一加一大於二：兩種防禦機制的加成作用可有效的嚇阻捕食者

標題：Multicomponent deceptive signals reduce the speed at which predators learn that prey are profitable

摘要

Many prey use multicomponent deceptive signals to fool predators into mistaking them for inedible objects, toxic prey, or danger- ous animals. However, recent experiments have suggested that multicomponent deceptive signals are no more effective in deter- ring predators than single-component signals, making it difficult to understand how they have evolved. Here, we use an established experimental system in which naive domestic chicks are presented with models of snake-mimicking caterpillars to test the idea that multicomponent deceptive signals reduce the speed at which predators learn that prey are profitable. We presented chicks with a series of 4 trials in which they encountered a single type of caterpillar model. The type of model differed among our 4 experimental groups that were arranged in a 2 × 2 factorial design: models either possessed eyespots or did not and were in either the resting or defensive posture. Chicks’ responses to the same model prey were then retested following an extended 72-h retention period. Chicks rapidly attacked prey with no defensive traits and initially showed similar levels of wariness to prey with either 1 or 2 deceptive traits. However, chicks learned that single-trait caterpillars were profitable more quickly than 2-trait caterpillars and retained their learned responses better. This suggests that prey with multicomponent deceptive signals may have a selective advantage over prey with single-component deceptive signals when predators repeatedly encounter such prey.

這個研究的概念跟討論Battus philenor的是不是每個部分的顏色對於警戒都有貢獻的概念有點類似，就是說通常有警戒斑紋的物種，也會有其他的第二防禦機制，像是某種行為配上顏色，這樣可能對於嚇阻捕食者更有效率。但就演化上來說，如果警戒色或行為已經夠有效率的話，又為何需要花更多的時間與資源來發展第二防禦？這真的會比較有效嗎？

文中透過行為學的實驗來討論這個問題，捕食者使用的是出生七天內的小家雞，獵物則是用麵粉配上食用色素，做成像Papilio canadiensis或是P. glaucus的幼蟲樣，這又分成兩個行為的型態：休息姿勢與防禦姿勢，配上有眼睛與沒眼睛，就是一個2x2因子的實驗設計。結果顯示沒有任何防禦機制（沒眼睛+休息姿勢）的最快被攻擊，只有眼睛或只有防禦姿勢在一開始的測試中與沒有防禦機制相似，之後的攻擊延遲上升，而有兩種防禦機制（有眼睛+防禦姿勢）攻擊延遲最長。結果顯示有兩種防禦機制能有效的讓捕食者遲疑。

～個人觀感～
這類實驗通常能解釋某些行為學上的觀點，可是他們覺得這個樣子的幼蟲之所以有防禦的樣子，不是單純的因為眼斑，而是因為他們認為這是有關蛇的擬態（snake mimicry）（嗯........不像吧，難道加拿大的蛇長這樣嗎？）


另一個點是這個研究團隊通常用剛出生的小雞做實驗，為的是剛出生時沒有任何經驗，很善良天真，餵什麼吃什麼，而且很可愛（誤），可以避免某些因為經驗帶來的實驗誤差，也很容易看的到學習曲線。但相對來說，因為這個年齡算是未成年，也就是思想不成熟，腦類發育可能未完全，這樣考量的話，使用這個年紀的雞來做實驗得到的結果，是不是真能夠反映自然界的狀況呢？

至於他們用黏土做的手工獵物可以看這裡，不得不說這看起來根本是另一個什麼（雖然捏的是挺可愛），然後這些笨雞還真的學到了什麼（到底學到什麼啊？）