吃軟不吃硬：警戒色昆蟲的神祕次級防禦機制

Abstract

1. 抵禦掠食者攻擊是獵物適應上重要的一環。獵物的次級防禦機制 （例如毒性） 常伴隨著警戒色的出現，並可藉由避免被捕食讓掠食者與獵物彼此獲利。因此警戒色獵物被預期具有和警戒色彩一起演化的有效次級防禦機制。 2. 早在將近一百五十年前，華萊士就已經提出球背象鼻蟲 (Pachyrhynchus) 以鮮豔的花紋來警告掠食者，並以堅硬的外骨骼作為防禦的假說；然而這項假說至今依然無實驗證據驗證。本研究檢驗球背象鼻蟲的次級防禦機制假說，並探討堅硬外骨骼在球背象鼻蟲身上所扮演的生態功能。 3. 我們以斯文豪氏攀木蜥蜴（Japalura swinhonis）作為掠食者，實驗設計四組象鼻蟲處理（軟/硬；有足鉤/無足鉤）進行操控性行為實驗測試其存活率。並以比較破壞球背象鼻蟲的外骨骼，和掠食者共域昆蟲獵物所需的力量 (牛頓)，與掠食者的咬合力大小，來了解球背象鼻蟲的硬度在生態上的重要性。 最後用氣相色譜質譜法 (GC-MS) 分析球背象鼻蟲體內是否有毒性或驅避性的化合物。 4. 所有「硬」球背象鼻蟲被蜥蜴攻擊後都立即被吐出且存活。相較之下「軟」球背象鼻蟲則在被多次咀嚼後吞下。此外不論足鉤存在與否，蜥蜴對球背象鼻蟲的捕食結果都與上述相同。 5. 「硬」球背象鼻蟲的硬度顯著高於共域昆蟲的平均硬度和當地蜥蜴族群的平均咬合力。 6. GC-MS 分析出球背象鼻蟲體內四種可能的化合物（ethyl propyl ether、diethylene glycol、butylated hydroxy toluene 以及cadalene） 均無毒性或對脊椎動物有驅避的功能。 7. 本研究指出具警戒色物種的硬度是一項有效抵禦捕食者的次級防禦機制，亦提供ㄧ個探討脊椎動物掠食者與具有警戒色獵物之間時空互動的研究模式。

1. Anti-predator strategies are significant components of adaptation for prey species. The advance of the secondary defenses like toxins have led to the evolution of warning colours that benefit both prey and predators by mutual avoidance. Therefore aposematic prey are expected to possess effective secondary defenses evolved together with their warning colours. 2. This study tested the hypothesis of secondary defensive function and ecological significance of the hard body in aposematic Pachyrhynchus weevils, which was pioneered by Alfred Russel Wallace nearly 150 years. 3. We used predation trials of Japalura tree lizards to assess the survivorship of ‘hard’ (mature) vs. ‘soft’ (teneral) and ‘claw’ (intact) vs. ‘clawless’ (surgically removed) weevils. The ecological significance of the weevil’s hard body was evaluated by assessing the hardness of the weevils, local insect prey, and the bite forces of lizard populations. The existence of toxins or deterrents of the weevil was examined by Gas Chromatography-Mass Spectrometry (GC-MS). 4. All ‘hard’ weevils were spat out and survived the attack by the lizards. By contrast, the ‘soft’ weevils were chewed and subsequently swallowed. The results were the same regardless of presence or absence of the weevil’s tarsal claws. 5. The hardness of ‘hard’ Pachyrhynchus weevil was significantly higher than the average hardness of insect prey in the same habitat and mean bite forces of local lizards. 6. The four candidate compounds of the weevil identified by GC-MS (ethyl propyl ether, diethylene glycol, butylated hydroxy toluene and cadalene) had no known toxic or repellent functions against the vertebrates. 7. These results reveal that the hardness of aposematic prey functions as effective secondary defense, and provide a framework for understanding spatio-temporal interactions between vertebrate predators and aposematic insect prey.