Cyclic Changes in the Chemical Defense of the Eastern North American Population of the Monarch Butterfly

帝王蝶毒性的季节性急剧增加似乎对它们在春季和夏季在北美重新定居至关重要。黑脉金斑蝶的幼虫以马利筋植物为食，并从这些植物中吸收有毒的菊石。这些化学物质保护蝴蝶免受捕食，因为它们是苦涩的，会导致鸟类呕吐。帝王蝶每年秋天迁徙到墨西哥，然后在第二年春天重新入侵北美东部。由于在每年的迁徙周期中，核苷会发生变化，以不同种类马利筋为食的帝王蝶会以不同的速度失去它们的威慑能力。秋季候鸟以一种弱吐的马利筋为食；在墨西哥，随后的核内酯损失导致它们的化学防御系统部分崩溃，在那里，黄鹂和雀嘴鸟每年冬天都会吃掉数百万只蝴蝶。然而，幸存下来并迁徙回美国的帝王蝶在剧毒的南方乳草上产卵，产生新一代明显有毒的蝴蝶，完成春季向北迁徙到加拿大。利用鸟类进行的生物测定和对大榄香内酯进行的化学测定将证明假定的春季代毒性增加。研究结果应该提供明确的证据，证明黑脉金斑蝶基于乳草的化学防御是一种可预测的动态现象，与它的年度迁徙周期密不可分。墨西哥中部杉木林的砍伐正在破坏黑脉金斑蝶越冬的栖息地。正因为如此，黑脉金斑蝶在北美东部的迁徙已被指定为一种濒危生物现象，这是生物多样性保护的一个新类别。这项研究计划将在未来10-15年黑脉金斑蝶数量预计会减少之前，记录下春季迁徙的生物学情况。这也与北美东部马利筋栖息地的保护有关，以维持帝王蝶在春夏季的种群数量。

A dramatic seasonal increase in toxicity of monarch butterflies appears crucial to their spring and summer recolonization of North America. Monarchs larvae feed on milkweed plants and sequester poisonous cardenolides from them. These chemicals protect the butterflies from predation because they are bitter and cause birds to vomit. Monarch butterflies migrate each fall to Mexico, then reinvade eastern North America the following spring. Since cardenolides change during the annual migration cycle, monarchs feeding on different milkweed species lose their deterrent qualities at different rates. Fall migrants feed on a weakly emetic milkweed; subsequent cardenolide loss leads to a partial breakdown of their chemical defense in Mexico, where orioles and grosbeaks eat millions of butterflies each winter. However, monarchs that survive and migrate back to the USA lay their eggs on extremely poisonous southern milkweeds, producing a new generation of apparently toxic butterflies that complete the spring migration northward to Canada. Bioassays using birds and chemical assays of the cardenolides will document the postulated increase in toxicity of the spring generation. The results should provide definitive evidence that the milkweed-based chemical defense of the monarch is a predictably dynamic phenomenon inextricably entwined with its annual migration cycle. %%% Logging of the fir forests in central Mexico is destroying the overwintering habitat of the monarch butterfly. Because of this, the monarch's migration in Eastern North America has been designated an endangered biological phenomenon, a new category of biodiversity conservation. This research program will document the biology of the spring remigration before the predicted collapse of the monarch population over the next 10-15 years. It is also relevant to conservation of milkweed habitat in eastern North America for the maintence of monarch population in spring & summer.