冠羽畫眉之偶外配對繁殖策略

Abstract

配對系統中，與配偶之外個體交配所生之子代稱為偶外配對子代（extra pair paternity, EPP），目前研究認為偶外配對是雌鳥策略下的產物，而偶外配對的發生亦可能源於雌鳥的混合繁殖策略（mixed reproductive strategy），雌鳥透過偶外配對應可獲得不同的直接或間接繁殖利益。雌鳥選擇假說（female choice hypothesis）和雄鳥交換假說（trade-off hypothesis）中，分別對偶外配對行為中有不同的預測。根據雌鳥選擇假說，雌鳥能選擇行偶外配對及行偶外配對的對象。冠羽畫眉（Yuhina brunneiceps）屬於合作生殖（cooperative breeding system）中共用一巢制（joint-nesting system）的單配對鳥種，群體內多於兩隻以上的雌鳥在同一巢中下蛋，群成員共同分擔築巢、孵蛋、孵雛及育雛等工作。在野外的行為觀察中，曾紀錄到低位階雄鳥與高位階雌鳥交配的非正常配對行為，經親緣鑑定（paternity analysis）證實，冠羽畫眉有25.5％的子代是由偶外配對所生，包括群內偶外配對與群外偶外配對兩種情形。資料蒐集自2001–2006年的繁殖季，野外工作除行為觀察、監測及紀錄各繁殖巢，同時進行繫放成鳥與幼鳥以及胚胎採樣。親緣分析結果顯示，41.2％的巢含有1隻以上的偶外配對子代，偶外配對的子代佔所有子代的21.7％。冠羽畫眉雌鳥選擇群外偶外配對的主要對象為高位階雄鳥，其偶外配對行為也較符合雌鳥選擇假說；雌鳥可能藉由群外偶外配對產生較優秀的子代，以獲得間接利益；冠羽畫眉雌鳥以不同的配對方式作為投資策略，以獲得最高的總繁殖利益。繁殖群內的不同個體可能因需求與目的不同，而採行不同的繁殖策略，因而造成不同位階及性別的成鳥選擇不同配對方式，由於行偶外配對可為個體帶來額外的繁殖利益，因此推測偶外配對是冠羽畫眉的一種繁殖策略。

Extra-Pair paternity (EPP) is defined as the offspring resulting from copulations outside the social pair bonds in the mating system. Field studies and comparative analyses have shown that EPP is due to female strategy. Females might obtain direct or indirect benefits through EPP. Female choice hypothesis predicts that females can pursue EPP and choose the preferred EPP males. By the contrary, EPP is constrained by males mate guarding behavior and females show no preference in the trade-off hypothesis. Taiwan yuhinas（Yuhina brunneiceps）is a joint-nesting, cooperatively breeding bird. The reproductive skew is low (0.19). Individuals in the breeding group share the load of nesting, the incubation and the provisioning. EPP has been recorded by observation in Taiwan yuhinas and there are 25.5% of EPP offspring have been recognized by paternity analysis. Data was selected from the breeding seasons during 2001-2006. Results showed that females prefer high-ranking males as EPP males and females might obtain indirect benefits from producing EPP offspring. Each member in the breeding group with its own demand and aim adapts different breeding strategies to get the highest reproductive benefits. Therefore, EPP is one kind of reproductive strategy in Taiwan yuhinas.