergent fighting contexts hypothesis)。雄性鍬形蟲具有特化的大顎做為武器，大顎尺寸及咬合力經常是打鬥勝負的決定因子。不同於多數鍬形蟲大顎型態受體型影響，鬼豔鍬形蟲(Odontolabis siva)的大型個體出現長牙型(major)及短牙型(minor)，因此鬼豔鍬形蟲提供了檢驗物種在體型相近的情況下，不同武器型態如何影響打鬥行為的良好案例。本研究測試打鬥情境分化假說並探討大顎型態如何影響咬合力及打鬥行為，研究預期體型相近的短牙型比長牙型有較大的咬合力，而較長的大顎則有利於鉗著對手身體並將對手拋開。研究採集鬼豔鍬形蟲野外個體，先進行異速生長分析確認型態與型值的關係，之後進行咬合力測量與體型相近配對的打鬥實驗，記錄打鬥影片各階段打鬥行為發生頻率與持續時間，利用複迴歸模型分析型態對咬合力及打鬥行為發生頻率的相關性，並以行為序列分析建立各形態組間的打鬥行為序列。結果指出，鬼豔鍬形蟲長牙型為等速生長，而短牙型為正異速生長，顯示短牙型中的大型個體對武器發育的投資比例相對身體增加。相同體型的大型個體中短牙型咬合力大於長牙型，短牙型發生纏鬥(tussle)的機率高於長牙型。長牙型傾向以鉗著頭胸(clamp-HT)的打鬥策略拋開對手，而短牙型則使用纏鬥(tussle)或鉗著大顎(clamp-M)的打鬥策略，研究結果支持打鬥情境分化假說，武器型態為特定打鬥策略的適應演化結果。Animal weapons are highly diverse in forms, but the evolutionary drivers for weapon diversity remain unexplored. One of the most compelling explanations for weapon diversity posits that weapon forms is adaptations of species-specific fighting styles, and selection for optimizing weapon performance under different fighting styles can drive evolutionary changes in weapon forms (divergent fighting contexts hypothesis). The study tested the prediction from this hypothesis that male morphs with different weapon forms should exhibit distinct fighting styles to enhance combat performance in Odontolabis siva parryi stag beetles. This study examined mandible allometries, measured bite forces and characterized fighting behaviors of stag beetles in size-matched contests by controlling the effect of body sizes on contest outcomes. The results show that O. siva parryi consists of two clearly differentiated male morphs and weapon forms: majors with long, slender and smooth mandibles; and minors with short, wide and tusked mandibles. A positive relationship between bite force and mandible size in minors suggests that weapons size is an honest signal of bite performance in minors. While negative relationship between bite force and mandible size in majors indicates weapon size cannot predict bite performance in majors. Majors primarily use their long and slender mandibles to grasp the head or thorax of rivals and lift them (fighting style of grasp and lift). In contrast, minors with short and wide mandibles mostly tussle or clamp onto the mandibles of rivals and lift them (fighting style of tussle and lift). By adopting morph-specific fighting styles to win the contests, majors have a mechanical advantage of longer reaches, while minors enjoy a physical advantage of higher bite forces than their rivals. Remarkably, majors with longer mandibles and minors with higher bite forces each win an approximately equal proportion of size-matched contests, indicating that two distinct weapon forms and fighting styles are probably equally effective in combats. Our research provides the first empirical evidence in stag beetles suggesting that weapon forms adapt behavior to enhance combat performance under specific fighting styles.